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100 Chapter 4

4.1 Studying Atoms

Key ConceptsWhat was Dalton’s theoryof the structure of matter?

What contributions didThomson and Rutherfordmake to the developmentof atomic theory?

Vocabulary◆ nucleus

Reading StrategySummarizing Copy thetable. As you read,complete the table aboutatomic models.

Studying the structure of atoms is a little like studying wind. Becauseyou cannot see air, you must use indirect evidence to tell the directionof the wind. You might notice which way fallen leaves move as they arepushed by the wind, and infer that the leaves and wind are moving inthe same direction.

Atoms pose a similar problem because they are extremely small.Even with a microscope, scientists cannot see the structure of an atom.In this chapter, you will find out how John Dalton, J. J. Thomson,Ernest Rutherford, Niels Bohr, and other scientists used evidence fromexperiments to develop models of atoms.

Ancient Greek Models of Atoms If you cut a piece of aluminum foil in half, you have two smaller piecesof the same shiny, flexible substance. You could cut the pieces again andagain. Can you keep dividing the aluminum into smaller pieces? Greekphilosophers debated a similar question about 2500 years ago.

The philosopher Democritus believed that all matter consisted ofextremely small particles that could not be divided. He called theseparticles atoms from the Greek word atomos, which means “uncut” or“indivisible.” He thought there were different types of atoms with spe-cific sets of properties. The atoms in liquids, for example, were roundand smooth, but the atoms in solids were rough and prickly.

Aristotle did not think there was a limit to the number of timesmatter could be divided. Figure 1 shows the model Aristotle used todescribe matter. For many centuries, most people accepted Aristotle’sviews on the structure of matter. But by the 1800s, scientists hadenough data from experiments to support an atomic model of matter.

Rutherford

Ratio of massesin compounds

Positive, densenucleus

Scientist Evidence Model

b. ?a. ?

Deflected beam

e. ?

d. ?c. ?

Fire

Water

DryHot

ColdW

et

Air Earth

Figure 1 Aristotle thought thatall substances were built up fromonly four elements—earth, air,fire, and water. These elementswere a combination of fourqualities—hot, cold, dry, and wet.Fire was a combination of hot anddry. Water was a combination ofcold and wet.

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FOCUS

Objectives4.1.1 Describe ancient Greek models

of matter.4.1.2 List the main points of Dalton’s

atomic theory and describe hisevidence for the existence ofatoms.

4.1.3 Explain how Thomson andRutherford used data fromexperiments to produce theiratomic models.

Build VocabularyLatin Plural Forms Explain that theword nucleus comes from a Latin wordmeaning “kernel.” Explain that a kernelis a grain or seed. Ask students to discusshow the definition of the term nucleusrelates to its Latin origin. (Like the kernelof a nut, the nucleus is a small, massivecenter of the atom.) Remind students thatthe plural of the word nucleus is nuclei.

Reading Strategya. Dalton b. Indivisible, solid spheresc. Thomson d. Negative charges evenlyscattered through a positively chargedmass of matter (plum pudding model)e. Deflection of alpha particles passingthrough gold foil

INSTRUCT

Ancient Greek Models of AtomsUse VisualsFigure 1 Have students examine thediagram in Figure 1 that lists thequalities of each of Aristotle’s fourelements. Ask, What qualities didAristotle use to describe air? (Air is acombination of hot and wet.) Whatelement was a combination of dryand cold? (Earth) Is “wet and cold” an accurate description of water?(Wet describes water, but water isn’talways cold.)Visual

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Reading Focus

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Section 4.1

Print• Reading and Study Workbook With

Math Support, Section 4.1 • Transparencies, Chapter Pretest and

Section 4.1

Technology• Interactive Textbook, Section 4.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 4.1• Go Online, NSTA SciLinks, Atomic theory

Section Resources

Dalton’s Atomic TheoryJohn Dalton was born in England in 1766. He was a teacher who spenthis spare time doing scientific experiments. Because of his interest inpredicting the weather, Dalton studied the behavior of gases in air.Based on the way gases exert pressure, Dalton correctly concluded thata gas consists of individual particles.

Evidence for Atoms Dalton gathered evidence for the existenceof atoms by measuring the masses of elements that combine whencompounds form. He noticed that all compounds have something incommon. No matter how large or small the sample, the ratio of themasses of the elements in the compound is always the same. In otherwords, compounds have a fixed composition.

For example, when magnesium burns, as shown in Figure 2, itcombines with oxygen. The product of this change is a white solidcalled magnesium oxide. A 100-gram sample of magnesium combineswith 65.8 grams of oxygen. A 10-gram sample of magnesium com-bines with 6.58 grams of oxygen. The ratio of the mass of magnesiumto the mass of oxygen is constant in magnesium oxide.

Dalton’s Theory Dalton developed a theory to explain why theelements in a compound always join in the same way. Dalton pro-posed the theory that all matter is made up of individual particlescalled atoms, which cannot be divided. The main points of Dalton’stheory are as follows.

■ All elements are composed of atoms.

■ All atoms of the same element have the same mass, and atoms ofdifferent elements have different masses.

■ Compounds contain atoms of more than one element.

■ In a particular compound, atoms of different elements alwayscombine in the same way.

In the model of atoms based on Dalton’s theory, the elementsare pictured as solid spheres like those in Figure 3. Each type ofatom is represented by a tiny, solid sphere with a different mass.

Recall that a theory must explain the data from many exper-iments. Because Dalton’s atomic theory met that goal, the theorybecame widely accepted. Over time, scientists found that not allof Dalton’s ideas about atoms were completely correct. But this didnot cause later scientists to discard the atomic theory. Instead, theyrevised the theory to take into account new discoveries.

What did Dalton notice that all compounds havein common?

Figure 2 Magnesium reacts withoxygen to form the compoundmagnesium oxide. The ratio ofmagnesium to oxygen, by mass, inmagnesium oxide is always about3 : 2. Observing What color ismagnesium oxide?

Atomic Structure 101

Figure 3 Dalton made thesewooden spheres to represent theatoms of different elements.

Dalton’s AtomicTheoryBuild Science SkillsEvaluating As students read Chapter 4,have them evaluate what portions ofDalton’s model were accurate and whatportions needed to be revised. (Daltondid not discuss subatomic particles, which are smaller components of atoms.Atoms of the same element can havedifferent masses.)Logical

Many students have trouble differen-tiating compounds from mixtures.Remind students that Dalton noticedthat the ratio of masses of elements in a compound is always the same.Compounds are distinguished frommixtures and solutions by their fixedcompositions. Have students recallexamples of mixtures and describe how their compositions can vary. Verbal

Use VisualsFigure 3 Have students examine thewooden spheres shown in Figure 3. Ask, Why do you think there are holes in Dalton’s wooden spheres?(One acceptable answer is that Daltonused the holes in the spheres to connectspheres together to construct models of compounds.)Visual, Logical

FYINot every scientist was convinced thatDalton had the physical evidence tosupport the assumption that elements arecomposed of indivisible particles calledatoms. William Whewell (1784–1868)argued that particles could combine infixed proportions in compounds, but stillnot be indivisible.

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Customize for English Language Learners

Using Visual Aids Have students draw simple illustrations for the models of atoms according to Dalton,Thompson, and Rutherford. Have them

describe each model to a partner using theillustrations as a visual aid. Then, have eachpair of students discuss the similarities anddifferences among the three models. Answer to . . .

Figure 2 White

Dalton noticed that the ratio of masses of

elements in a compound is always the same.

0098_hsps09te_Ch04.qxp 4/18/07 10:46 AM Page 101

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Thomson’s Model of the AtomWhen some materials are rubbed, they gain the ability to attract orrepel other materials. Glass and the amber in Figure 4 have this prop-erty. Based on their behavior, such materials are said to have either apositive or a negative electric charge. Objects with like charges repel,or push apart. Objects with opposite charges attract, or pull together.

Some charged particles can flow from one location to another.A flowof charged particles is called an electric current. When you turn on anappliance such as a hair dryer, a current flows from the wall socketthrough the appliance. Joseph John Thomson (1856–1940), better knownas J. J. Thomson, used an electric current to learn more about atoms.

Thomson’s Experiments Thomson used a device like the oneshown in Figure 5A. At the center of the device is a sealed glass tubefrom which most of the air has been removed. There is a metal disk ateach end of the tube. Wires connect the metal disks to a source of elec-tric current. When the current is turned on, one disk becomes negativelycharged and the other disk becomes positively charged. A glowingbeam appears in the space between the disks.

Thomson hypothesized that the beam was a stream of charged par-ticles that interacted with the air in the tube and caused the air to glow.In one experiment Thomson did to test his hypothesis, he placed a pairof charged metal plates on either side of the glass tube, as shown inFigure 5B. The plates caused the beam to deflect, or bend, from itsstraight path. Thomson observed that the beam was repelled by thenegatively charged plate and attracted by the positively charged plate.

Investigating Charged Objects

Materialstransparent tape, metric ruler, scissors

Procedure1. Cut two 10-cm pieces of tape. Fold over 1 cm

of tape at one end of each piece of tape toform a “handle.”

2. Hold the pieces of tape by their folded endsso that they are hanging straight down.Then, without letting the pieces of tape touch,slowly bring their sticky sides close together.Record your observations.

3. Place one piece of tape on a clean surface withthe sticky side facing down.

4. Place the second piece, sticky side down,directly over the first piece, as shown. Pressdown firmly so the pieces stick together.

5. Remove the joined strips from the table.Slowly peel the strips apart.

6. Bring the separated strips close togetherwithout touching. Record your observations.

Analyze and Conclude1. Drawing Conclusions What can you

conclude about the charges on the twopieces of tape after they are separated?

2. Inferring What other objects have youobserved that became charged?

Sticky sides down

Figure 4 Amber is the hardenedform of a sticky, viscous liquidthat protects trees from insectsand disease. If amber is rubbedwith wool, it becomes chargedand can attract a feather.Predicting What will happento the feather if the amber losesits charge?

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Thomson’s Model of the Atom

Investigating Charged Objects

Objective After completing this activity, studentswill be able to• explain that like charges repel and

unlike charges attract.

Skills Focus Observing, DrawingConclusions, FormulatingHypotheses

Prep Time 5 minutes

Class Time 10 minutes

Expected Outcome Students willdiscover that the two pieces of tape areattracted to one another after they arepulled apart and then brought near oneanother.

Analyze and Conclude 1. The two pieces of tape have oppositecharges because they attract whenbrought close together. 2. Possible answers include clothes thatcling together when removed from adryer, and the charge that builds upwhen a person walks across a carpet(which is demonstrated by the sparkthat occurs when the person touches a doorknob). Kinesthetic, Logical

FYIIf you do not want to present all theexperimental evidence for the atomictheory, be sure students understandattraction and repulsion of chargedparticles and Rutherford’s nucleus modelof the atom.

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Section 4.1 (continued)

Charge People have known for thousands ofyears that amber can attract other materials after it has been rubbed with fur. Plato evenrefers to these attractive powers of amber in one of his dialogues.

The labels positive and negative werearbitrarily assigned by Benjamin Franklinduring his studies of electric charge andelectric current. (He also was the first to usethe terms battery and conductor.)

Facts and Figures

Evidence for Subatomic Particles Thomson concludedthat the particles in the beam had a negative charge because they wereattracted to the positive plate. He hypothesized that the particles camefrom inside atoms. He had two pieces of evidence to support hishypothesis. No matter what metal Thomson used for the disk, the par-ticles produced were identical. The particles had about the massof a hydrogen atom, the lightest atom.

Thomson’s discovery changed how scientists thought about atoms.Before his experiments, the accepted model of the atom was a solid ballof matter that could not be divided into smaller parts. Thomson’sexperiments provided the first evidence that atoms are made of evensmaller particles. Thomson revised Dalton’s model to account for thesesubatomic particles.

Thomson’s Model An atom is neutral, meaning it has neither anegative nor a positive charge. How can an atom contain negative par-ticles and still be neutral? There must be some positive charge in theatom. In Thomson’s model of the atom, the negative charges wereevenly scattered throughout an atom filled with a positively chargedmass of matter. The model is called the “plum pudding” model, aftera traditional English dessert.

You might prefer to think of Thomson’s model as the “chocolatechip ice cream” model. Think of the chocolate chips in Figure 6 as thenegative particles and the vanilla ice cream as the positively chargedmass of matter. When the chocolate chips are spread evenly through-out the ice cream, their “negative charges” balance out the “positivecharge” of the vanilla ice cream.

How do objects with the same charge behavewhen they come close to one another?

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Sealed tube filled withgas at low pressure

Source ofelectric current

Metal disk

–

+

Metal disk

Glowingbeam

A

Figure 5 Thomson used a sealed tube of gas in his experiments.A When the current was on, the disks became charged and aglowing beam appeared in the tube. B The beam bent toward apositively charged plate placed outside the tube. Inferring What was the charge on the particles in the beam?

Figure 6 A scoop of chocolatechip ice cream can representThomson’s model of the atom.The chips represent negativelycharged particles, which arespread evenly through a mass ofpositively charged matter—thevanilla ice cream.


Positiveplate

Negativeplate

B

Source ofelectric current

Metal disk

–

+

FYIThomson used the speed of an electron,its angle of deflection, and the strengthof the current to determine the charge-to-mass ratio of an electron. RobertMilliken determined the actual mass of an electron through his oil-dropexperiment.

The current that flows through anappliance is an alternating current. Thecurrent Thomson used in his experimentwas a direct current. Not all movementof charge is a current. Charge can flowto or from a balloon (or between a handand a doorknob). With a current, chargemust flow continuously (at least until the circuit is interrupted). The conceptsof electric charge and current areaddressed in depth in Chapter 20.

Use Community ResourcesHave a physics or chemistry professor visitthe class and demonstrate Thomson’sexperiment using a cathode ray tube.Encourage students to think of questionsto ask about how the experimentdemonstrates the properties of electrons. Interpersonal, Visual

Build Reading LiteracyCompare and Contrast Refer topage 226D in Chapter 8, whichprovides the guidelines for comparingand contrasting.

Have students read about the differentatomic models described in Section 4.1.Then, have students create a chart thatcompares and contrasts each model.Logical

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Answer to . . .

Figure 4 The feather will no longerbe attracted to the amber and willdrop to the ground.

Figure 5 Negative

Objects with the samecharge repel.


Expected results

Gold atoms

Actual results

Nucleus

Alphaparticles

Alphaparticles

Rutherford’s Atomic TheoryWhen you try something new, you may have expectations about theoutcome. Does the outcome always meet your expectations or are yousometimes surprised? Scientists can also be surprised by the results oftheir experiments, but unexpected results can lead to important dis-coveries. This is what happened to Ernest Rutherford (1871–1937).

Rutherford’s Hypothesis In 1899, Ernest Rutherford discov-ered that uranium emits fast-moving particles that have a positivecharge. He named them alpha particles. In 1909, Rutherford asked oneof his students, Ernest Marsden, to find out what happens to alphaparticles when they pass through a thin sheet of gold.

Recall that in Thomson’s model of the atom, the mass and positivecharge are evenly spread throughout an atom. Based on this model,Rutherford hypothesized that the mass and charge at any location inthe gold would be too small to change the path of an alpha particle. Hepredicted that most particles would travel in a straight path from theirsource to a screen that lit up when struck. Those few that did not passstraight through would be deflected only slightly.

The Gold Foil Experiment Marsden used the equipmentshown in Figure 7. He aimed a narrow beam of alpha particles at thegold. The screen around the gold was made of a material that produceda flash of light when struck by a fast-moving alpha particle. By observ-ing the flash, Marsden could figure out the path of an alpha particleafter it passed through the gold.

Some of the locations of the flashes on the screen did not supportRutherford’s prediction. More particles were deflected than he expected.About one out of every 20,000 was deflected by more than 90 degrees.Some of the alpha particles behaved as though they had struck an objectand bounced straight back.

Figure 7 The path of an alphaparticle can be detected by thelocation of a flash on a screen.Rutherford expected the paths ofthe positively charged alphaparticles that were aimed at thethin gold foil to be affected onlyslightly by the gold atoms. Butmore particles were deflectedthan expected and some particlesbounced straight back.

For: Links on atomic theory

Visit: www.SciLinks.org

Web Code: ccn-1041

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Screen

Undeflectedparticle

Gold foil

90�

Source ofalpha particles

Slit

Beam ofalpha

particles

Deflectedparticle

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Rutherford’s Atomic TheoryFYIBased on Figure 7, students mightconclude that Marsden used a circularscreen. The screen in the diagramrepresents multiple positions of a smallerscreen that was moved from one positionto another while data was collected.Alpha particles will be identified ashelium nuclei in Chapter 10.

Comparing Atomic Models

Purpose Students will comparedifferent atomic models.

Materials 3 clear, round bowls;flavored gelatin mix; canned blueberries;maraschino cherries

Procedure The day before, prepare theflavored gelatin mix. Divide the liquidgelatin evenly into the three bowls. Chillslightly. Drain the blueberries and addthem to one of the bowls so that theyare distributed as evenly as possible inthe nearly gelled mix. Return the bowlsto the refrigerator. Right before class,place a cherry in the center of the thirdbowl. Have students discuss whichatomic models are represented by eachbowl of gelatin.

Expected Outcome The gelatin alonerepresents Dalton’s atomic model. Thegelatin with blueberries representsThomson’s “plum-pudding” atomicmodel, with the berries representingelectrons. The gelatin with the cherryrepresents Rutherford’s atomic model.Visual, Logical

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In His Own Words In a lecture Rutherfordgave at Cambridge in 1936, he recalled hisreaction when Geiger told him about alphaparticles being scattered backward by the

gold foil. Rutherford described his reaction. “It was almost as incredible as if you fired a15-inch shell at a piece of tissue paper and itcame back and hit you.”

Facts and Figures

Download a worksheet on atomictheory for students to complete,and find additional teacher supportfrom NSTA SciLinks.

Section 4.1 Assessment

Reviewing Concepts1. What theory did Dalton propose about

the structure of matter?

2. What evidence did J. J. Thomson provideabout the structure of an atom?

3. What did Rutherford discover about thestructure of an atom?

4. What evidence did Thomson have that hisglowing beam contained negative particles?

5. Why was Dalton’s model of the atom changedafter Thomson’s experiment?

Critical Thinking6. Comparing and Contrasting Explain why

scientists accepted Dalton’s atomic theory butnot the idea of an atom proposed by theGreek philosophers.

7. Drawing Conclusions If you observeda beam of particles being bent toward anegatively charged plate, what mightyou conclude?

8. Relating Cause and Effect In theRutherford experiment, why weren’t allthe alpha particles deflected?

Discovery of the Nucleus The alpha particles whosepaths were deflected must have come close to another chargedobject. The closer they came, the greater the deflection was.But many alpha particles passed through the gold withoutbeing deflected. From these results, Rutherford con-cluded that the positive charge of an atom is not evenlyspread throughout the atom. It is concentrated in avery small, central area that Rutherford called thenucleus. The nucleus is a dense, positively chargedmass located in the center of the atom. (The plural ofnucleus is nuclei.)

Because Thomson’s model no longer explainedall the evidence, Rutherford proposed a new model.

According to Rutherford’s model, all of an atom’spositive charge is concentrated in its nucleus. The alphaparticles whose paths were deflected by more than 90 degreescame very close to a nucleus. The alpha particles whose paths werenot bent moved through the space surrounding the nuclei withoutcoming very close to any nucleus.

Figure 8 shows the inside of the Astrodome, a domed stadium inHouston, Texas. The roof of the stadium rises to a height of 202 feetabove the center of the field. If an atom had the same volume as the sta-dium, its nucleus would have the volume of a marble. The total volumeof an atom is about a trillion (1012) times the volume of its nucleus.


Writing to Persuade Imagine you live inancient Greece. Assume all you know aboutmatter is what you can observe with your fivesenses. You have heard the views of bothDemocritus and Aristotle about matter. Writea paragraph supporting one of their views.

Figure 8 The Houston Astrodomeoccupies more than nine acres andseats 60,000 people. If the stadiumwere a model for an atom, amarble could represent its nucleus. Using Analogies In the model,where would the marble have tobe located in the stadium torepresent the nucleus?

Build Science SkillsUsing Models Have students model thegold foil experiment by shooting marblesacross the floor at an arrangement ofwidely spaced, small objects—such asbeads that are glued to a flat surface—and recording the angle of the marblesthat are deflected. Discuss the results oftheir experiment in light of Marsden’sfindings. Kinesthetic, Interpersonal

Integrate BiologyCells in most living organisms have acentral structure called a nucleus. Thisorganelle contains the cell’s genetic, orhereditary, material. Ask, How are anatom’s nucleus and a cell’s nucleussimilar? (They are both central structuresof a basic unit.)Logical

ASSESSEvaluate UnderstandingAsk groups of students to summarizeDalton’s, Thompson’s, and Rutherford’satomic theories. Have them come upwith simple word phrases or mnemonicdevices to help them easily distinguishamong the three theories. For example,Dogs Sort Socks � Dalton’s Solid Sphere;Turtles Play Ping-pong � Thomson’sPlum Pudding; and Rats Poke Noodles �Rutherford’s Positive Nucleus.

ReteachUse the Science and History time line on p. 114 to present and discuss asummary of the three models.

Students might argue that theproperties Democritus assigned toatoms match observed properties ofmatter, such as smoothness androughness. Students might argue thatthe properties Aristotle assigned toelements serve a similar purpose, and his system also seems to account forchanges between types of matter.

If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 4.1.

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5. Dalton assumed atoms were solid,indivisible particles. Thomson had evidencethat smaller particles existed inside atoms. 6. Dalton had data from experiments tosupport his theory, whereas the Greeks did not have data.7. The particles have a positive charge.8. The nucleus is small compared with theatom as a whole. Very few of the alphaparticles came close enough to a gold nucleusto be deflected.


1. All matter is composed of individualparticles called atoms, which cannot bedivided.2. Thomson provided the first evidence thatatoms are made from even smaller particles.3. All of the positive charge of an atom isconcentrated in its nucleus. 4. The beam was attracted to a positivelycharged plate and repelled by a negativelycharged plate.

Answer to . . .

Figure 8 The marble would have tobe located in the center of the stadium.


Small-Scale ConstructionSome scientists and engineers think of atoms and molecules asconstruction materials for building very small objects.

Building from the bottom upWith a scanning tunnelingmicroscope, it is possible to moveindividual atoms or molecules.This figure, made of linked carbonmonoxide molecules, is just fivenanometers (0.000005 mm) tall. In1990, scientists built this figure todemonstrate bottom-upconstruction methods.

Building from the top downGears are toothed disks that aredesigned to fit together so that themotion of one gear controls themotion of another. These silicon gearsare among the smallest objects evermade from the top down.

The field of science called nanotechnology is named for a unitof measurement—the nanometer. A billion nanometers (109 nm)can fit in one meter. The diameter of a human hair is about80,000 nm. Scientists and engineers who use nanometers asmeasuring units are building miniature versions of objectssuch as motors.

There are two general methods for building any object.You can start with more material than you need andshape the object by removing matter or you canbuild up the object from smaller pieces. When youshape your nails with an emery board, you are using atop-down construction method. When you see “someassembly required” on the side of a box, the manufacturerexpects you to use a bottom-up method of construction.

Silicon gearassembly

Dust mite

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Small-Scale ConstructionBackgroundThe diameter of one of the silicon gearsis 50 microns (50,000 nanometers).Despite their relative sizes on the page, the silicon gear is much largerthan the model of nanogears. (Theatomic radius of a carbon atom is only770 nanometers.)

In 1959, Richard Feynman suggestedthe possibility of nanometer-scaleconstruction when he said, “Theprinciples of physics, as far as I can see,do not speak against the possibility ofmaneuvering things atom by atom.”

The uses for small-scale constructionare potentially very diverse, thoughcurrently they are very limited. Biologistsalready use nanotechnology to maketiny labels for use in diagnostic andpharmaceutical research. Suchmolecular labels can be injected orabsorbed in one part of the body andthen traced as they travel throughoutthe body. One company has even madea sunscreen with nanometer-sizedparticles that scatter harmful light rayswhile transmitting visible light, whichmakes the cream clear instead of white.

Build Science Skills

Using Models

Purpose Students will explore the difference between top-down and bottom-up methods of construction.

Materials modeling clay, plastic knives


Procedure Have students use the top-down method to construct a model of agear like the one shown in the dust mitephoto. Consider bringing in samples ofgears for students to observe. Then,have students use the bottom-upmethod to construct a model of thestick figure shown in the feature.

Expected Outcome Students willmake a gear by starting with a lump ofclay and then removing clay to shapethe gear (a top-down constructionmethod). They will make a stick figureby joining small pieces of clay together(a bottom-up method of construction).Kinesthetic, Visual

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� Research proposed uses of nanotechnology.Make a poster describing one proposed use.Explain the advantage ofusing small objects in thisapplication. What problemsmust be solved before theapplication can be used?

� Take a Discovery ChannelVideo Field Trip by watching“Go For Gold.”

Going Further

Video Field Trip

NanogearsThis image of nanogears was produced with a computer program designed to make models of molecules. Hollow tubes(nanotubes) made from sheets of carbonatoms do exist. So do the rings containingcarbon and hydrogen atoms, which are usedfor the “teeth” of the gears. But researchersneed to figure out how to get the “teeth”to attach to the tubes.

The future ofnanotechnology

Potential applications fornanotechnology include

medical diagnostic tools and atomic-level electronic devices that

assemble themselves. If such devicesprove successful, perhaps someday

the surgical robot will be built.

Sheet of carbon atomsrolled into a tube

Futuristic model of ananorobot performing

surgery in a blood vessel.

Ring of carbon atoms withhydrogen atoms attached


Going FurtherPossible uses include bar codes thatattach to molecules and are used to monitor biological processes, devices that deliver drugs to specificlocations in the body at specific times,and self-assembling products. Oneoverall challenge is the construction of interfaces between humans and tiny devices. Verbal, Visual


After students have viewed the Video Field Trip,ask them the following questions: What did theancient Egyptians know about how gold reactswith air? (They realized that gold does not reacteasily with air and does not corrode like many othermetals.) Name a practical reason why templeroofs were often covered with gold in countrieswith hot climates. (Because gold is an excellent

reflector of sunlight, it kept the interiors cooler thanthey would have been otherwise.) How do theproperties of gold make it useful on spacemissions? (Gold is often used to coat space vehiclesin order to reflect the intense sunlight. A thin layer ofgold on the goggles used by astronauts reduces theintensity of light reaching the astronauts’ eyes.)How can the arrangement of atoms on thesurface of gold be observed? (Electronmicroscopes are designed to trace the arrangementof particles on a surface through the up-and-downmovement of a probe tip.)

Video Field Trip

Go For Gold


4.2 The Structure of an Atom

Reading StrategyMonitoring Your Understanding Beforeyou read, copy the table. List what you knowabout atoms and what you would like to learn.After you read, list what you have learned.

Key ConceptsWhat are three subatomicparticles?

What properties can beused to compare protons,electrons, and neutrons?

How are atoms of oneelement different fromatoms of other elements?

What is the differencebetween two isotopes ofthe same element?

Vocabulary◆ proton◆ electron◆ neutron◆ atomic number◆ mass number◆ isotopes

Beams like the ones Thomson produced create the images on manytelevision screens. When a beam sweeps across the screen, spots on thescreen light up in the same way the screen in the gold-foil experimentlit up when struck by an alpha particle. In a color television, there arethree beams, one for each primary color of light—red, green, and blue.The particles in these beams are subatomic particles.

Properties of Subatomic ParticlesBy 1920, Rutherford had seen evidence for the existence of two sub-atomic particles and had predicted the existence of a third particle.

Protons, electrons, and neutrons are subatomic particles.

Protons Based on experiments with elements other than gold,Rutherford concluded that the amount of positive charge varies amongelements. Each nucleus must contain at least one particle with a posi-tive charge. Rutherford called these particles protons. A proton is apositively charged subatomic particle that is found in the nucleus of anatom. Each proton is assigned a charge of 1�. Some nuclei containmore than 100 protons.

Electrons The particles that Thomson detected were later namedelectrons. Electron comes from a Greek word meaning “amber.” Anelectron is a negatively charged subatomic particle that is found in thespace outside the nucleus. Each electron has a charge of 1�.

What I Know What I Would What I HaveAbout Atoms Like to Learn Learned

Figure 9 This 45-foot-tall steelsculpture of a clothespin is inPhiladelphia, Pennsylvania. ClaesOldenburg made the clothespinin 1976 from 10 tons of steel. If aproton had a mass of 10 tons,then an electron would have amass of about 5 kilograms.

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Objectives4.2.1 Identify three subatomic

particles and compare theirproperties.

4.2.2 Distinguish the atomic numberof an element from the massnumber of an isotope, and usethese numbers to describe thestructure of atoms.

Build VocabularyWord-Part Analysis Have studentslook up the term isotope in a dictionarythat provides word prefixes. Have themuse the prefix iso- to help themunderstand the term. (The prefix iso-means “same.” Isotopes of an elementhave the same atomic number, butdifferent numbers of neutrons.)

Reading StrategyMost students will know that atoms arethe “building blocks” of matter, andsome may know that atoms containsubatomic particles. Students may saythat they want to learn more about thestructure of atoms.

INSTRUCT

Properties ofSubatomic ParticlesUse VisualsFigure 9 Have students examine thephoto and the caption that describes it in Figure 9. Suggest some familiarobjects that have a mass of 5 kg, such as a 12-pack of 16-ounce beveragecontainers. Ask, If a proton’s mass was10 tons and an electron’s mass was 5 kg, what mass would represent themass of a neutron? (10 tons)Logical

FYIDifferent books use different conventionsfor symbols for subatomic particles. Forexample, some texts use only the letters e, p, and n. Others include a superscriptzero on the n to indicate the lack ofcharge. Electrons, protons, and neutronsare not the only subatomic particles.Quarks will be discussed in Chapter 10.

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Section 4.2

Print• Reading and Study Workbook With

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Section Resources

Figure 10 This table lists thesymbol, the relative charge, therelative mass, and the actual massof an electron, a proton, and aneutron. The Model columnshows the colors used in this bookto represent the subatomicparticles. Calculating What isthe difference in actual massbetween a proton and a neutron?

Designing an Atomic Exhibit

You work as a volunteer at the local sciencemuseum. You are asked to design an exhibit thatcompares the size of a lithium atom to the size ofits nucleus. A lithium atom has a diameter of about3 � 102 picometers. The nucleus of a lithium atomhas a diameter of about 5 � 10

_3 picometers.There are a trillion (1012) picometers in a meter.

Defining the Problem State the problem inyour own words. What decisions will you needto make before you can proceed?

Organizing Information How many timeslarger is the lithium atom than its nucleus? Findseveral objects that could represent the nucleusin your exhibit and measure their diameters.

Creating a Solution Pick one of the objects youmeasured to represent the nucleus in your atomicexhibit. Figure out how far away from the objectyou would have to place a marker so that peoplecould visualize the relative sizes of the atom andthe nucleus.

Presenting Your Plan Write a proposal topresent to the committee that approves projects.Tell them where you would place the nucleus andwhere you would have to place the marker. Beprepared to explain why your exhibit needs thespace you are requesting.

Particle Symbol RelativeCharge

Relative Mass(proton � 1)

Actual Mass(g)

Electron

Proton

Neutron

e�

p�

n

1�

1�

0

1

1

9.11 � 10�28

1.674 � 10�24

1.675 � 10�24

11836

Model

Properties of Subatomic Particles


Neutrons In 1932, the English physicist James Chadwick designedan experiment to show that neutrons exist. Chadwick concluded thatthe particles he produced were neutral because a charged object didnot deflect their paths. A neutron is a neutral subatomic particle thatis found in the nucleus of an atom. It has a mass almost exactly equalto that of a proton.

Comparing Subatomic ParticlesFigure 10 summarizes some properties of protons, electrons, and neu-trons. Protons, electrons, and neutrons can be distinguished bymass, charge, and location in an atom. Protons and neutrons havealmost the same mass. But the data in Figure 10 show that it wouldtake about 2000 electrons to equal the mass of one proton. Electronshave a charge that is equal in size to, but the opposite of, the charge ofa proton. Neutrons have no charge. Protons and neutrons are found inthe nucleus, but electrons are found in the space outside the nucleus.

For: Articles on atomic chemistry

Visit: PHSchool.com

Web Code: cce-1042

Comparing SubatomicParticlesBuild Science SkillsCalculating Have students confirm the relative masses given in Figure 10 bydividing the mass given for an electronby the mass given for a neutron. (9.11� 10�28/1.675 � 10�24 = 5.44 � 10�4; 1/1836 = 5.45 � 10�4)These numbers are almost equal. Notethat the masses given are in grams. Logical

Designing an Atomic Exhibit

Defining the Problem To design an exhibit that compares the size of alithium atom to the size of its nucleus,students must decide what materials touse and where to locate the exhibit.

Organizing Information It is 60,000times larger than its nucleus.

Creating a Solution If a standardmarble with a 5/8-inch diameter (1.6 cm)represents the nucleus, the marker shouldbe at a distance of 960 m to representthe outer limit of the atom.

Presenting Your Plan The proposalshould explain why the exhibit requiresat least two locations that are about akilometer apart to make a model of thelithium atom in which the nucleus is thesize of a marble. Encourage students tolocate two familiar area landmarks thatare the appropriate distance apart basedon the item chosen for the nucleus.Visual, Logical

For Extra HelpBe sure that students multiply by theconversion factor that has the given unitin the denominator and the desired unitin the numerator.Logical

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Customize for Inclusion Students

Visually ImpairedProvide visually impaired students with atactile model that represents the difference in mass between a proton and an electron.Count out (or have a group of students countout) 1836 small beads and place them into a

resealable plastic bag. Have them feel thedifference in mass between the bag of 1836 beads and an identical bag containingonly one bead. Point out that the masses they sense include the masses of the bags.

Science News provides studentswith current information onatomic chemistry.

Answer to . . .

Figure 10 0.001 �10�24 g


Everything scientists know about the nucleus and subatomic par-ticles is based on how the particles behave. Scientists still do not havean instrument that can show the inside of an atom. But they dohave microscopes that can show how atoms are arranged on the sur-face of a material. The How It Works box on page 111 describes one ofthose microscopes.

Atomic Number and Mass NumberDalton predicted that the atoms of any element are different from theatoms of all other elements. With the discovery of subatomic particles,scientists were able to describe those differences.

Atomic Number The atoms of any given element alwayshave the same number of protons. For example, there is oneproton in the nucleus of each and every hydrogen atom.Therefore, hydrogen is assigned the atomic number 1. Theatomic number of an element equals the number of protonsin an atom of that element.

Hydrogen atoms are the only atoms with a single proton.Atoms of different elements have different numbers of

protons. The sulfur shown in Figure 11A is assigned atomicnumber 16 because a sulfur atom has 16 protons. You can useatomic numbers to refer to elements, like names and symbols,because each element has a unique atomic number.

Each positive charge in an atom is balanced by a negativecharge because atoms are neutral. So the atomic number of anelement also equals the number of electrons in an atom. Eachhydrogen atom has one electron. Each sulfur atom has 16.

Mass Number The atomic number tells you the numberof protons in an atom’s nucleus. It does not give you any infor-mation about the number of neutrons in an atom. For thatinformation, you need to know the atom’s mass number. Themass number of an atom is the sum of the protons and neu-trons in the nucleus of that atom. An atom of aluminum with13 protons and 14 neutrons has a mass number of 27. If youknow the atomic number and the mass number of an atom,you can find the number of neutrons by subtracting.

Number of NeutronsNumber of neutrons � Mass number � Atomic number

Which scientist demonstrated the existence of neutrons?

Figure 11 Each element has adifferent atomic number. A Theatomic number of sulfur (S) is 16.B The atomic number of iron (Fe)is 26. C The atomic number ofsilver (Ag) is 47. Applying Concepts How manyprotons are there in each atom ofsulfur, iron, and silver?

110 Chapter 4

A

B

C

110 Chapter 4

Atomic Number andMass NumberFYIThe atomic mass unit will be introducedin Section 5.2, when atomic masseslisted in the periodic table are discussed.The force that binds protons andneutrons together in the nucleus iscalled the strong nuclear force and isaddressed in Chapter 10, as is the effectof the size of a nucleus on its stability.

Particles and Numbers

Purpose Students will observe therelationship between number ofprotons, number of neutrons, atomicnumber, and mass number.

Materials overhead projector, red andgreen gummy candies

Procedure Explain that the greencandies represent neutrons and the redcandies represent protons. Model alithium-7 nucleus by placing a group of three red candies and four greencandies on the overhead. Ask studentsto count the number of candies(particles) to determine the massnumber of the lithium atom. Then,remove (subtract) the green candies(neutrons) to get the atomic number.Perform a similar demonstration withoxygen-16 (eight protons and eightneutrons) and boron-11 (five protonsand six neutrons).

Expected Outcome Students shouldgain a familiarity with determining massnumbers and atomic numbers. Visual

Build Reading LiteracyIdentify Main Idea/Details Refer to page 98D in this chapter, whichprovides the guidelines for identifyingmain ideas and details.

Have students read Atomic Number andMass Number on p. 110. Ask them toidentify the main idea of each paragraph.Point out that the main idea is usuallywithin the first or second sentence of aparagraph. Encourage students toinclude this exercise in the notes they use to study. Verbal

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Scanning TunnelingMicroscopeA probe is moved back and forth across the surface of asample. When electrons jump, or tunnel, across the gapbetween the sample and the probe, an electric current isproduced. A computer uses data about changes in theprobe’s position to produce an image of the sample’ssurface. Interpreting Diagrams How is the distancebetween the probe tip and the sample kept constant?

Scanning probeAs the probe is moved

over the sample, currentflows between the probetip and the sample. Theprocessor holds the tip at aconstant distance from thesample by keeping theelectric current constant.Thus, changes in the verticalposition of the probe willfollow the contours of thesample’s surface.

ProcessorThe processor sends,

receives, and recordsinformation about themovement of the probe.

Electricalsignal fromprobe

Scanning tunnelingmicroscope Modern scanning tunnelingmicroscopes produce images ofmetal samples or biologicalspecimens such as DNA.

Probe tip The tip ofthe probe is only one ortwo atoms in width.

Electricalsignal fromprocessor

Goldsample

Scanningdevice Thisdevice raisesand lowersthe probe.

Computer A computerassembles a map of the

sample’s surface, using datareceived from the processor.Color was added to theimage shown on thecomputer screen.

Electron flowElectrons flow across a gapof about one nanometer(0.000001 mm)between theprobe tip andthe sample,producingan electriccurrent.


Scanning TunnelingMicroscopeThe scanning tunneling microscope(STM) is used to obtain high-resolutionimages of solid surfaces. This technologyallows scientists and researchers to viewa three-dimensional profile of a surface,which can give information aboutsurface textures and crystal structure.STM data is initially displayed as a blackand white image that is colorized tohighlight different features.

In 1986, Gerd Binnig of Germany andHeinrich Rohrer of Switzerland shared theNobel Prize in Physics with Germany’sErnst Ruska for designing the scanningtunneling microscope.

Interpreting Diagrams The processormaintains a constant electric currentbetween the probe tip and the sample,which keeps the distance between thetip and sample constant. Logical

For EnrichmentEncourage students to explore the useof scanning tunneling microscopes inresearch on surface textures, crystalstructure, or molecular shape. Havethem present their findings to the classin the form of a poster. Visual

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Coining Terms The English physicianWilliam Gilbert (1544–1603) introduced theterm electric, which is based on the Greekword for amber. (William Gilbert was thepersonal physician to Queen Elizabeth I and

a pioneer in the study of magnetism. A unit ofmagnetic force is named for him.) Credit fornaming the electron goes to G. JohnstoneStoney, an Irish physicist who suggested thename in 1891.

Facts and Figures

Answer to . . .

Figure 11 There are 16 protons in asulfur atom, 26 in an iron atom, and47 in a silver atom.

James Chadwick


112 Chapter 4


Reviewing Concepts1. Name three subatomic particles.

2. Name three properties you could use todistinguish a proton from an electron.

3. Which characteristic of an atom alwaysvaries among atoms of different elements?

4. How are the isotopes of an elementdifferent from one another?

5. What do neutrons and protons have incommon? How are they different?

6. How can atoms be neutral if they containcharged particles?

7. What is the difference between atoms ofoxygen-16 and oxygen-17?

Critical Thinking8. Comparing and Contrasting What

property do protons and electrons havethat neutrons do not?

9. Applying Concepts Explain why it isn’tpossible for an atom to have a mass numberof 10 and an atomic number of 12.

IsotopesIn Dalton’s atomic theory, all the atoms of a given element areidentical. Every atom of a given element does have the samenumber of protons and electrons. But every atom of a givenelement does not have the same number of neutrons. Isotopesare atoms of the same element that have different numbers ofneutrons and different mass numbers. Isotopes of an ele-ment have the same atomic number but different massnumbers because they have different numbers of neutrons.

For example, every atom of oxygen has 8 protons. Some oxygenatoms have 8 neutrons and a mass number of 16. Some oxygen atomshave 9 neutrons and a mass number of 17. Some oxygen atoms have10 neutrons and a mass number of 18. When it is important to distin-guish one oxygen isotope from another, the isotopes are referred to asoxygen-16, oxygen-17, and oxygen-18. All three oxygen isotopes canreact with hydrogen to form water or combine with iron to form rust.

With most elements, it is hard to notice any differences in the phys-ical or chemical properties of their isotopes. Hydrogen is an exception.Hydrogen-1 has no neutrons. (Almost all hydrogen is hydrogen-1.)Hydrogen-2 has one neutron, and hydrogen-3 has two neutrons.Because a hydrogen-1 atom has only one proton, adding a neutrondoubles its mass. Water that contains hydrogen-2 atoms in place ofhydrogen-1 atoms is called heavy water. Figure 12 compares somephysical properties of ordinary water and heavy water.

Elements In Section 2.1, you were toldthat elements contain only one type ofatom. How would you define “type ofatom” to account for the existenceof isotopes?

Property

Meltingpoint

Boilingpoint

Density(at 25�C)

Ordinary Water

0.00�C

100.00�C

0.99701 g/cm3

Heavy Water

3.81�C

101.42�C

1.1044 g/cm3

Comparing Ordinary Waterand Heavy Water

Figure 12 Heavy water containshydrogen-2 atoms, which havetwice the mass of hydrogen-1atoms. Using Tables At whattemperature would a sample ofheavy water freeze?

IsotopesBuild Science SkillsCalculating Uranium-238 has a massnumber of 238 with 146 neutrons in thenucleus. Uranium-235 has 143 neutronsin the nucleus. Ask, What is the atomicnumber of uranium? (92)Logical

Many students think that isotopes aredifferent from “ordinary” or “regular”atoms. To challenge this misconception,have students read the text on this pageand examine the data presented inFigure 12. Ask, How are the composi-tions of heavy water and ordinarywater similar? (Both contain hydrogenand oxygen atoms.) What type ofhydrogen atoms does ordinary watercontain? (Hydrogen-1 atoms) What typeof hydrogen atoms does heavy watercontain? (Hydrogen-2 atoms) Comparethe properties of heavy water andordinary water. (They have differentmelting points, boiling points, anddensities.) Logical

ASSESSEvaluate UnderstandingHave students write three reviewquestions for this section. Studentsshould then break into groups of three orfour and ask each other their questions.

ReteachRevisit Figure 10 to review the differencesamong protons, neutrons, and electrons.

Students might say that “type of atom”refers to the atomic number of the atomor to the number of protons in the atom.


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the atom. Protons are charged particles.Neutrons are neutral particles. 6. The positive charge of the protons in thenucleus is balanced by the negative charge of the electrons. 7. Each oxygen-17 atom has one moreneutron than each oxygen-16 atom. 8. Protons and electrons are charged particles.Neutrons have no charge. 9. An atom with an atomic number of 12 has12 protons. Because the mass number is thesum of the protons and neutrons, the massnumber would need to be at least 12.


1. Proton, electron, and neutron2. Mass, charge, and location in an atom 3. The atoms of any element have a differentnumber of protons than the atoms of all other elements. 4. Isotopes of an element have the sameatomic number but different mass numbersbecause they have different numbers of neutrons. 5. Protons and neutrons have almost the samemass and are both located in the nucleus of

112 Chapter 4

Answer to . . .

Figure 12 3.81°C

4.3 Modern Atomic Theory

Reading Strategy Sequencing Copy the flowchart. After youread, complete the description of how a gainor loss of energy affects atoms.

Key ConceptsWhat can happen toelectrons when atomsgain or lose energy?

What model do scientistsuse to describe howelectrons behave in atoms?

What is the most stableconfiguration of electronsin an atom?

Vocabulary◆ energy levels◆ electron cloud◆ orbital◆ electron

configuration◆ ground state

Electrons and Energy Levels

Emitsenergy

Excitedstate

a. ? b. ?

Have you ever wondered what produces the different colors ina fireworks display? Why does one explosion produce red lightand another explosion produce green light? The people who makefireworks know that certain compounds will produce certaincolors of light when they are heated. For example, compoundscontaining the element strontium produce red light when theyare heated. Compounds containing barium produce green light.

You have seen two things that can happen when atoms absorbenergy—an increase in kinetic energy or a phase change. But thereis another possibility. The energy may be temporarily absorbed bythe atom and then emitted as light. The colors in a fireworks dis-play are a clue to how electrons are arranged in atoms.

Bohr’s Model of the AtomYou may have seen diagrams of an atom that look like a solarsystem with planets revolving around a sun. These diagrams arebased on a model of the atom that was developed by Niels Bohr(1885–1962), a Danish physicist who worked for a while withRutherford. Bohr agreed with Rutherford’s model of a nucleussurrounded by a large volume of space. But Bohr’s model didsomething that Rutherford’s model did not do. It focused on theelectrons. A description of the arrangement of electrons in anatom is the centerpiece of the modern atomic model.

Figure 13 Fireworks are often displayed above theLincoln Memorial in Washington, D.C. The red lightwas produced by a strontium compound.


FOCUS

Objectives4.3.1 Describe Bohr’s model of the

atom and the evidence forenergy levels.

4.3.2 Explain how the electron cloud model represents thebehavior and locations ofelectrons in atoms.

4.3.3 Distinguish the ground state from excited states of an atom based on electron configurations.

Build VocabularyLINCS Have students use the LINCSstrategy to learn the terms energy levels,electron cloud, orbital, electron configu-ration, and ground state. In LINCSexercises, students List what they knowabout each term, Imagine a picture thatdescribes the term, Note a reminding“sound-alike” word, Connect the termsto the sound-alike word by making up a short story, and then perform a briefSelf-test.

Reading Strategya. Electron moves to higher energy level.b. Electron moves to lower energy level.

INSTRUCT

Bohr’s Model of the AtomBuild Reading LiteracyRelate Text and Visuals Refer topage 190D in Chapter 7, whichprovides the guidelines for relating text and visuals.

Have students read Bohr’s Model of theAtom on pp. 113–116. Then, havestudents examine the diagram of Bohr’smodel in the time line on p. 115. Ask,What do the circles around the nucleusrepresent? (They represent energy levels.)Visual

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Print• Laboratory Manual, Investigations 4A

and 4B• Reading and Study Workbook With

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Section Resources

Section 4.3


Energy Levels In Bohr’s model, electrons move with constantspeed in fixed orbits around the nucleus, like planets around a sun.Each electron in an atom has a specific amount of energy. If an atomgains or loses energy, the energy of an electron can change. The possi-ble energies that electrons in an atom can have are called energy levels.

To understand energy levels, picture them as steps in a staircase.As you move up or down the staircase, you can measure how yourposition changes by counting the number of steps you take. You mighttake one step up, or you might jump two steps down. Whether you aregoing up or down, you can move only in whole-step increments. Justas you cannot stand between steps on a staircase, an electron cannotexist between energy levels.

1803 John Daltonpictures atoms astiny, indestructibleparticles, with nointernal structure.

1897 J. J. Thomson, a Britishscientist, discovers the electron,leading to his “plum-pudding”model. He pictures electronsembedded in a sphere ofpositive electric charge.

1904 HantaroNagaoka, a Japanesephysicist, suggests thatan atom has a centralnucleus. Electronsmove in orbits like therings around Saturn.

Models of the AtomThe development of scientific ideas onthe structure of atoms has passed severalkey milestones during the last 200 years.

1911 New ZealanderErnest Rutherfordstates that an atom hasa dense, positivelycharged nucleus.Electrons moverandomly in the spacearound the nucleus.

Tiny,solid sphereDALTON

MODEL

Sphere withpositive chargethroughout

The positive charge of thesphere balances the negativecharge of the electrons.

THOMSONMODEL

RUTHERFORDMODEL

Nucleus

Path of amovingelectron

1900 1905 1910

Negativelycharged particle(electron)

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-

---

- -

-++ +

+ ++

+ +

1805 18951800

114 Chapter 4

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- -

-

--

--

-

114 Chapter 4

Students may think that electrons travelaround the nucleus in fixed orbits, likeplanets orbiting the sun. Challenge thismisconception by having studentscompare Bohr’s model and the electroncloud model. Explain that Bohr’s modelcorrectly introduced the concept ofenergy levels, but energy levels cannotbe used to describe the actual locationof an electron. The electron cloud modelcan be used to model the probabilitythat an electron is in a certain location.The exact speed and location of a singleelectron cannot be determined.Verbal

FYIThe usefulness of Bohr’s model waslimited. The model could be used todescribe the behavior of the singleelectron in a hydrogen atom quiteaccurately. However, this model could not be applied to atoms withmultiple electrons.

Integrate Space SciencePlanets in the solar system travel in fixed orbits around the sun. Becausemost of the orbits are nearly circular, the difference between the distance tothe sun when a planet is closest andwhen it is farthest away is not great(given the magnitude of distances inspace). Pluto is an exception. Its orbit isso elliptical that there are times duringPluto’s journey around the sun (249Earth days) when it is closer to the sunthan Neptune is. This switch in order of proximity to the sun lasts 20 years. It last happened between 1979 and1999. Have students research when it will happen again. Logical

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Customize for English Language Learners

Think-Pair-Share Have students work in pairs to think ofstructures that can serve as analogies forenergy levels. Examples include rungs of aladder, guitar frets, and the series of holes on abelt or shoe strap. Note, however, that in all ofthese models, the intervals are equal, which is

not true of the intervals between energy levels.Provide pictures of dressers that have drawersof different sizes or bookshelves that haveadjustable shelves, which might better modelthe intervals of energy levels. Strengthendiscussion skills by having students share their examples with the class.

The landing at the bottom of the staircase is like the lowest energylevel in an atom. Each step up represents a higher energy level. Thedistance between two steps represents the difference in energy betweentwo energy levels. To continue the analogy, there would need to be adifferent staircase for each element because no two elements have thesame set of energy levels.

An electron in an atom can move from one energy level toanother when the atom gains or loses energy. An electron may moveup two energy levels if it gains the right amount of energy. An elec-tron in a higher energy level may move down two energy levels if itloses the right amount of energy. The size of the jump between energylevels determines the amount of energy gained or lost.

1932 JamesChadwick, a Britishphysicist, confirms theexistence of neutrons,which have no charge.Atomic nuclei containneutrons and positivelycharged protons.

1924 Frenchman Louisde Broglie proposes thatmoving particles likeelectrons have someproperties of waves.Within a few years,evidence is collected tosupport this idea.

1926 Erwin Schrödingerdevelops mathematicalequations to describe themotion of electrons inatoms. His work leads tothe electron cloud model.

ELECTRONCLOUDMODEL

The electron cloud isa visual model of theprobable locations ofelectrons in an atom.The probability offinding an electron ishigher in the denserregions of the cloud.

The nucleuscontains protonsand neutrons.

1915 1920 1925 1930 1935

Electrons gain orlose energywhen they movebetween fixedenergy levels.

- --

-

--

-

-

1913 In NielsBohr’s model, theelectrons move inspherical orbits atfixed distancesfrom the nucleus.

BOHRMODEL

Nucleus

Electron

Summary Select a scientistmentioned on the time line.Research and write a para-graph about the scientist’searly years. What experiencesled to his interest in science?Was he the first in his familyto be interested in science?What subjects did he studyat school?


+

FYIIn Section 6.1, students will learn thatelectrons sometimes gain enoughenergy to escape from an atom.

Models of the AtomHave groups of students build or drawmodels that represent the changes overtime in scientists’ understanding ofatomic structure. Have them make athree-dimensional version of the timeline shown and display it as a mobile ordiorama. Have students note the timescale on the time line. Explain that thebreak between 1805 and 1895 allowsthe milestone in 1803 to be included.Group, Visual

There will be more information for somescientists than for others. The exercisefocuses on early experiences becausestudents will not understand most ofwhat is written about the careers ofthese scientists. By pooling theirresearch, students will see that scientistscan emerge from diverse backgrounds.

Students may learn that Dalton was the son of a weaver and de Broglie was the son of a duke; that Daltonbegan his teaching career at the age of 12; that Schrödinger was an onlychild, but Rutherford had 11 siblings; or that Chadwick was shy andRutherford charming. Verbal

Use Community ResourcesHave a female scientist visit the class todiscuss the experiences that led to herinterest in science. Have studentsprepare questions similar to those askedin the Writing in Science feature.Interpersonal, Visual

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De Broglie and Schrödinger In 1924,Louis de Broglie, a French graduate student,derived an equation that describes thewavelength of a moving particle. Using de Broglie’s equation, an electron has awavelength of about 2 � 10�10 cm. In 1926,Erwin Schrödinger, an Austrian physicist,wrote a mathematical equation to describe the location and energy of the electron in a

hydrogen atom. When the equation is solved(using advanced calculus), it produces a seriesof wave functions that describe the behaviorof electrons. The quantum mechanical modelof atoms is based on these wave functions.Atomic physicists define an orbital as thespace-dependent part of the Schrödinger wavefunction of an electron in an atom or molecule.

Facts and Figures


Evidence for Energy Levels What evidence is there that elec-trons can move from one energy level to another? Scientists canmeasure the energy gained when electrons absorb energy and move toa higher energy level. They can measure the energy released when theelectron returns to a lower energy level.

The movement of electrons between energy levels explains the lightyou see when fireworks explode. Light is a form of energy. Heat pro-duced by the explosion causes some electrons to move to higher energylevels. When those electrons move back to lower energy levels, theyemit energy. Some of that energy is released as visible light. Becauseno two elements have the same set of energy levels, different elementsemit different colors of light.

Electron Cloud ModelLike earlier models, Bohr’s model was improved as scientists madefurther discoveries. Bohr was correct in assigning energy levels to elec-trons. But he was incorrect in assuming that electrons moved likeplanets in a solar system. Today, scientists know that electrons move ina less predictable way.

Scientists must deal with probability when trying to predict thelocations and motions of electrons in atoms. An electron cloud is avisual model of the most likely locations for electrons in an atom. Thecloud is denser at those locations where the probability of finding anelectron is high. Scientists use the electron cloud model todescribe the possible locations of electrons around the nucleus.

Figure 14 provides an analogy for an electron cloud. When the pro-peller of an airplane is at rest, you can count the number of blades.When the propeller is moving, the blades spin so fast that you see onlya blur. You know that the blades are located somewhere in the blur,but at any specific moment in time you can’t be exactly sure whereeach blade is located.

What determines the amount of energy gained orlost when an electron moves between energy levels?

Figure 14 When the propeller ofan airplane is at rest, you can seethe locations of the blades. Whenthe propeller is moving, you seeonly a blur that is similar to adrawing of an electron cloud.Comparing and ContrastingDescribe one difference betweenthe motion of a propeller and themotion of an electron.

116 Chapter 4

For: Links on energy levels

Visit: www.SciLinks.org

Web Code: ccn-1043

Electron Cloud ModelBuild Science SkillsUsing Models Have students examinethe propeller in Figure 14. Ask, How isthe moving propeller similar to anelectron cloud? (You cannot be sure atany specific moment where the propellerblades or electrons are located. However,the central part of the propeller and thenucleus of an atom are in fixed locations.)What other examples can you think ofthat could model the concept of anelectron cloud? (Acceptable answersinclude a ceiling fan, or moths flyingaround a light bulb.) Visual

Electron Cloud Model

Purpose Students will use a model to describe the probable position ofelectrons.

Materials small, round balloon; large, round balloon; 10 beads with 4-mm diameter; 5 beads with 2-mm diameter

Procedure Put the 4-mm beads intothe small balloon. Tell students that thesmall balloon represents the nucleus of aboron atom (five neutrons, five protons).Put the 2-mm beads into the largeballoon. Explain that the beads representelectrons and the balloon represents theelectron cloud. Slightly inflate the smallballoon and push it completely into thelarge balloon. Inflate the large balloonand tie the end. Agitate the balloon sothat the small beads are in constantmotion.

Expected Outcome The preciselocation of a bead at a specific time isunknown, but the probability that it is in the large balloon is quite high. Kinesthetic

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Emission and Absorption Spectra Whenthe energy gained or lost by an atom is lightenergy, each frequency (or wavelength) oflight corresponds to a movement of anelectron between two energy levels in theatom. An element can be identified by thefrequencies of light that are absorbed or

emitted by its atoms because no two elementshave the same set of energy levels. Forexample, the element helium was discoveredon the sun in 1868 before it was discovered onEarth. The spectrum of light emitted by gaseson the surface of the sun contained a yellowline that did not match a known element.

Facts and Figures

116 Chapter 4

Download a worksheet on energylevels for students to complete,and find additional teacher supportfrom NSTA SciLinks.


Atomic Orbitals The electron cloud represents all the orbitals in an atom. An orbital isa region of space around the nucleus where an electron is likely to befound. To understand the concept of an orbital, imagine a map of yourschool. Suppose you mark your exact location with a dot once every10 minutes over a period of one week. The places you visit the most—such as your classrooms, the cafeteria, and the area near yourlocker—would have the highest concentration of dots. The places youvisit the least would have the lowest concentration of dots.

The dots on your map are a model of your “orbital.” They describeyour most likely locations. There are some locations in your orbitalthat you may not visit every week—such as the principal’s office or theauditorium. These locations may not be represented by a dot on yourmap. Despite such omissions, the dots on your map are a good modelof how you usually behave in your orbital. An electroncloud is a good approximation of how electrons behave intheir orbitals.

The level in which an electron has the least energy—thelowest energy level—has only one orbital. Higher energylevels have more than one orbital. Figure 15 shows thenumber of orbitals in the first four energy levels of an atom.Notice that the maximum number of electrons in an energylevel is twice the number of orbitals. Each orbital can containtwo electrons at most.

Comparing Excited States

Materials fluorescent (“neon”) markers, glow-in-the-darktoy, ultraviolet (UV) lamp

Procedure 1. Use the fluorescent markers to draw a picture

on a piece of paper.

2. With the room darkened, observe yourdrawing under a UV lamp. CAUTION Do notlook directly at the light. Remove the drawingfrom under the UV light and observe it again.Record your observations.

3. Observe the glow-in-the-dark toy under theUV light. Remove the toy from the light andobserve it again. Record your observations.

Analyze and Conclude1. Observing How did the glow of the toy

differ from the glow of your drawing?

2. Formulating Hypotheses Use the conceptsof ground and excited states to explain howUV light caused your drawing and the toyto glow.

3. Drawing Conclusions In which object, yourdrawing or the toy, do the atoms have excitedstates that are more stable, or less likely tochange? Explain your answer.

Figure 15 The table lists thenumber of orbitals in the first fourenergy levels of an atom. It alsolists the maximum number ofelectrons in each energy level.Inferring How many electronscan be in each orbital?

EnergyLevel

Number ofOrbitals

Maximum Numberof Electrons

1

2

3

4

1

4

9

16

2

8

18

32

Energy Levels, Orbitals,and Electrons

Atomic Orbitals

Comparing Excited States

Objective After completing this activity, studentswill be able to • explain how UV light causes objects

to glow.• use the persistence of light to

compare excited states.

Skills Focus Observing, FormulatingHypotheses

Prep Time 5 minutes


Safety Check the MSDS for themarkers to make sure that they are lowin VOCs (volatile organic compounds).Students should not look directly at UVlight, which is harmful to the eyes.Demonstrate safe use of the UV lampsbefore allowing students to use them.

Teaching Tips• Do this lab only after you teach

ground state and excited states.

Expected Outcome Fluorescent inkwill emit brilliant visible light under UVlight. This fluorescence will instantlycease when the UV light is removed.Phosphorescent (glow-in-the-dark)objects will continue to emit visible lighteven after the UV light is removed.

Analyze and Conclude 1. The toy still glowed after the UV lightwas removed. The drawing did not. 2. The drawing and the toy absorbedenergy from the UV light. When electronsmoved to higher energy levels, the atomswere in an excited state. When electronsreturned to lower energy levels, energywas released as visible light. 3. The fact that the toy’s glow persistedsuggests that the excited state of atomsin the toy was more stable than theexcited state of atoms in the drawing. Visual, Kinesthetic

L2


Answer to . . .

Figure 14 The propeller blades havea single, set path, and the blades stopmoving when the engine is shut off.

Figure 15 Two

The size of the jumpbetween energy levels

FYIAccording to the quantum mechanical model, an orbital is the mathematical function thatdescribes the behavior of an electron in space.


118 Chapter 4


Reviewing Concepts1. When is an electron in an atom likely to

move from one energy level to another?

2. What model do scientists use to describehow electrons move around the nucleus?

3. Describe the most stable configurationof the electrons in an atom.

4. What did Bohr contribute to modernatomic theory?

5. What does an electron cloud represent?

Critical Thinking6. Comparing and Contrasting A boron

atom has two electrons in the first energy leveland three in the second energy level. Comparethe relative energies of the electrons in thesetwo energy levels.

7. Making Judgments Was Rutherford’s modelof an atom incorrect or incomplete? Explainyour answer.

8. Posing Questions Apply what you knowabout charged particles to the modern modelof the atom. Is there anything about thebehavior of electrons in atoms that isunexpected? Explain your answer.

Electron Configurations How are the seats in your classroom arranged? Are they lined up neatlyin rows, or are they grouped in clusters? A configuration is an arrange-ment of objects in a given space. Some configurations are more stablethan others, meaning that they are less likely to change. The positionof the gymnast on the balance beam in Figure 16 is not very stablebecause the beam is only 10 centimeters wide.

An electron configuration is the arrangement of electrons in theorbitals of an atom. The most stable electron configuration is theone in which the electrons are in orbitals with the lowest possibleenergies. When all the electrons in an atom have the lowest possible

energies, the atom is said to be in its ground state.For example, lithium is a silvery-white metal with an atomic

number of 3, which means that a lithium atom has three electrons. Inthe ground state, two of the lithium electrons are in the orbital of thefirst energy level. The third electron is in an orbital of the secondenergy level.

If a lithium atom absorbs enough energy, one of its electrons canmove to an orbital with a higher energy. This configuration is referredto as an excited state. An excited state is less stable than the groundstate. Eventually, the electron that was promoted to a higher energylevel loses energy, and the atom returns to the ground state. Helium,neon, argon, krypton, and xenon atoms returning from excited statesto the ground state emit the light you see in “neon” lights.

Figure 16 A gymnast on abalance beam is like an atom inan excited state—not very stable.

Describing Energy Levels Use a bookcase asan analogy for the energy levels in an atom.Use the analogy to write a paragraph aboutelectrons and energy levels. (Hint: Reread thestaircase analogy on pages 114 and 115.)

118 Chapter 4

ElectronConfigurationsUse VisualsFigure 16 Extend the analogy of thegymnast on the balance beam by havingstudents consider a gymnast doing anentire routine on equipment such as abalance beam, a pommel horse, parallelbars, or uneven bars. In the analogy,when is the configuration of thegymnast like an atom in an excitedstate? (When the gymnast is in aprecarious position, such as when thegymnast is not in direct contact with the equipment) In the analogy, when is the gymnast most like an atom in its ground state? (The gymnast is in her most stable position when she is standing on the floor.)Logical

ASSESSEvaluate UnderstandingHave students draw and label a diagramthat represents Bohr’s model of anatom. Then, have students explain howthe electron cloud model differs fromBohr’s model.

ReteachUse the diagrams on p. 115 in theScience and History feature to reviewBohr’s model, energy levels, andelectron clouds.

The shelves in a bookcase can representenergy levels in an atom. If studentsknow about potential energy, they maycompare what happens to the energy ofa book as it is moved between shelves to the difference in energy betweenelectrons in different energy levels.


L1

L2

3

L1


5. An electron cloud represents the mostprobable locations of an electron in an atom.6. The electrons in the second energy level willhave more energy than the electrons in thefirst energy level.7. Rutherford’s description of an atom wascorrect, but incomplete. It did not provide asmuch information about the behavior of theelectrons as later models.8. Students may ask why the negativelycharged electrons are not drawn into thenucleus by the positively charged protons.


1. Electrons are likely to move from oneenergy level to another when atoms gain orlose energy. 2. The electron cloud model3. The most stable configuration is the one inwhich the electrons are in orbitals with thelowest possible energy. 4. Bohr contributed the idea that electronshave energy levels with specific amounts of energy.

C H A P T E R

98 Chapter 4

Atomic Structure

These images of carbon were magnified as �much as 20 million times. Color was added tothe images to highlight features.

How do science concepts apply to yourworld? Here are some questions you’ll beable to answer after you read this chapter.

■ What uses are there for objects that are not visible to the unaided eye? (page 106)

■ Which subatomic particle produces the images on many television screens and computer monitors? (Section 4.2)

■ How does the type of hydrogen atom in water affect the properties of water? (Section 4.2)

■ How do fireworks produce the colors you see when the fireworks explode? (Section 4.3)

98 Chapter 4

ASSESS PRIORKNOWLEDGE Use the Chapter Pretest below to assessstudents’ prior knowledge. As needed,review these Science Concepts andMath Skills with students.

Review Science ConceptsSection 4.1 Encourage students torecall that compounds have fixedcompositions. Remind students that an atom is the smallest particle of an element.

Section 4.2 Have students review theterms mass, volume, and density and theunits used for each.

Section 4.3 Review two things that canhappen when matter absorbs energy.

Review Math SkillsScientific Notation Students willneed to know how to use scientificnotation to understand values for certainproperties of subatomic particles.

Direct students to the Math Skills in theSkills and Reference Handbook at theend of the student text.

CHEMISTRY

Chapter 4

Chapter Pretest

1. True or False: Compounds have fixedcompositions. (True)2. What is an atom? (An atom is the smallestparticle of an element.) 3. Which of the following units is a unit of mass? (c)

a. mL b. °Cc. g d. cm

4. Volume is (c)a. the straight-line distance between

two points.b. the quantity of matter in an object.c. the amount of space taken up by

an object.d. a representation of an object or event.

5. What is density? (Density is the ratio of themass of a substance to its volume.)

6. Which two of the following events cantake place when a liquid absorbs energy? (a and d)

a. The average kinetic energy of theparticles in the liquid increases.

b. The temperature decreases.c. The liquid freezes.d. The liquid changes to a gas.

4.1 Studying Atoms



Chapter Preview

Video Field Trip

Go For Gold

CHEMISTRY

How Can You Study Objects That Are Not Visible?Procedure1. Make and record observations about the

contents of two sealed bags. Use your sensesof touch, smell, and hearing to help you makeyour observations.

2. Predicting Based on your observations,make a prediction about what objects could bein each bag. Decide whether there is a singleobject or more than one object in each bag.

3. Your teacher will list on the chalkboard all ofthe predictions from the class.

Think About It1. Inferring What evidence did you use to

predict what objects were in the bags andhow many objects were in the bags?

2. Evaluating and Revising Record one of thepredictions listed that fits your observations aswell as or better than your own prediction.

3. Designing Experiments Propose anexperiment that would test the prediction.

Atomic Structure 99

How Can You Study Objects That Are Not Visible?Purpose In this activity, students makeinferences based on their observationsand conclude that indirect evidencemust be used to study the structure ofobjects that are too small to see.

Skills Focus Observing, Inferring

Prep Time 5 minutes

Materials 2 sealed, brown paper bags

Advance Prep Place a single type ofsmall object in each bag. Possibleobjects that can be identified byproperties other than sight are lemonslices (odor), pennies (shape), rubberbands (elasticity), sandpaper squares(texture), and bells (sound). Use a lettercode to identify the object in each bag.Seal the bags before distributing themto students.


Teaching Tips• Use more than two types of objects so

each group of students does not havean identical set.

• To model the study of atoms, askstudents not to open the bags at theend of the activity.

• Ask students how they could applywhat they have learned in this activityto the study of atoms, which are toosmall to see.

Expected Outcome Students shouldrecognize that detailed observations canreveal information about objects andevents that cannot be observed directly.

Think About It1. Acceptable student answers mayinclude sound, texture, hardness, smell,and how objects move inside the bags.2. Students may choose a predictionthat they did not record in Step 2 if thisprediction is a better fit for the evidencethey list for Question 1. 3. Answers should include a practical testthat would clearly support or contradictthe prediction chosen in Question 2. Logical

L2

Atomic Structure 99

Encourage students to view the Video Field Trip“Go For Gold.”

ENGAGE/EXPLORE

Video Field Trip

Go For Gold



Forensic scientists use various approaches todistinguish different substances. In this lab, you will observe the flame colors of several substancesand use the data to determine the identity of anunknown substance.

Problem How can the color of a flame beused to distinguish substances?

Materials

Skills Observing, Predicting, Using Data Tables

Procedure

Part A: Observing Flame Colors1. Make a copy of the data table shown.

2. Light the Bunsen burner. CAUTION Puton safety goggles and a lab apron. Tie backloose hair and clothing before working witha flame.

3. Dip the wire loop into the calcium chloridesolution and then place the loop in the flameas shown. Observe and record the color ofthe flame.

• solutions ofcalcium chloride,boric acid,potassium chloride,copper(II) sulfate,sodium chloride,and an unknown

• Bunsen burner• nichrome wire loop• dilute solution of

hydrochloric acid• wash bottle with

distilled water

Using Flame Tests

Data Table

Calcium chloridePotassium chlorideBoric acidCopper(II) sulfateSodium chlorideUnknown

Identity of Unknown

Solution FlameColor

4. Clean the loop by dipping it into hydrochloricacid. Then, while holding the loop over a sink,rinse away the acid with distilled water.CAUTION Keep hydrochloric acid away from yourskin and clothing. Do not breathe in its vapor.

5. Repeat Steps 3 and 4 with each of the othersolutions. Be careful not to transfer anysolution from one container to another.CAUTION These chemicals are poisonous. Do not let them get on your skin.

Part B: Examining an Unknown Solution6. Obtain the unknown solution from your teacher.

7. Repeat Steps 3 and 4 using the unknownsolution. Compare your observations withthe other data you recorded to identify theunknown. CAUTION Wash your handsthoroughly before leaving the laboratory.

Analyze and Conclude1. Comparing and Contrasting Is there a

relationship between the color of the flameand the color of the solution?

2. Formulating Hypotheses How do thesesubstances produce light of different colors?

3. Drawing Conclusions A forensic scientistdoes a flame test on a substance that wasfound at a crime scene. What might thescientist conclude if the flame turns green?

There is another test that youcan use to distinguish elements

by color. With your teacher supervising, dip a wireloop in borax. Heat the loop in a flame until theborax melts. Remove the loop from the flame andlet the borax cool. It will form a clear glass bead.Dip the bead in a tiny sample of solid coppersulfate and return the loop to the flame for a fewseconds. Remove the loop and observe the colorof the bead as it cools.

Go Further

Using Flame TestsObjectiveAfter completing this activity, studentswill be able to• observe that different substances

produce different colors when placedin a flame.

Skills Focus Observing, Predicting,Using Data Tables, DrawingConclusions

Prep Time 20 minutes

Advance Prep Provide each lab groupwith a sample of each solution andunknown in a small, labeled containerwith a lid. Film canisters work well andthey can be obtained free of chargefrom any photo shop. Prepare solutionsthat are about 0.1 M. Provide 0.1 M HClin a glass container.


Safety Students need to wear safetygoggles and lab aprons. Review proper use of the Bunsen burner andappropriate safety precautions for usingflames. Make sure long hair is tied backand loose clothing is not worn. Some ofthe substances are extremely toxic ifingested. Make sure students wash theirhands thoroughly before leaving thelaboratory.

Teaching Tips• Demonstrate proper technique for

performing flame tests and cleaningthe wire loop between samples. Stressthe importance of not contaminatingone sample with another.

• Have students test the sodiumchloride solution last because it canremain on the loop and make itdifficult to see the other colors.

• Stress the importance of recordingspecific colors, for example, “fire-engine red” instead of simply “red.”

Expected Outcome Students shouldbe able to make accurate observationsof flame colors and use them to identifytheir unknown solutions.

Sample Data The following flame colors arecharacteristic: calcium: orange;potassium: violet; boron: light-green;copper: green; sodium: yellow-orange.

Go FurtherThe copper-glass bead will change fromgreen to blue as it cools.Kinesthetic, Visual

L2


Analyze and Conclude1. There is no relationship between flame colorand solution color.2. When the compounds are placed in the flame,atoms absorb energy and electrons move tohigher energy levels. As these electrons moveback to lower energy levels, they release energyas visible light. The color of light produceddepends on the difference in energy betweentwo specific energy levels in an atom.3. The green flame indicates that the substancemay contain copper, barium, or boron. Visual, Logical


98A Chapter 4

Planning Guide

SECTION OBJECTIVES STANDARDS ACTIVITIES and LABSNATIONAL STATE

A-1, B-1, B-2,E-1, G-1, G-2,G-3

A-1, A-2, B-1,B-2, G-1, G-2,G-3

A-1, A-2, B-1,B-2, B-6, E-2,G-1, G-2, G-3

4.2 The Structure of an Atom, pp. 108–112

1 block or 2 periods

4.2.1 Identify three subatomic particles andcompare their properties.

4.2.2 Distinguish the atomic number of anelement from the mass number of anisotope, and use these numbers todescribe the structure of atoms.

4.3 Modern Atomic Theory, pp. 113–118


4.3.1 Describe Bohr’s model of the atom andthe evidence for energy levels.

4.3.2 Explain how the electron cloud modelrepresents the behavior and locations ofelectrons in atoms.

4.3.3 Distinguish the ground state from excited states of an atom based onelectron configurations.

4.1 Studying Atoms, pp. 100–105


4.1.1 Describe ancient Greek models of matter.

4.1.2 List the main points of Dalton’s atomictheory and describe his evidence for theexistence of atoms.

4.1.3 Explain how Thomson and Rutherfordused data from experiments to producetheir atomic models.

TE Teacher Demo: Particles and Numbers, p. 110 L2

SE Quick Lab: Comparing Excited States, p. 117

SE Forensics Lab: Using Flame Tests, p. 119

TE Teacher Demo: Electron Cloud Model, p. 116

LM Investigation 4A: Constructing a Model of an Atom

LM Investigation 4B: Modeling an Electron Cloud L1

L2

L2

L2

L2

SE Inquiry Activity: How Can You Study Objects That Are Not Visible? p. 99

SE Quick Lab: Investigating ChargedObjects, p. 102

TE Teacher Demo: Comparing AtomicModels, p. 104 L2

L2

L2

Easy Planner Teacher Express

Atomic Structure 98B

Quantities for each group

STUDENT EDITION

Inquiry Activity, p. 992 sealed, brown paper bags

Quick Lab, p. 102transparent tape, metric ruler, scissors

Quick Lab, p. 117fluorescent (“neon”) markers,glow-in-the-dark toy, ultraviolet(UV) lamp

Forensics Lab, p. 119solutions of calcium chloride,boric acid, potassium chloride,copper(II) sulfate, sodiumchloride, and an unknown;Bunsen burner; nichrome wireloop; dilute solution ofhydrochloric acid; wash bottlewith distilled water

TEACHER’S EDITION

Teacher Demo, p. 1043 clear, round bowls; flavoredgelatin mix; canned blueberries;maraschino cherries

Build Science Skills, p. 106modeling clay, plastic knives

Teacher Demo, p. 110overhead projector, red andgreen gummy candies

Teacher Demo, p. 116small, round balloon; large,round balloon; 10 beads with4-mm diameter; 5 beads with2-mm diameter

Materials for Activities and Labs

Chapter Assessment

CHAPTER ASSESSMENT

SE Chapter Assessment, pp. 121–122

CUT Chapter 4 Test A, BCTB Chapter 4iT Chapter 4PHSchool.com GOWeb Code: cca-1040

STANDARDIZED TEST PREP

SE Chapter 4, p. 123TP Diagnose and Prescribe

Go online for these Internet resources.

Web Code: cca-1040

Web Code: cce-1042

Web Code: ccn-1041Web Code: ccn-1043

Interactive Textbook withassessment at PHSchool.com

Ability Levels Components

For students who need additional help

For all students

For students who need to be challengedL3

L2

L1 SE Student EditionTE Teacher’s EditionLM Laboratory ManualPLM Probeware Lab

Manual

RSW Reading & StudyWorkbook

MSPS Math Skills &Problem SolvingWorkbook

CUT Chapter & Unit TestsCTB Computer Test BankTP Test Prep ResourcesDC Discovery Channel

Videotapes & DVDs

T TransparenciesiT Interactive TextbookP Presentation Pro

CD-ROMGO Internet Resources

RSW Section 4.1

DC Go For Gold

T Chapter 4 Pretest

Section 4.1

P Chapter 4 Pretest

Section 4.1

GO Atomic theory L2

L2

L2

L2

L2

L2

L1 SE Section 4.1Assessment, p. 105

iT Section 4.1

RSW Section 4.2

RSW Math Skill

T Section 4.2

P Section 4.2

GO Atomic

chemistry L2

L2

L2

L2


iT Section 4.2

RSW Section 4.3

T Section 4.3

P Section 4.3

GO Energy levels L2

L2

L2


iT Section 4.3

RESOURCES SECTION

PRINT and TECHNOLOGY ASSESSMENT

98C Chapter 4

Chemistry Refresher

Ancient Models of Atoms 4.1

Democritus of Abdera thought that the shapes and sizes of theatoms in a material determined the properties of the material.Atoms were seen as constantly moving in space, sometimes col-liding and forming groups. Later, Aristotle gained authority forhis idea that all substances were built up from only four elementsbecause his proposals fit with the existing worldview throughoutthe Middle Ages. Much of alchemy was based on the theory thatelements could be transformed into other elements through thequalities they possessed in common.

Evidence for Dalton’s Theory 4.1

In 1802, John Dalton proposed the law of partial pressures ofgases: A gas in a mixture of gases contributes the same pressureit would produce if it were the only gas in a container. In 1803,Dalton proposed the law of definite proportions, which statesthat elements form compounds in certain fixed ratios. Dalton’swork with gas pressure and with the composition of compoundsled him to consider the fundamental nature of matter. He pre-sented his atomic theory in a series of lectures in 1807.

Subatomic Particles 4.1 and 4.2

J.J. Thomson studied the flowof current through gases in asealed tube. He used a magnetto deflect the cathode rays. Hecould not use an electric fieldbecause the passage of the raysthrough the tube caused thegas to become a conductor.The conducting gas screenedthe particles in the rays fromthe effect of the external elec-tric field. When Thomsonremoved almost all the gasfrom the tube, the effect of thefield could be seen.

Before you teach

Many students think thatisotopes are different from“ordinary” or “regular”atoms. However, isotopesare atoms of an elementthat have differing nuclearcompositions. For astrategy to overcome thismisconception, seeAddress Misconceptionson page 112.

Big IdeasOnce students have some understanding of the propertiesthat are used to describe and classify matter, they needtheories to explain why matter behaves in predictableways. In Chapter 3, students used kinetic theory to explainthe behavior of solids, liquids, and gases. The atomictheory presented in Chapter 4 will be used in Chapters 6and 7 to explain bonding and chemical reactions.

Matter and Change In the nineteenth and earlytwentieth centuries, scientists refined models of theatom based on strong indirect evidence. Section 4.1describes some of the methods scientists used to gatherevidence about atomic structure. You can use Section 4.1to help students understand the relationship betweenscientific laws (which summarize observed patterns innature) and theories (which explain these observedpatterns). Dalton’s atomic theory is a case in point. In histheory, Dalton described properties of atoms that couldexplain the law of conservation of mass and the law ofdefinite proportions.

Some people think that a theory must be discarded ifevidence is discovered that contradicts the theory. Moreoften, a theory is revised to account for new evidence.For example, the discovery of isotopes disprovedDalton’s assumption that every atom of an element hadthe same mass. Subsequently, the atomic theory wasrevised to state that every atom of an element has thesame number of protons, but not necessarily the samenumber of neutrons.

Forces and Motion The idea that oppositelycharged particles attract and that similarly chargedparticles repel is a fundamental concept in science. Theattraction of particles to a positive plate in Thomson’sexperiment demonstrated the negative charge of theseparticles (electrons). The repulsion of positively chargedalpha particles in the gold foil experiment demonstratedthat the nucleus of an atom has a positive charge.

From the AuthorDavid FrankFerris State University

Dalton’s Symbols for Some ElementsSymbol Chemical Name Symbol Chemical Name

Hydrogen Carbon

Azote (Nitrogen) Oxygen

In 1909, Ernest Rutherford (who had worked with Thomson)asked Ernest Marsden (an undergraduate student from NewZealand) to find out whether any alpha particles were scattered bymore than a few degrees as they passed through foil. Marsden useda sheet of gold foil that was only 0.00004 (4 � 10�5) cm thick.After World War I, Rutherford experimented with bombardingnitrogen atoms with alpha particles. Some of the nitrogen atomsreleased hydrogen nuclei and transmuted into oxygen atoms. Heconcluded that these nuclei were released from the nitrogen nucleiand that they were fundamental particles. He is usually creditedwith naming these particles protons, after the Greek word protos,meaning “first.” Although Rutherford predicted the existence ofneutrons in 1920, it was his research assistant, James Chadwick,who provided the supporting evidence.

Electrons and Energy Levels 4.3

In a staircase, the distancebetween steps is (ideally) con-stant from step to step. In anatom, the difference in energybetween energy levels is notconstant. It decreases as thequantum number of the energylevel (n) increases. The specificamount of energy that isabsorbed or released when anelectron moves between energylevels is called a quantum ofenergy. When the energyabsorbed or released by an atomis light energy, the quanta arecalled photons.

Build Reading Literacy

Identify Main Idea/Details

Locating Topic Sentences in ParagraphsStrategy Help students understand and remember the mostimportant information about a topic. This strategy can beapplied to short segments of text, such as a single paragraph ora subsection, to find the main idea and details that support thetopic. Assign students a short passage to read, such as The GoldFoil Experiment on p. 104.

Example1. Tell students that many paragraphs have a topic sentence thatexpresses the paragraph’s main idea. Topic sentences are oftenthe first or second sentence in the paragraph. Sometimes,however, they are the last sentence, or even a sentence in themiddle of the paragraph.2. Then, explain that the rest of the paragraph contains details,or additional facts and examples about the main idea.3. Direct students’ attention to a paragraph with a clearly statedtopic sentence, and ask students which sentence best states themain idea of the paragraph.4. Next, help students find sentences with details that explainmore about the main idea.5. Have students organize the main idea and details visually in aweb format.6. Have students work in pairs to identify the main idea anddetails in another passage and organize them in a web.

See p. 110 for a script on how to use the identify mainideas/details strategy with students. For additional BuildReading Literacy strategies, see pp. 103 and 113.

Students may think thatelectrons travel around the nucleus in fixed orbits,like planets orbiting the sun. However, theexact path or location of an electron cannot be determined. For astrategy to overcome this misconception, seeAddress Misconceptionson page 114.

Atomic Structure 98D

n = 1

Decreasing photon wavelength

Incr

easi

ng

en

erg

y

Ener

gy

leve

l

Photon Emission as a Result of Energy Level Transitions

n = 2

n = 3

n = 4

n = 5

ΔE2

ΔE3

ΔE4

For: Teaching methods for atomic structureVisit: www.SciLinks.org/PDLinksWeb Code: ccn-0499

120 Chapter 4

CHAPTER

4 Study Guide4.1 Studying Atoms

Key Concepts

• Dalton proposed the theory that all matter is madeup of individual particles called atoms, whichcannot be divided.

• Thomson’s experiments provided the first evidencethat atoms are made of even smaller particles.

• According to Rutherford’s model, all of an atom’spositive charge is concentrated in its nucleus.

Vocabulary

nucleus, p. 105


Key Concepts

• Protons, electrons. and neutrons aresubatomic particles.

• Protons, electrons, and neutrons can bedistinguished by mass, charge, and location in an atom.

• Atoms of different elements have differentnumbers of protons.

• Isotopes of an element have the same atomicnumber but different mass numbers becausethey have different numbers of neutrons.

Vocabulary

proton, p. 108

electron, p. 108

neutron, p. 109

atomic number, p. 110

mass number, p. 110

isotopes, p. 112


Key Concepts

• An electron in an atom can move from oneenergy level to another when the atom gainsor loses energy.

• Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus.

• An electron cloud is a good approximation of howelectrons behave in their orbitals.

• The most stable electron configuration is the onein which the electrons are in orbitals with thelowest possible energies.

Vocabulary

energy levels, p. 114

electron cloud, p. 116

orbital, p. 117

electron configuration, p. 118

ground state, p. 118

Table of Properties Use information from thechapter to complete the table below.

Thinking Visually

Symbol

Relative charge

Relative mass

1�

a. ?

Particle Proton Electron Neutron

e� n

11836

b. ? c. ?

d. ? e. ?

120 Chapter 4

Study Guide

Study TipOrganize New InformationTell students to organize the keyinformation from the chapter into onecomprehensive document. They shouldconsider creating an outline, a chart, a set of flashcards, a time line, or a conceptmap to help them visualize the relation-ships. For example, they might wish toorganize all of the information in thischapter on an annotated time line. Theycould include drawings that illustrateeach atomic model. They could also labelthe drawings with vocabulary terms andtheir definitions.

Thinking Visuallya. p�

b. 1�c. 0d. 1e. 1

Chapter 4

Print• Chapter and Unit Tests, Chapter 4

Test A and Test B• Test Prep Resources, Chapter 4

Technology• Computer Test Bank, Chapter Test 4• Interactive Textbook, Chapter 4• Go Online, PHSchool.com, Chapter 4

Chapter Resources


CHAPTER

4 Assessment

Choose the letter that best answers the question orcompletes the statement.

1. One of the first people to state that matter ismade up of atoms was

a. Democritus. b. Aristotle.c. Dalton. d. Rutherford.

2. Dalton’s model of an atom is best described as a. a solar system. b. a solid sphere.c. a plum pudding. d. an electron cloud.

3. Who provided the first evidence that atomscontain subatomic particles?

a. Dalton b. Rutherfordc. Thomson d. Bohr

4. Almost all the mass of an atom is located in itsa. protons. b. electrons.c. electron cloud. d. nucleus.

5. An electron is a particle witha. a negative charge, found in the nucleus.b. a positive charge, found in the nucleus.c. no charge, found outside the nucleus.d. a negative charge, found outside

the nucleus.

6. Which particle is the least massive?a. proton b. electronc. neutron d. nucleus

7. All atoms of an element have the same a. mass number. b. number of isotopes.c. atomic number. d. number of neutrons.

8. The number of neutrons in an atom equals thea. mass number minus atomic number.b. atomic number plus number of electrons.c. mass number plus atomic number.d. atomic number minus mass number.

9. The atomic number of sulfur is 16. How manyelectrons are there in an atom of sulfur-34?

a. 16 b. 34c. 18 d. 50

10. Atoms emit energy as light whena. electrons move to a higher energy level.b. electrons move to a lower energy level.c. protons move to a higher energy level.d. protons move to a lower energy level.

Reviewing Content

11. Why must indirect evidence be used to study thestructure of atoms?

12. What evidence convinced Dalton that elementsmust be made of individual particles called atoms?

13. In Thomson’s experiment, why was the glowingbeam repelled by a negatively charged plate?

14. What evidence supported Thomson’s hypothesisthat the negative particles he observed camefrom inside atoms?

15. Compare the mass and volume of the nucleus tothe total mass and volume of an atom.

16. Compare the relative masses of protons,neutrons, and electrons in an atom.

17. What is the difference between the atomicnumber of an atom and its mass number?

18. If the atomic number of an atom is 11, howmany electrons does the atom have? Explain.

19. If an atom has an atomic number of 6 and amass number of 14, how many protons,electrons, and neutrons are in the atom?

20. What part of Dalton’s theory was modified afterthe discovery of isotopes?

21. Which isotope of oxygen is represented by thedrawing—oxygen-16, oxygen-17, or oxygen-18?Assume that all the protons and neutrons in thenucleus are visible in the drawing. Give a reasonfor your answer.

22. What is the main difference between Bohr’smodel of the atom and the atomic theory that iscurrently accepted?

23. What does it mean to say that an atom is in anexcited state?

Understanding Concepts

Proton

NucleusElectron cloud

Neutron

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Assessment

If your class subscribes to the Interactive Textbook, your students can go online to access aninteractive version of the StudentEdition and a self-test.

Reviewing Content1. a 2. b 3. c4. d 5. d 6. b7. c 8. a 9. a

10. b

Understanding Concepts11. Atoms are too small to observedirectly.12. The ratio of the mass of elements ina compound is always the same. 13. The beam contained negativelycharged particles, and like charges repel. 14. The particles were much less massivethan the lightest known atom. The sameparticles were produced no matterwhich metal was used as the origin ofthe particles. 15. Almost all of the mass of an atom islocated in the nucleus. The volume ofthe nucleus is much smaller than thevolume of the atom as a whole.16. Protons and neutrons have almost thesame mass, which is about 2000 timesgreater than the mass of an electron. 17. The atomic number represents thenumber of protons or electrons in theatom. The mass number represents thenumber of protons and neutrons. 18. Because an atom must be neutral, ithas 11 electrons to balance the chargeon the 11 protons. 19. Six protons, six electrons, eight neutrons20. All atoms of an element are identical. 21. Oxygen-17 because there are eight protons and nine neutrons22. Bohr assumed that electronstraveled in orbits around the nucleus.Current atomic theory assumes thatelectrons do not travel in fixed paths. 23. One or more of the electrons in anatom have moved from the groundstate to an orbital with a higher energy.


Homework GuideSection

4.14.24.3

Questions1–2, 11–14, 24–25, 343–9, 15–21, 26–33, 3710, 22–23, 35–36

PPLS


122 Chapter 4

CHAPTER

4 Assessment (continued)

24. Controlling Variables Look at the drawing ofthe experimental setup in Figure 5A. Explain howthe setup is a control for the setup in Figure 5B.

25. Predicting How would the results of Thomson’sexperiment change if the beam were a stream ofneutrons instead of a stream of electrons?

26. Interpreting Diagrams The atomic number ofcarbon is 6. The atomic number of nitrogen is 7.The atomic number of oxygen is 8. Name theisotope represented by the drawing.

27. Hypothesizing Why were the proton andelectron discovered before the neutron?

28. Applying Concepts Explain why a neutralatom cannot have one proton, one neutron,and two electrons.

29. Classifying The nucleus of an atom contains sixneutrons and six protons. The nucleus of a secondatom contains six neutrons and five protons. Arethey atoms of different elements or isotopes of thesame element? Explain your answer.

30. Calculating The atomic number for iron is 26.How many neutrons are in the nucleus of an ironatom with a mass number of 57? How manyelectrons does the iron atom have?

31. Applying Concepts If a potassium atom hasan atomic number of 19 and a mass number of39, how many protons, electrons, and neutronsare in the atom?

32. Applying Concepts A helium-4 atom has twiceas many protons as a hydrogen atom. How manyprotons and how many neutrons are in thenucleus of a helium-4 atom?

Math Skills

Critical Thinking

33. Comparing and Contrasting The compoundin blood that carries oxygen to cells throughoutthe body contains iron. Iron has an atomic numberof 26. Iron-59 is used to diagnose disorders in theblood. How is iron-59 different from all otherisotopes of iron? How is it the same?

34. Using Analogies Scientists working in the fieldof nanotechnology use either a top-down orbottom-up approach to construct tiny objects.Give an example of a visible structure that wasmade using the bottom-up approach and onethat was made using the top-down approach.

35. Inferring If you see a green color whenfireworks explode, can you be certain that thefireworks contained a barium compound? Givea reason for your answer.

36. Relating Cause and Effect Brightly coloredneon lights consist of tubes filled with a gas.When an electric current passes through thetubes, different colors are emitted. Why mightyou conclude that the tubes in a multicoloreddisplay contain more than one element?

37. Writing in Science Better technology leads toan increase in scientific knowledge. An increase inknowledge allows for the invention of newtechnology. Write a paragraph discussing thesestatements. Use a scanning tunneling microscopeas your example.

Preparing a Survey Write ten questions you couldask to find out what people know about the modernmodel of an atom. Figure out the best order for thequestions to test someone’s knowledge fairly. Beprepared to explain your choices.

Performance-Based Assessment

Concepts in Action

Proton

NucleusElectron cloud

Neutron

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122 Chapter 4

Critical Thinking24. There are no external charged platesin setup A. 25. Because neutrons have no charge,the charged plates would not deflect the beam. 26. Carbon-1427. A neutral particle is more difficult todetect than a charged particle becausecharged plates do not deflect its path. 28. The atom would have a negativecharge, and atoms are neutral. 29. They are atoms of different elementsbecause they have different numbers ofprotons. All atoms of a given elementhave the same number of protons.

Math Skills 30. 31 neutrons; 26 electrons31. 19 protons, 19 electrons, 20 neutrons32. Two protons and two neutrons

Concepts in Action33. Iron-59 is different from other isotopes of iron because it contains 33 neutrons. It is the same as otherisotopes because they all contain 26 protons. 34. Possible choices for the top-downanalogy include a stone sculpture, a claypot, or a carved wooden object, such as a totem pole. Choices for the bottom-upanalogy include a brick wall, a beadnecklace, a tile floor, or a patchwork quilt.35. You cannot be certain becauseelements other than barium also mayproduce a green color. 36. Each element used in neon lightsproduces a distinctive color when its atoms are excited. If there aremultiple colors, there must be multipleelements present. 37. With a scanning tunnelingmicroscope, scientists can gatherpreviously unavailable data about atoms,such as how atoms are arranged on the surface of materials. An electronmicroscope could not be developed untilscientists knew that electrons existed.

Chapter 4

Performance-Based AssessmentStudents should ask at least one question aboutthe overall structure of an atom, the properties ofsubatomic particles, and the behavior ofelectrons. Students should be able to explain theorder they have chosen.

Your students can independentlytest their knowledge of the chapterand print out their test results foryour files.


Standardized Test PrepTest-Taking Tip

Using Data TablesWhen presented with a question that is relatedto a data table, read the title of the table to seewhat type of data it contains. Then look at theheadings of the columns and rows to see howthe data are organized. The table below listsproperties for subatomic particles. There is a rowfor each particle and a column for each property.Read the question to find out which data youwill need to answer the question. In this case,you will need the data on relative charge.

Which of the following statements is true? (A) The charge on a proton is larger than the

charge on an electron.(B) The charge on a proton is smaller than the

charge on an electron.(C) The charge on a proton is identical to the

charge on an electron (D) The charge on a proton is equal in size but

opposite to the charge on an electron.(E) A proton is a neutral particle.

(Answer: D)

Particle Symbol RelativeCharge

Relative Mass(proton = 1)

Electron

Neutron

e�

n

1�

Proton p� 1�

0

1_____1836

1

1

Properties of Subatomic Particles

Choose the letter that best answers the question orcompletes the statement.

1. J. J. Thomson demonstrated that electrons(A) have a negative electric charge.(B) have a positive electric charge. (C) are repelled by a positively charged object.(D) are attracted to a negatively charged object.(E) do not have an electric charge.

2. According to Dalton’s atomic theory, an atom is (A) made of smaller particles.(B) a particle with a positive charge. (C) the smallest particle of an element.(D) in constant motion.(E) a particle with a negative charge.

3. Electrons in the first energy level of an atom(A) have no energy. (B) have the lowest possible energy.(C) have the highest possible energy.(D) are in an excited state.(E) are in an unstable state.

4. Most alpha particles pass through a thin layer ofgold without deflection because gold atoms(A) are filled with positively charged matter. (B) have no overall charge.(C) have a negatively charged nucleus. (D) do not have a nucleus.(E) have a dense nucleus surrounded by space.

Use the data table to answer Question 5.

5. What is the mass number of oxygen-18? (A) 8(B) 10(C) 16(D) 18(E) 0.205

6. An electron configuration describes(A) regions of space around the nucleus of

an atom.(B) possible energies that an electron can have.(C) the arrangement of electrons in an atom.(D) the emission of light from an excited atom. (E) the number of possible orbitals in an atom.

Property Oxygen-16 Oxygen-18

Protons

Neutrons

Electrons

Percentagein nature

8

8

8

99.757

8

10

8

0.205

Comparison of Oxygen Isotopes

Standardized Test Prep 1. A 2. C 3. B 4. E 5. D 6. C



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