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.

108 Chapter 4

108 Chapter 4

FOCUS

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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Reading Focus

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Section 4.2

Print• Guided Reading and Study Workbook

With Math Support, Section 4.2 and Math Skill: Electrons and Orbitals

• Transparencies, Section 4.2

Technology• iText, Section 4.2• Presentation Pro CD-ROM, Section 4.2• Go Online, Science News, Atomic chemistry

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

Atomic Structure 109

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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Atomic Structure 109

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 �1�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 anyinformation about the number of neutrons in an atom. Forthat information, you need to know the atom’s mass number.The mass number of an atom is the sum of the protons andneutrons in the nucleus of that atom. An atom of aluminumwith 13 protons and 14 neutrons has a mass number of 27. Ifyou know the atomic number and the mass number of anatom, 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?

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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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Section 4.2 (continued)

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.

Atomic Structure 111

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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Atomic Structure 111

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

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Section 4.2 Assessment

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.

If your class subscribes toiText, use it to review key concepts inSection 4.2.

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Section 4.2 (continued)

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.

Section 4.2 Assessment

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