Mixed Groups

7f)w You'll Learn• Distinguish among metals,nonmetals, and metalloids.

• Describe the nature of allotropes.• Recognize the significance ofdifferences in crystal structurein carbon.

• Understand the importanceof synthetic elements.

It's ImportantThe elements in mixed groups affectyour life every day, because they arein everything from the computer youuse to the air you breathe.

Review Vocabularysubstance: element or compoundthat cannot be broken down intosimpler components

New Vocabulary• metalloid• allotrope• semiconductor• transuranium element

The Boron Group

5B

13AI r

31Ga

49 /JIn"'HI 81 II I

TI I I

Figure 16 Aluminum is used fre-quently in the construction of air-planes because it is light and strong.

Properties of MetalloidsCan an element be a metal and a nonmetal? In a sense, some

elements called metalloids are. Metalloids share unusual charac-teristics. Metalloids can form ionic and covalent bonds withother elements and can have metallic and nonmetallic proper-ties. Some metalloids can conduct electricity better than mostnonmetals, but not as well as some metals, giving them thename semiconductor. '>\lith the exception of aluminum, themetalloids are the elements in the periodic table that are locatedalong the stair-step line. The mixed groups-13, 14, 15, 16, and17-contain metals, nonmetals, and metalloids.

The Boron GroupBoron, a metalloid, is the first element in Group 13. If you look

around your home, you might find two compounds of boron. Oneof these is borax, which is used in some laundryproducts to soften water. The other is boric acid, amild antiseptic. Boron also is used as a grindingmaterial and as boranes, which are compounds usedfor jet and rocket fuel.

Aluminum, a metal in Group 13, is the mostabundant metal in Earth's crust. It is used in soft-drink cans, foil wrap, cooking pans, and as siding.Aluminum is strong and light and is used in the con-struction of airplanes such as the one in Figure 16.

584 CHAPTER 19 Elements and Their Properties

Silicon is used to make the chips that allow this computer to run.

The Carbon GroupEach element in Group 14, the carbon family, has four elec-

trons in its outer energy level, but this is where much of the sim-ilarity ends. Carbon is a nonmetal, silicon and germanium aremetalloids, and tin and lead are metals. Carbon occurs as an ele-ment in coal and as a compound in oil, natural gas, and foods.Carbon in these materials can combine with OA)'gen to producecarbon dioxide, CO2, In the presence of sunlight, plants utilizeCO2 to make food. Carbon compounds, many of which areessential to life, can be found in you and all around you. Allorganic compounds contain carbon, but not all carbon com-pounds are organic.

Silicon is second only to oxygen in abundance in Earth'scrust. Most silicon is found in sand, Si02, and almost all rocksand soil. The crystal structure of silicon dioxide is similar to thestructure of diamond. Silicon occurs as two allotropes.Allotropes, which are different forms of the same element, havedifferent molecular structures. One allotrope of silicon is a hard,gray substance, and the other is a brown powder.

Whatareallotropes?

Silicon is the main component in semiconductors-ele-ments that conduct an electric current under certain conditions.Many of the electronics that you use every day, like the computerin Figure 17, need semiconductors to run. Germanium, the othermetalloid in the carbon group, is used along with silicon in mak-ing semiconductors. Tin is used to coat other metals to preventcorrosion, like the tin cans in Figure 17. Tin also is combinedwith other metals to produce bronze and pewter. Lead was usedwidely in paint at one time, but because it is toxic, lead no longeris used.

Figure 17 Elements in Group 14have many uses.

These tin cans are made of steelwith a tin coating.

The Carbon Group6(

1451

32Ge505n

\'\ I 82 / I I\

I Pb l I

SECTION 3 Mixed Groups 585

Figure 18 Three allotropesof carbon are depicted here.Identify the geometric shapesthat make up each allotrope.

Topic:BuckminsterfullereneVisit gpscience.com for Web linksto information about thiscompound.

Activity Research this com-pound and describe some of thequalities that make it unique.

'\Scienc line

Allotropes of Carbon What do the diamond in a diamondring and the graphite in your pencil have in common? They areboth carbon. Diamond, graphite, and buckminsterfullerene,shown in Figure 18, are allotropes of an element.

A diamond is clear and extremely hard. In a diamond, eachcarbon atom is bonded to four other carbon atoms at the ver-tices, or corner points, of a tetrahedron. In turn, many tetra-hedrons join together to form a giant molecule in which theatoms are held tightly in a strong crystalline structure. Thisstructure accounts for the hardness of diamond.

Graphite is a black powder that consists of hexagonal layersof carbon atoms. In the hexagons, each carbon atom is bondedto three other carbon atoms. The fourth electron of each atomis bonded weakly to the layer next to it. This structure allows thelayers to slide easily past one another, making graphite an excel-lent lubricant. In the mid-1980s, a new allotrope of carboncalled buckminsterfullerene was discovered. This soccer-ball-shaped molecule, informally called a buckyball, was named afterthe architect-engineer R. Buckminster Fuller, who designedstructures with similar shapes.

In 1991, scientists were able to use the buckyballs to synthe-size extremely thin, graphitelike tubes. These tubes, called nan-otubes, are about 1 billionth of a meter in diameter. That meansyou could stack tens of thousands of nanotubes just to get thethickness of one piece of paper. Nanotubes might be llsed some-day to make computers that are smaller and faster and to makestrong building materials.

..Graphite Diamond Buckminsterfullerene

586 CHAPTER 19 Elements and Their Properties

The Nitrogen GroupThe nitrogen family makes up Group 15. Each element has

five electrons in its outer energy leveL These elements tend toshare electrons and to form covalent compounds with other ele-ments. Nitrogen often is used to make nitrates (which are com-pounds that contain the nitrate ion, N03-) and ammonia, NH3'both of which are used in fertilizers. Nitrogen is the fourth mostabundant element in your body. Each breath you take is about80 percent gaseous nitrogen in the form of diatomic molecules,N 2. Yet you and other animals and plants can't use nitrogen inits diatomic form. The nitrogen must be combined into com-pounds, such as amino acids.

7The Nitrogen N

Group 15p

33 """As

51 I/JSb, I I 83 / I'J Bi / -1

-

3600

Use Circle Graphs

CRUST ELEMENTS OAI'gen, the predominant element in Earth's crust, makes upapproximately 46.6 percent of the crust. If you were to show this information on acircle graph, how many degrees would be used to represent oxygen?

IIlImiID known values and unknown values

Identify the known values:OAl'gen is approximately 46.6%circle contains 3600

Identify the unknown values:how many degrees OAl'gen represents

E!lD the problem46.6 _ x100

x = = 167.760 or 1680

ImD the answer

Does your answer seem reasonable? Check your answer by dividing youranswer by 3,60, then multiplying by 100.

1. The percentages of remaining elelnents in Earth's crust are: silicon, 27.7; alUlninum,8.1; iron, 5.0; calcium, 3.6; sodium, 2.8; potasSiUIl1, 2.6; magnesium, 2.1; and otherelements, 1.5. Illustrate in a circle graph.

2. The approximate percentages of the elements in the human body are: oxygen, 65;carbon, 18; hydrogen, 10; nitrogen, 3; calcium, 2; and other elements which accountfor 2. lIlustrate these percentages in a circle graph.

For more practice problems, go to page 834, and visit gpscience.com/extra_problems.

SECTION 3 Mixed Groups 587

The Oxygen 8Group 0

16

l- S34SeS2 i/jTe LL

I r r 84 /1 rIJ IT r ,-

Po

Figure 19 Group 16 com-pounds have avariety of uses.

Uses of the Nitrogen Group Phosphorus is a nonmetalthat has three allotropes. Phosphorous compounds can be usedfor many things from water softeners to fertilizers, match heads,and even in fine china. Antimony is a metalloid, and bismuth isa metal. Both elements are used with other metals to lower theirmelting points. Because of this property, the metal in automaticfire-sprinkler heads contains bismuth.

Why is bismuth used in fire-sprinkler heads?

The Oxygen GroupGroup 16 on the periodic table is the oAi'gen group. You can

live for only a short time without oA)'gen, which makes up about21 percent of air. Oxygen, a nonmetal, exists in the air asdiatomic molecules, 02' During electrical storms, some oA)'genmolecules, 02' change into ozone molecules, 03' Oxygen alsohas several uses in compound form, including the one shown atleft in Figure 19.

Nearly all living things on Earth need 02 for respiration.Living things also depend on a layer of 03 around Earth for pro-tection from some of the Sun's radiation.

The second element in the OA)'gen group is sulfur. Sulfur is anonmetal that exists in several allotropic forms. It exists asdifferent-shaped crystals and as a noncrystalline solid. Sulfurcombines with metals to form sulfides of such distinctive colorsthat they are used as 'pigments in paints.

The nonmetal selenium and two metalloids-tellurium andpolonium-are the other Group 16 elements. Selenium is themost common of these three. This element is one of several thatyou need in trace amounts in your diet. Many multivitaminscontain this nonmetal as an ingredient. But selenium is toxic iftoo much of it gets into your system. Selenium also is used inphotocopiers like the one in Figure 19.

Solutions of hydrogen peroxide,H202, are used to clean minorwounds.

Selenium is used inxerography to makephotocopies.

588 CHAPTER 19 Elements and Their Properties

--r--Smoke

£i'---T--- Alarm

Battery---+---'"

+-;:=:-__

Smokeparticle

Smokedetector

Figure 20 The americium usedin smoke detectors is a syntheticelement that has saved lives.

-_,.."

Synthetic ElementsIf you made something that always fell apart, you might

think you were not successful. However, nuclear scientists arelearning to do just that. By smashing existing elements with par-ticles accelerated in a heavy ion accelerator, they have been suc"-cessful in creating elements not typically found on Earth. Exceptfor technetium 43 and promethium 61, each synthetic elementhas more than 92 protons.

Bombarding uranium with neutrons can make neptunium,element 93. Half of the synthesized atoms of neptunium disin-tegrate in about two days. This may not sound useful, but whenneptunium atoms disintegrate, they form plutonium. Thishighly toxic element has been produced in control rods ofnuclear reactors and is used in bombs. Plutonium also can bechanged to americium, element 95. This element is used inhome smoke detectors such as the one in Figure 20. In smokedetectors, a small amount of americium emits charged particles.An electric plate in the smoke detector attracts some of thesecharged particles. When a lot of smoke is in the air, it interfereswith the electric current, which immediately sets off the alarmin the smoke detector.

The Transuranium Elementsr -

,

Transuranium Elements Elements having more than 92 pro-tons, the atomic number of uranium, are called transuraniumelements. These elements do not belong exclusively to the metal,nonmetal, or metalloid group. These are the elements toward thebottom of the periodic table. Some are in the actinide series, andsome are on the bottom row of the main periodic table. All of thetransuranium elements are synthetic and unstable, and many ofthem disintegrate quickly.

SECTION 3 Mixed Groups 589

ONATIONALGEOGRAPHIC VISUALIZING THE DISCOVERY OF ELEMENTS

Why make elements? Figure 21 shows when some of theelements were discovered throughout history. The processesused to discover these elements have varied widely. The mostrecently discovered elements are synthetic. By studying how thesynthesized elements form and disintegrate, you can gain anunderstanding of the forces holding the nucleus together. Whenthese atoms disintegrate, they are said to be radioactive.

Radioactive elements can be useful. For example, technetium'sradioactivity makes it ideal for many medical applications. At thistime, many of the synthetic elements last only small fractions ofseconds after they are constructed and can be made only in smallamounts. However, the value of applications that might be dis-covered easily could offset their costs.

Seeking Stability Element 114, discovered in 1999, appears tobe much more stable than most synthetic elements of its size. Itlasted for 30 s before it broke apart. This may not seem like long,but it lasts 100,000 times longer than an atom of element 112.Perhaps this special combination of 114 protons and 175 neutronsallows the nucleus to hold together despite the enormous repul-sion between the protons.

In the 1960s, scientists theorized that stable synthetic elementsexist. Finding one might help scientists understand how the forcesinside the atom work. Perhaps someday you'll read about some ofthe everyday uses this discovery has brought.

SciencTopic: Synthetic ElementsVisit gpscience.com for Web linksto information and an onlineupdate about synthetic elements.

Activity Find what some ofthe latest developments are insynthetic elements. Collect infor-mation on the one that interestsyou most and explain what youthink is the most intriguing prop-erty of this element.

SummaryProperties of Metalloids• Metalloids are elements that can form ionicand covalent bonds with other elements andcan have metallic and nonmetallic properties.

Carbon Group• The elements in Group 14 have four electronsin their outer energy levels.

Nitrogen Group• The elements in Group 15 tend to share elec-trons and form covalent bonds.

Synthetic Elements• Synthetic elements are elements that are nottypically found on Earth.

• By synthesizing elements, scientists may under-stand how the forces inside the atom work.

Self Check1. Explain why Groups 14 and 15 are better representa-tives of mixed groups than Groups 13 and Group 16.

2. Describe how allotropes of silicon differ in appearance.3. Explain how an element is classified as a transuraniumelement.

4. Describe what type of structure a diamond has. Howwould you build a model of this?

5. Think Critically Graphite and a diamond are bothmade of the element carbon. Why is graphite a lubri-cant and diamond the hardest gem known?

6. Calculate Element 114 lasted 30s before falling apart.It lasted 100,000 times longer than element 112. Howlong did element 112 last?

\Selene gpscience.com/seICcheck_quiz SECTION 3 Mixed Groups 591

592 CHAPTER 19 Elements and Their Properties

....

Form aBased on your understanding of how carbonatoms bond, form a hypothesis about the rela-tionship of graphite's molecular structure toits physical properties.

Real-WorldOften, a lubricant is needed when two metalstouch each other. For example, a sticky locksometimes works better with the addition ofa small amount of graphite. What gives thisallotrope of carbon the slippery property of alubricant? Why do certain arrangements ofatoms in a material cause the material to feelslippery?

StipperY Carbon

Use care when workingwith scissors anduncooked spaghetti.

Safety PrecautionsE:::.. 'Win IE

Goals• Make amodel thatwill demonstrate themolecular structure ofgraphite.

• Compare and contrastthe strength of thedifferent bonds ingraphite.

• Infer the relationshipbetween bonding andphysical properties.

Possible Materialsthin spaghettismall gumdropsthin polystyrene sheetsflat cardboardscissors

Test YourMake APlan1. As a group, agree upon a logical hypothesis statement.2. As a group, sequence and list the steps you need to take to test yourhypothesis. Be specific, describing exactly what you will do at each step tomake a model of the types of bonding present in graphite.

3. Remember from Figure 18 that graphite consists of rings of six carbons bondedin a flat hexagon. These rings are bonded to each other. In addition, the flatrings in one layer are weakly attached to other flat layers.

4. list possible materials you plan to use.5. Read over the experiment to make sure that all steps are in logical order.6. Will your model be constructed with materials that show weak and strongattractions?

Follow Your Plan1. Make sure your teacher approves your plan before you start.2. Have you selected materials to use in your model that demonstrate weak andstrong attractions? Carry out the experiment as planned.

3. Once your model has been constructed, list any observations that you make andinclude a sketch in your Science Journal.

Analyze Your1. Compare your model with designs and results of other groups.2. How does your model illustrate two types of attractions found in the graphitestructure?

3. How does the bonding of graphite that you explored in the lab explaingraphite's lubricating properties? Write your answer in your Science Journal.

Conclude andApply--1. Describe the results you obtained from yourexperiment. Did the results support yourhypothesis?

2. Describe why graphite makes a good lubricant.3. Explain what kinds of bonds you think adiamond has.

ommunicating.. .Explain to a friend why graphite makes agood lubricant and how the two types ofbonds make a difference.

LAB 593

. . •

1. Nonmetals can be brittle and dull. They arealso poor conductors of electricity.

2. As a typical nonmetal, hydrogen is a gasthat forms compounds by sharing electronswith other nonmetals and by forming ionicbonds with metals.

3. All the halogens, Group 17, have sevenouter electrons and form covalent and ioniccompounds, but each halogen has someproperties that are unlike each of the othersin the group.

I:ElI!D Use the Foldable that you made at the begin-ning of this chapter to help you review elements and theirproperties.

2. Allotropes are forms of the same elementhaving different molecular structures.

3. The properties of three forms of carbon-graphite, diamond, and buckminster-fullerene-depend upon the differences intheir crystal structures.

4. All synthetic elements are short-lived. Exceptfor technetium-43 and promethium-61, theyhave atomic numbers greater than 92 andare referred to as transuranium elements.These elements are found toward the bot-tom of the periodic table.

Mixed Groups1. Groups 13 through 16 include metals, non-metals, and metalloids.

4. The noble gases, Group 18, are elementswhose properties and uses"are related totheir chemical stability.

Nonmetals

MetalsSection 1

1. A typical metal is a hard, shiny solidthat, due to metallic bonding, ismalleable, ductile, and a goodconductor.

2. Groups 1 and2 are the alkaliand alkalineearth metals,which havesome similar andsome contrastingproperties.

3. The iron triad,the coinage met-als, and the ele-ments in Group12 are examplesof transitionelements.

4. The lanthanides and actinides have atomicnumbers 58 through 71 and 90 through103, respectively.

\Sc:ienc: II gpscience.comlintera ctive_tuto r CHAPTER STUDY GUIDE 595

Complete each sentence with the correct vocabll-lary word(s).

1. The are located to the left of thestair-step line on the periodic table.

2. Different structural forms of the sameelement are called _

3. Positively charged ions are surrounded byfreely moving electrons in _

4. A(n) is a molecule comprised oftwo atoms.

5. The are in Groups 3 through 12on the periodic table.

() americium0) plutonium

12. Which elements are least reactive?A) metals () noble gasesB) halogens 0) actinides

13. What element is formed when neptuniumdisintegrates?A) ytterbiumB) promethium

9. Which set of elements makes up the mostreactive group of all metals?A) iron triadB) coinage metalsC) alkali metals0) alkaline earth metals

10. Which element is the most reactive of allnonmetals?A) fluorine C) hydrogenB) uranium 0) oxygen

11. Which element is always found in naturecombined with other elements?A) copper C) magnesiumB) gold 0) silver

radioactive element p.572salt p.580semiconductor p.585sublimation p.581transition element p.574transuranium element

p.589

•allotrope p.585diatomic molecule p.579ductile p.570malleable p.570metal p.570metallic bonding p.571metalloid p.584nonmetal p.578

() covalent0) diatomic

14. Copy and complete the concept map usingthe following: trallsitioll elements, hydrogen,metals, illHer transition metals, noble gases.

areoand

Halogensexceptastatineand

are

Alkali metalsand alkalineearth metal

andoand

7. What type of bond is found in a piece ofpure gold?A) metallicB) ionic

Choose the word or phrase that best allswers theI]lIestioll.

6. When magnesium and fluorine react, whattype of bond is formed?A) metallic C) covalentB) ionic 0) diatomic

8. Because electrons move freely in metals,which property describes metals?A) brittle C) dullB) hard 0) conductors

ec mg Concepts

596 CHAPTER REVIEW'1

Scienc gpscience.com/vocabulary_puzzlemaker

Gas Volume%

CO 6.8

H) 47.3

CH4 33.9

CO) 2.2

N) 6

Other 3.8

25. Interpret Data When coke-oven gas isburned in an industrial process, severalgases are produced in the reaction. If385 grams of coke-oven gas are con-sumed in this reaction, how manygrams of CH4 (methane) are produced?

26. Use Percentage Chloroform has thechemical formula, CHCl3, and a molec-ular weight of 119.39 g. What is thepercentage of Cl (chlorine) present inthis compound?

27. Use Numbers Calculate the molecularweight of gallium bromide (GaBr3).

Gas Analysis

Use the following table to answer question 25.

22. Compare and Contrast Explain why aluminumis a metal and carbon is not.

23. Explain What is metallic bonding? Explainhow this affects conductivity.

24. Describe the geometric shapes of the carbonallotropes.

•

17. Explain why mercury is rarely used in ther-mometers that take body temperatures.

18. Explain The density of hydrogen is lowerthan air and can be used to fill balloons.Why is helium used instead of hydrogen?

19. Explain Copper is a good choice for use inelectrical wiring. What type of elementswould not work well for this purpose?Why?

20. Explain why various silver compounds areused in photography.

21. Describe Like selenium, chromium is poi-sonous but is needed in trace amounts inyour diet. How would you apply thisinformation in order to use vitamin andmineral pills safely?

16. Make and Use Tables Use the periodic tableto classify each of the following as alanthanide or actinide: californium,europium, cerium, nobelium, terbium,and uranium.

15. Concept Map Copy and complete the conceptmap using the following: Nn, Fe, Actinides,Hg, Bn, Alknli, nnd Inner tmnsition.

,Scienc gpscience.com!chapterJeview CHAPTER REVIEW 597

- ----, - _. -, --

Use the illustration below to answer questions 5 and 6,

Elements in the Human Body

0<:- ",<:- ",<:-,;;/:>>1;

""" "Elements

70,-----------------,6050..4030-....2010o

",<:-

Record your allswers all the answer sheetprovided by your teacher or on a sheet ofpaper.1. Which of these elements is the main com-ponent of steel, and the most widely usedof all metals?A. iron Co cadmiumB. aluminum D. magnesium

2. What term describes the Group 1 elementslithium, sodium, and potassium?A. alkali metalsB. radioactive elementsCo lanthanidesD. transition metals

Co liquidD. plasma

Use the illustration below to answer questions 3 and 4.

26 27 28Fe Co Ni r

;:,

3. What name is given to these three elementswhich are used in processes that create steeland other metal mixtures?A. halogens Co actinidesB. the coin metals D. the iron triad

s. Which of these is a property of the ele-ments that make up 98 percent of thehuman body?A. malleabilityB. poor electrical and heat conductivityCo shiny appearanceD. ductility

6. In which of these phases does the elementpresent in the highest percentage in thehuman body exist?A. gasB. solid

Co transition elementsD. alkali metals

Co nitrogenD. carbon

4. To which major group do these elementsbelong?A. nonmetalsB. noble gases

7. Which element is present in all organiccompounds?A. siliconB. m.'ygen

Eliminate Choices If you don't know the answer to amultiple-choice question eliminate as many incorrect choicesas possible.

8. Which of these is NOT a property oftransuranium elements?A. occur naturallyB. have greater than 92 protonsCo are syntheticD. are unstable

598 STANDARDIZED TEST PRACTICE

Record YOl/r al/slI'ers 01/ the allSlI'er sheetprovided by YOl/r teacher or 01/ a sheet ofpaper.9. Define the general properties of metalswhich make them useful and versatilematerials.

10. Use the electron configuration of the ele-ments sodium and potassium to explainwhy these elements do not occur in naturein elemental form.

Use the illustrations below to answer questions 11and 12.

11. Define the term allotrope, and identifythese allotropes of carbon.

12. Compare the structures of these carbonallotropes and relate the structures to theproperties of these materials.

13. Describe some unique properties ofhydrogen.

14. Identify and describe the uses of some ofthe halogens obtained from seawater.

15. Compare the two types of bonds whichnonmetals can form.

Record YOl/r nllSIvers on the answer sheetprovided by your teacher or on a sheet ofpaper.16. Recent Federal Drug Administration state-

ments advise limiting consumption of tunaand salmon. Which transitional element isthe source of the problem? Explain whythis element poses a potential risk.

17. Use the properties of metallic bonds toexplain why metal hammered into sheetsdoes not break, as well as why metals con-duct electricity.

18. Based on its electron configuration andposition in the periodic table, explain whyfluorine is the most chemically active ofall elements.

Use the illustration below to answer question 19.

19. Identify the gas which enables this blimpto remain suspended in the atmosphere.Why would it be dangerous to use hydro-gen for this purpose?

20. Explain the importance of organisms thatconvert nitrogen from its diatomic forminto other compounds.

\

Selene 9pscience.com/standa rd ized_test STANDARDIZED TEST PRACTICE 599

..section

•I

Stability in Bonding

WwYou'll Learn• Describe how a compound differsfrom its component elements.

• Explain what a chemical formularepresents.

• Explain that the electric forcesbetween oppositely charged elec-trons and protons are essential toforming compounds

• State a reason why chemicalbonding occurs.

W'v It's ImportantThe millions of different kindsof matter around us are a resultof chemical bonds.

Review Vocabularycompound: substance formed fromtwo or more elements in which theexact combination and proportion ofelements is always the same

New Vocabulary• chemical formula• chemical bond

Figure 1 The differencebetween the elemental coppermetal and the copper compoundformed on the Statue of Libertyis striking.

Elementalcopper

Surface coated with a coppercompound

602 CHAPTER 20 Chemical Bonds

Combined ElementsHave you ever noticed the color of the Statue of Liberty? Why

is it green? Did the sculptor purposely choose green? Why wasn'twhite, or tan, or even some other color like purple chosen? Vvasit painted that way? No, the Statue of Liberty was not painted.The Statue of Liberty is made of the metal copper, which is anelement. Pennies, too, are made of copper. Wait a minute, yousay. Copper isn't green-it's ... well, copper colored.

You are right. Uncombined, elemental copper is a bright,shiny copper color. So again the question arises: Why is theStatue of Liberty green?

Compounds Some of the matter around you is in the form ofuncombined elements such as copper, sulfur, and o},,)'gen. But,like many other sets of elements, these three elements unitechemically to form a compound when the conditions are right.The green coating on the Statue of Liberty and some old pen-nies is a result of this chemical change. One compound in thiscoating, seen in contrast with elemental copper in Figure 1, is anew compound called copper sulfate. Copper sulfate isn't shinyand copper colored like elemental copper. Nor is it a pale-yellowsolid like sulfur or a colorless, odorless gas like o>..)'gen. It has itsown unique properties.

Figure 2 Sodium is a soft,silvery metal that combines withchlorine, a greenish-yellow gasrepresented here as only one atom,to form sodium chloride, which is awhite crystalline solid.Describe how the properties oftable salt are different from thoseofsodium and eMorine.

Sodium chloride

Chemical Name Formula

Silicon dioxide Si02

Magnesium hydroxide Mg(OH)2

Sucrose CnH22011Calcium oxide CaO

Acetic acid CHJCOOH

Dinitrogen oxide N20

Ethanol C2HsOH

Sulfuric acid H2SO4

Hydrochloric acid HCI

Chlorine

Stomach acid

Battery acid

Grain alcohol

Laughing gas

Vinegar

lime

Table 1 Some Familiar Compounds

Cane sugar

Milk ofmagnesia

Sand

Familiar Name

+

".Na + :(1"Sodium

Describe whata ehemi-., .cal formula tells you.

FormulasThe chemical symbols Na and CI represent the elements

sodium and chlorine.When written as NaCl, the symbols make upa formula, or chemical shorthand, for the compound sodiumchloride. A chemical formula tells what elements a compoundcontains and the exact number of the atoms of each element in aunit of that compound. The compoundthat you ate probably most familiar with isH20, more commonly known as water. Thisformula contains the symbols H for theelement hydrogen and ° for the elementoxygen. Notice the subscript number 2written after the H for hydrogen. Subscriptmeans "written below." A subscript writtenafter a symbol tells how many atoms of thatelement are in a unit of the compound. If asymbol has no subscript, the unit contains.only one atom of that element. A unit ofH20 contains two hydrogen atoms and oneOA!'gen atom.

Look at the formulas for each com-pound listed in Table 1. What elementscombine to form each compound? Howmany atoms of each element are requiredto form each of the compounds?

New Properties Oneinteresting observation youwill make is that the com-pound formed when ele- II""'"ments combine often hasproperties that aren't any-thing Ilke those of the indi-vidual elements. Sodiumchloride, for example,shown in Figure 2, is acompound made from theelements sodium and chlo-rine. Sodium is a shiny,soft, silvery metal thatreacts violently with water.Chlorine is a poisonous greenish-yellow gas. Would you haveguessed that these elements combine to make ordinary table salt?

SECTION 1 Stability in Bonding 603

1 2 13 14 15 16 17 18

2

3

li Be

• ••• ••• •

Na Mg

•• •• •• •••• ..• • • •••• •• •• ••

B C N 0 F Ne

•• •• • •• • •• • •• •• ••.. .. •• •• •• ••• • • • • •• • • • • • •• •• •• •• ••

AI Si P S CI Ar

• • • ••• •• •• •• •• •• •• •• •• •• •• •••• •• •• •• •• ••• • • • • • • • • • • • • •• • • • • • • • • • •• • •• • • • • • •• • • •• • • • • • • • • •

• • • •• •• ••

Figure 3 Note in this simplifiedrepresentation of electron distribu-tion, that the number of electronsin each group's outer levelincreases across the table, until thenoble gases in Group 18 have acomplete outer energy level.Analyze Which of the groupspictured could form adiatomiccompound with the elements inGroup 21

Figure 4 Electron dot diagramsof noble gases show that they allhave a stable, filled outer energylevel.

e ee _

He :Kr:e e

e e e e

:Ne: :Xe:_ e _ e

e _ _ e

:Ar: :Rn:e e

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604 CHAPTER 20 Chemical Bonds

Atomic Stability\ilJhy do atoms form compounds? The electric forces between

oppositely charged electrons and protons hold atoms and mole-cules together, and thus are the forces that cause compounds toform. The periodic table on the inside back cover of your booklists the known elements, most of which can combine with otherelements. However, the six noble gases in Group 18 seldom formcompounds. Why is this so? Atoms of noble gases are unusuallystable. Compounds of these atoms rarely form because they arealmost always less stable than the original atoms.

The Unique Noble Gases To understand the stability of thenoble gases, it is helpful to look at electron dot diagrams.Electron dot diagrams show only the electrons in the outerenergy level of an atom. They contain the chemical symbol forthe element surrounded by dots representing its outer electrons.How do you know how many dots to make? For Groups 1 and 2and 13 through 18, you can use a periodic table or the portionof it shown in Figure 3. Look at the outer ring of each of the ele-ments. Group I has one outer electron. Group 2 has two. Group13 has three, Group 14, four, and so on to Group 18, the noblegases, which have eight.

Chemical Stability An atom is chemically stable when itsouter energy level is complete. Recall that the outer energy lev-els of helium and hydrogen are stable with two electrons. Theouter energy levels of all the other elements are stable when theycontain eight electrons. The noble gases are stable because theyeach have a complete outer energy level. Figure 4 shows electrondot diagrams of some of the noble gases. Notice that eight dotssurround Kr, Ne, Xe, Ar, and Rn, and two dots surround He.