Section 6.1 6.1 Ionic Bondingmrbakerphysical.weebly.com/uploads/3/0/8/0/30809369/ch6_book_ps_… ·...

158 Chapter 6

6.1 Ionic Bonding

Reading StrategySequencing Copy the concept map. As youread, complete the concept map to showwhat happens to atoms during ionic bonding.

Key ConceptsWhen is an atom unlikelyto react?

What is one way inwhich elements canachieve stable electronconfigurations?

How does the structure ofan ionic compound affectits properties?

Vocabulary◆ electron dot

diagram◆ ion◆ anion◆ cation◆ chemical bond◆ ionic bond◆ chemical formula◆ crystals

The handle of the titanium mug in Figure 1 was joined to the bodyby welding. The pieces were heated until their surfaces fused together.The welding of titanium does not take place in air. At the temperatureat which welding occurs, titanium becomes hot enough to react withoxygen in the air, forming an oxide. The oxide makes the weld morebrittle and likely to break. Because titanium does not react with a noblegas such as argon, the welding of titanium usually takes place in anargon atmosphere.

Argon’s name is a reminder of its inactivity. It comes from theGreek word argos, which means “idle” or “inert.” Why is argon veryinactive yet oxygen is highly reactive? Chemical properties, such asreactivity, depend on an element’s electron configuration.

Stable Electron ConfigurationsThe highest occupied energy level of a noble gas atom is filled.

When the highest occupied energy level of an atom is filled withelectrons, the atom is stable and not likely to react. The noble gaseshave stable electron configurations with eight valence electrons (or twoin the case of helium).

The chemical properties of an element depend on the number ofvalence electrons. Therefore, it is useful to have a model of atoms thatfocuses only on valence electrons. The models in Figure 2 are electrondot diagrams. An electron dot diagram is a model of an atom in whicheach dot represents a valence electron. The symbol in the center rep-resents the nucleus and all the other electrons in the atom.

loseselectrons

to

gainselectrons

to

a. ?

b. ?

Atom

Figure 1 The handle and body ofthis titanium mug were weldedtogether in an argon atmosphere.If titanium is allowed to reactwith oxygen in air, the compoundthat forms makes the weld morebrittle and more likely to break.

158 Chapter 6

FOCUS

Objectives6.1.1 Recognize stable electron

configurations.6.1.2 Predict an element’s chemical

properties using number ofvalence electrons and electrondot diagrams.

6.1.3 Describe how an ionic bondforms and how ionizationenergy affects the process.

6.1.4 Predict the composition of anionic compound from itschemical formula.

6.1.5 Relate the properties of ioniccompounds to the structure ofcrystal lattices.

Build VocabularyWord Forms Have students think ofword forms related to crystals such ascrystalline and crystallize. Have themdiscuss ways people commonly usethese words—and the word crystal—andcompare those usages to the waycrystals is defined in Section 6.1.

Reading Strategya. Form a cation b. Form an anion

INSTRUCT

Stable ElectronConfigurationsIntegrate Social StudiesIn 1902, G.N. Lewis proposed “the theoryof the cubical atom.” He illustrated histheory with drawings of cubes withvalence electrons placed at their corners.In his classic 1916 paper, “The Atom andthe Molecule,” Lewis simplified hisdiagrams by using dots to representelectrons and a symbol to represent thekernel of an atom.

Recreate the cube models for lithium and beryllium on the board or overheadprojector. (Draw cubes with one and twocorners circled, respectively.) Explain thateach circle represents a valence electron.Then, have students refer to Figure 2 anddraw their own cube models for boron,carbon, nitrogen, oxygen, and fluorine.Logical, Visual

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Section 6.1

Print• Reading and Study Workbook With

Math Support, Section 6.1• Math Skills and Problem Solving

Workbook, Section 6.1• Transparencies, Chapter Pretest and

Section 6.1

Technology• Interactive Textbook, Section 6.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 6.1• Go Online, NSTA SciLinks, Ionic bonds

Section Resources

Chemical Bonds 159

Group1A 2A 3A 4A 5A 6A 7A 8A

Electron Dot Diagrams for Some Group A Elements

H

Li

Na

K

Be

Mg

Ca

B

Al

Ga

C

Si

Ge

N

P

As

O

S

Se

F

Cl

Br

He

Ne

Ar

Kr

Figure 2 In an electron dotdiagram, each dot represents avalence electron. Observing Howmany valence electrons do sodiumand chlorine have?

For: Links on ionic bonds

Visit: www.SciLinks.org

Web Code: ccn-1061

Ionic BondsElements that do not have complete sets of valence electrons tend toreact. By reacting, they achieve electron configurations similar tothose of noble gases. Some elements achieve stable electronconfigurations through the transfer of electrons between atoms.

Transfer of Electrons Look at the electron dot diagram forchlorine in Figure 2. A chlorine atom has one electron fewer than anargon atom. If the chlorine atom were to gain a valence electron, itwould have the same stable electron arrangement as argon. Look atthe electron dot diagram for sodium. A sodium atom has one moreelectron than a neon atom. If a sodium atom were to lose this elec-tron, its highest occupied energy level would have eight electrons. Itwould then have the same stable electron arrangement as neon.

What happens at the atomic level when sodium reacts with chlorine?An electron is transferred from each sodium atom to a chlorine atom.Each atom ends up with a more stable electron arrangement than ithad before the transfer.

Formation of Ions When an atom gains or loses an electron,the number of protons is no longer equal to the number of electrons.The charge on the atom is not balanced and the atom is not neutral. Anatom that has a net positive or negative electric charge is called an ion.The charge on an ion is represented by a plus or a minus sign. Noticethe plus sign next to the symbol for sodium and the minus sign nextto the symbol for chlorine.

Na Cl+ Na+ Cl –

Build Science SkillsPredicting Emphasize that, except forhydrogen and helium, the dots in anelectron dot diagram do not representall of the electrons in an atom, just thevalence electrons.

Have students look at Figure 2. Ask themto predict the electron dot diagrams forrubidium, strontium, indium, tin, anti-mony, tellurium, iodine, and xenon.(These elements—Rb, Sr, In, Sn, Sb, Te, I,and Xe—have the same valence electronconfigurations as the elements directlyabove them in the periodic table.)Logical, Visual

Ionic Bonds

Many students think that objectsbecome positively charged because theygain protons. Challenge this misconcep-tion by explaining that electrons are theonly subatomic particles that can betransferred from an atom during achemical change. Have students recallhow electrons can move to higherenergy levels when an atom absorbsenergy. Remind students of the locationof protons in the nuclei of atoms.Logical

FYIOnce an ion has been defined as acharged particle, the term atom can bereserved for the neutral particle.

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Chemical Bonds 159

Customize for Inclusion Students

Visually ImpairedHave interested students listen to a recording ofIonisation, a musical work scored for percussionand sirens in 1931 by French-Americancomposer Edgard Varèse (1883–1965). There isno melody or harmony, just blocks of sound

produced by instruments including cymbals,maracas, and drums. The noises are supposedto represent what happens to electrons asionization occurs. Let students state theirreactions to the music. Ask them to discuss howappropriate the title is for this piece of music.

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

Answer to . . .

Figure 2 Sodium has one valenceelectron. Chlorine has seven.

0156_hsps09te_Ch06.qxp 4/18/07 11:40 AM Page 159

160 Chapter 6

The ion that forms when a chlorine atom gains an electron has 17 protons and 18 electrons. This ion has a charge of 1– because it hasone extra electron. The symbol for the ion is written Cl1�, or Cl� forshort. An ion with a negative charge is an anion (AN eye un). Anionslike the Cl� ion are named by using part of the element name plus thesuffix –ide. Thus, Cl� is called a chloride ion.

A sodium ion has 11 protons and 10 electrons. Because it has oneextra proton, the sodium ion has a charge of 1+. The symbol for theion is written Na1�, or Na� for short. An ion with a positive charge isa cation (KAT eye un). Naming a cation is easy. You just use the ele-ment name, as in the sodium ion.

Formation of Ionic Bonds Remember that a particle with anegative charge will attract a particle with a positive charge. When ananion and a cation are close together, a chemical bond forms betweenthem. A chemical bond is the force that holds atoms or ions togetheras a unit. An ionic bond is the force that holds cations and anionstogether. An ionic bond forms when electrons are transferred fromone atom to another.

What Determines the Size ofan Atom or Ion?

Scientists use atomic radii to compare the sizes ofatoms of different elements. Remember frommathematics that the radius of a sphere is thedistance from the center of the sphere to its outeredge. The radius is half the diameter of the sphere.Because atomic radii are extremely small, thesedistances are expressed in units called picometers(pm). As a comparison, there are one billion (109)picometers in a millimeter.

The table shows the atomic radius and ionicradius for six metals and six nonmetals. You willuse the data to relate the size of an element’satoms to the element’s location on the periodictable. You also will use the data to compare thesizes of atoms and their ions.

1. Using Tables Within a period, what happensto the atomic radius as the atomic number ofthe elements increases?

2. Using Tables Within Groups 1A, 2A, 6A, and7A, what happens to the atomic radius ofelements as the atomic number increases?

3. Inferring How does adding an occupiedenergy level affect the atomic radius?(Hint: Lithium is a Period 2 element andsodium is a Period 3 element.)

4. Comparing and Contrasting Compare theatomic and ionic radii for potassium (K), andfor bromine (Br).

5. Making Generalizations What happens tothe radius of an atom when the atom loseselectrons? When the atom gains electrons?

6. Relating Cause and Effect Explain thedifference in size between a metal atom andits cation.

1A 2A 7A6A

Li152

601+ 2+

2– 1–

1+ 2+2– 1–

1+ 2+2– 1–

112

31

66

140

64

136

186

95

160

65

103

184

99

181

227

133

197

99

117

198

114

195

Be F

Atomicradius

Ionicradius

O

Na Mg ClS

K Ca BrSe

Atomic and Ionic Radii

What Determines the Size of an Atom or Ion?Answers1. Within a period, the atomic radiusdecreases as the atomic numberincreases. 2. Within these groups, the atomic radiusincreases as the atomic number increases. 3. When the next higher energy level isoccupied, there is a significant increasein the atomic radius. 4. The ionic radius for potassium is muchsmaller than its atomic radius. The ionicradius for bromine is much larger than itsatomic radius.5. With the loss of valence electrons, theradius decreases. With the addition ofvalence electrons, the radius increases. 6. An energy level that was occupied is nolonger occupied and the size decreases.

For Extra HelpExplain that attractions between protonsand electrons largely determine theatomic radius of a specific atom. Verbal

FYIBecause an electron cloud does not havean outer edge, the radius for elementsthat form diatomic molecules is calcu-lated by measuring the distance betweenthe two nuclei in the molecule anddividing by two.

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160 Chapter 6

Atomic and Ionic Radii The radius of an atom decreases from left to right across aperiod because valence electrons are shieldedfrom the nucleus by electrons in lower energylevels. The amount of positive nuclear chargeexperienced by valence electrons is mainlydetermined by the difference in chargebetween the nucleus and the inner (or core)electrons. Because the number of coreelectrons does not change across a period,

as the number of protons increases the chargeexperienced by valence electrons increasesand the atomic radius decreases.

The radius of an anion is larger than theradius of its corresponding atom. Addingelectrons to the highest occupied energy levelincreases the repulsions among electrons. Theincrease in repulsions causes the electrons tospread out more in space.

Facts and Figures

Chemical Bonds 161

Ionization Energy An electron canmove to a higher energy level when an atomabsorbs energy. Cations form when elec-trons gain enough energy to escape fromatoms. The energy allows electrons to over-come the attraction of the protons in thenucleus. The amount of energy used toremove an electron is called ionizationenergy. It varies from element to element.The lower the ionization energy, the easier itis to remove an electron from an atom.

Figure 3 shows two trends for ionization energy. Ionization energiestend to increase from left to right across a period. It takes more energyto remove an electron from a nonmetal than from a metal in the sameperiod. Ionization energies tend to decrease from the top of a group tothe bottom. In Group 1A, potassium has a lower ionization energythan sodium. So it is easier to remove an electron from potassium thanfrom sodium, and potassium is more reactive than sodium.

Ionic CompoundsCompounds that contain ionic bonds are ionic compounds, which canbe represented by chemical formulas. A chemical formula is a notationthat shows what elements a compound contains and the ratio of theatoms or ions of these elements in the compound. The chemical for-mula for sodium chloride is NaCl. From the formula, you can tell thatthere is one sodium ion for each chloride ion in sodium chloride.

Based on the diagram in Figure 4, what would the formula formagnesium chloride be? A magnesium atom cannot reach a stableelectron configuration by reacting with just one chlorine atom. It musttransfer electrons to two chlorine atoms. After the transfer, the chargeon the magnesium ion is 2� and its symbol is Mg2�. The formula forthe compound is MgCl2. The 2 written to the right and slightly belowthe symbol for chlorine is a subscript. Subscripts are used to show therelative numbers of atoms of the elements present. If there is only oneatom of an element in the formula, no subscript is needed.

What is ionization energy?

Trends in Ionization Energy

Gen

eral

ly d

ecre

ases

Generally increases

Figure 3 Ionization energiesgenerally increase from left toright across a period.Interpreting Diagrams What isthe trend for ionization energywithin a group?

Figure 4 Magnesium chlorideforms when magnesium atomstransfer electrons to chlorine atoms.Magnesium chloride is used tocontrol dust that is stirred up bytraffic on unpaved roads.

Mg MgCl

Cl

Cl –

Cl –+ 2+

FYIMemory aids may help students distin-guish cations and anions. Tell studentsthat cation means “to go down,” whichmight make them think of losing anelectron, and that anion means “to goup,” which might make them think ofgaining an electron. Students could alsothink of related terms that start with thesame letter, such as cast off for cationand accept for anion. Finally, studentscould associate the phrase A NegativeION with anion.

Use VisualsFigure 3 Emphasize the differencebetween the trend in ionization energyacross a period and the trend within agroup. Ask, According to Figure 3,which element has a greater ionizationenergy, sodium or magnesium?(Magnesium) Which element has agreater ionization energy, potassiumor cesium? (Cesium) Logical

Ionic CompoundsBuild Reading LiteracyKWL Refer to page 124D in Chapter 5,which provides the guidelines for a KWL.

Before students read Ionic Compounds,have students construct a KWL chartwith three columns entitled, What IKnow, What I Want to Know, and What I Learned. Have them fill out the final column after they have readpp. 161–164. Intrapersonal

Build Math SkillsRatios and Proportions Whenelements combine to form compounds,they do so in specific whole numberratios. For example, water, or H2O, has ahydrogen to oxygen ratio of 2:1. Askstudents to calculate the followingratios: sodium to chlorine in sodiumchloride, NaCl; magnesium to chlorinein magnesium chloride, MgCl2; andsodium to oxygen in sodium oxide,Na2O. (1:1; 1:2; 2:1) Logical

Direct students to the Math Skills in the Skills and Reference Handbookat the end of the student text foradditional help.

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Chemical Bonds 161

Ionization Energy Ionization energy variesfor each valence electron in an atom. Theseenergies are referred to as first ionizationenergy, I1, second ionization energy, I2, and soforth. An I2 is always greater than an I1 for a

given element because the second electron isbeing removed from a positively charged ion.Because the positive charge continues toincrease with each subsequent removal of anelectron, an I3 is greater than an I2.

Facts and Figures

Answer to . . .

Figure 3 It generally decreases fromtop to bottom within a group.

The amount of energyneeded to remove an

electron from an atom


162 Chapter 6

Crystal Lattices A chemical formula for an ionic compound tellsyou the ratio of the ions in the compound. But it does not tell you howthe ions are arranged in the compound. If you looked at a sample ofsodium chloride with a hand lens or microscope, you would be able tosee that the pieces of salt are shaped like cubes. This shape is a clue tohow the sodium and chloride ions are arranged in the compound.

Figure 5A shows that the ions in sodium chloride are arranged in anorderly, three-dimensional structure. Each chloride ion is surroundedby six sodium ions and each sodium ion is surrounded by six chlorideions. Each ion is attracted to all the neighboring ions with an oppositecharge. This set of attractions keeps the ions in fixed positions in a rigidframework, or lattice. The repeating pattern of ions in the lattice is likethe repeating pattern of designs on the wallpaper in Figure 6.

Solids whose particles are arranged in a lattice structure are calledcrystals. Compare the cubic shape of the sodium chloride crystals inFigure 5B to the arrangement of ions in Figure 5A. The shape of anionic crystal depends on the arrangement of ions in its lattice. In turn,the arrangement of the ions depends on the ratio of ions and their rel-ative sizes. Crystals are classified into groups based on the shape oftheir crystals. Crystals of ruby have a six-sided, hexagonal shape. The

How It Works box on page 163 describes one way to make rubies.

What shape are sodium chloride crystals?

Figure 5 The structure and shapeof a crystal are related. A In asodium chloride crystal, each ionis surrounded by six oppositelycharged ions. B Sodium chloridecrystals are shaped like cubes.

Na� Cl�

A B

Figure 6 This wallpaper displays arepeating pattern of flower and fruitdesigns. Using Analogies How is thisarrangement of designs similar to thearrangement of ions in a crystal?

162 Chapter 6

Integrate Earth ScienceGeologists use a system to classifyminerals similar to the one used bychemists to classify compounds. Askstudents to use the library or Internet tofind photographs of minerals that havecubic crystals (like sodium chloride) orhexagonal crystals (like ruby). Somestudents may also want to find examplesof tetragonal, monoclinic, triclinic, andorthorhombic crystals.Visual

Use VisualsFigure 5 Have students examine Figure 5. Ask, How are sodium ionsrepresented in the figure? (Sodium ions are represented by the smaller, orange spheres.) How are chloride ions represented in the figure?(Chloride ions are represented by thelarger, green spheres.) What do younotice about the pattern of thelocations of positive and negative ions in the diagram? (No two neigh-boring ions have the same charge.)What similarity do you notice betweenthe diagram and the photograph of sodium chloride crystals? (Thestructures in both the diagram and thephotograph have a cubic shape.)Visual

Build Science SkillsApplying Concepts Help studentsunderstand the structure of a crystallattice by encouraging them to think of three-dimensional analogies forlattices, such as scaffolding on abuilding or cups and saucers stacked on a tray in a restaurant. Visual, Logical

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Crystal Systems Crystals are classified intoseven different crystal systems, which aredescribed by geometric figures with six faces.The figures are distinguished by the angles atwhich the faces meet and by how many edgeson a face are equal in length. Cubic: three equaledges, three 90º angles; tetragonal: two equal

edges, three 90º angles; orthorhombic: no equaledges, three 90º angles; monoclinic: no equaledges, two 90º angles; triclinic: no equal edges,no 90º angles; hexagonal: two equal edges, two 90º angles, one 120º angle; rhombohedral:three equal edges, two 90º angles.

Facts and Figures

Chemical Bonds 163

Synthetic Rubies

Making synthetic rubies One way of making synthetic rubiesis called the pulled-growth method.It was invented by Polish scientistJan Czochralski (1885–1953).

Rubies are mainly aluminum oxide, which iswhite. The substitution of a small percentage ofchromium ions for aluminum ions gives rubiestheir distinctive red color. Because natural rubies are rare, rubies are often manufactured.Interpreting Diagrams What substances are in the mixture used to make rubies?

Synthetic rubyA synthetic ruby boule has ahexagonal crystal structure identicalto the natural ruby gemstone. Itsshape is determined by the arrangement of ions in the crystal.

Forming a boule As therod is lifted higher, an

oblong-shaped crystal called aboule grows from the end. Oncecooled, the boule can be cut into different shapes.

Seed crystal Aluminumoxide and chromium(VI)

oxide are melted. A tiny piece ofruby, called a seed crystal, isattached to a rod and placedabove the molten mixture (melt).

Lowering into the meltThe rod is lowered until the

seed crystal touches the melt. Therod is slowly lifted, and ions in themelt begin to attach themselvesto the seed crystal to form a ruby.

Rubyboule

Rubyforming

Crucible

Rotating rod

Heater Furnace vessel

Aluminum andchromium oxides Melt

Seed crystal

SYNTHETICRUBY BOULE

HEXAGONAL CRYSTAL

STRUCTURE

NATURAL RUBY

Synthetic RubiesAnother technique used to createsynthetic rubies was first used in 1902. It is called the Verneuil flame-fusionprocess. It differs from the more expen-sive Czochralski pulled-growth processdescribed in the text in that the mixtureof aluminum and chromium oxides isheated in open air with a flame instead of melted in a crucible.

The mineral corundum consists ofaluminum oxide mixed with chromium,iron, or other mineral impurities. Thecolor of a corundum crystal depends on the type and amount of impurities.Crystals of red corundum are calledrubies, while crystals of other colors arecalled sapphires. In addition to the fam-iliar use of synthetic rubies in jewelry,some lasers use synthetic rubies toproduce light of a specific frequency.

Interpreting Diagrams Oxides ofaluminum and chromiumVisual

For EnrichmentInterested students can make a posterpresentation for the class explainingother methods for synthesizinggemstones. Some common methodsbesides the flame-fusion and pulled-growth methods include the flux-growth method, the solution-growthprocess, and horizontal crystallization. Visual, Portfolio

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Chemical Bonds 163

Answer to . . .

Figure 6 The pattern of designelements in the wallpaper repeats theway the arrangement of ions repeats ina crystal lattice.

They are cubes.


FYIReal crystals are not perfect. Sites in alattice may be vacant, sites may beoccupied by impurities, and occupiedsites may be squeezed in between regularsites in the lattice (an interstitial defect).In an ionic compound, a cation vacancyis balanced by a nearby anion vacancy orby an interstitial cation (which maintainsan overall balance of charge). Crystaldefects are largely responsible for howcrystals fracture under stress.

Use VisualsFigure 7 Have students compare thebefore-and-after diagrams. Ask, Whenthe hammer hits the crystal, whathappens to the positions of the ions?(Ions with similar charge are pushed nearone another.) How do objects with thesame charge behave? (They repel.) Visual

ASSESSEvaluate UnderstandingHave students describe the formation ofanions, cations, and ionic bonds.

ReteachUse the diagram at the bottom of p. 161to review the formation of cations,anions, and ionic bonds.

Potassium is more reactive than calciumbecause the amount of energy needed toremove a single valence electron from apotassium atom is much smaller than theamount of energy needed to remove twovalence electrons from a calcium atom.

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 6.1.

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164 Chapter 6

Section 6.1 Assessment

Reviewing Concepts1. When is an atom least likely to react?

2. Describe one way an element can achievea stable electron configuration.

3. What characteristic of ionic bondscan be used to explain the properties ofionic compounds?

4. Use ionization energy to explain why metalslose electrons more easily than nonmetals.

5. Why is a rock salt crystal likely to shatterwhen struck?

Critical Thinking6. Making Generalizations What will the ratio

of ions be in any compound formed from aGroup 1A metal and a Group 7A nonmetal?Explain your answer.

7. Drawing Conclusions Why do ioniccompounds include at least one metal?

8. Predicting Based on their chemical formulas,which of these compounds is not likely to bean ionic compound: KBr, SO2, or FeCl3?Explain your answer.

Properties of Ionic Compounds The properties of sodiumchloride are typical of an ionic compound. It has a high melting point(801°C). In its solid state, sodium chloride is a poor conductor of elec-tric current. But when melted, it is a good conductor of electric current.Sodium chloride crystals shatter when struck with a hammer. Theproperties of an ionic compound can be explained by the strongattractions among ions within a crystal lattice.

Recall that the arrangement of particles in a substance is the resultof two opposing factors. The first factor is the attractions among par-ticles in the substance. The second factor is the kinetic energy of theparticles. The stronger the attractions among the particles, the morekinetic energy the particles must have before they can separate.

For an electric current to flow, charged particles must be able tomove from one location to another. The ions in a solid crystal latticehave fixed positions. However, when the solid melts, the lattice breaksapart and the ions are free to flow. Melted, or molten, sodium chlorideis an excellent conductor of electric current.

Rock salt contains large crystals of sodium chloride. If you tappeda crystal of rock salt sharply with a hammer, it would shatter into manysmaller crystals. Figure 7 shows what happens to the positions of theions when the crystal is struck. Negative ions are pushed into positionsnear negative ions, and positive ions are pushed into positions nearpositive ions. Ions with the same charge repel one another and causethe crystal to shatter.

Hammer strikes crystal.

Ionic crystal shatterswhen struck.

Figure 7 When an ionic crystal isstruck, ions are moved from theirfixed positions. Ions with thesame charge repel one anotherand the crystal shatters.

Reactivity of Metals Use what youknow about how ionic bonds form toexplain the difference in reactivity betweenpotassium and calcium. If necessary, reread the description of Group 1A andGroup 2A properties in Section 5.3.

164 Chapter 6


5. When ions with the same charge are pushedclose together, they repel one another.6. The ratio will be one to one because aGroup 1A metal loses one electron and aGroup 7A nonmetal gains one electron toachieve a stable electron configuration.7. One element in an ionic compound must form cations, but nonmetals tend to form anions. 8. SO2 because sulfur and oxygen are bothnonmetals and unlikely to form cations


1. When the highest occupied energy level ofan atom is filled with electrons 2. Through the transfer of electrons betweenatoms3. The strong attractions among ions within acrystal lattice4. Metals have lower ionization energies thannonmetals. The lower the ionization energy, theeasier it is to remove an electron from an atom.


6.2 Covalent Bonding

Reading StrategyRelating Text and Visuals Copy the table. As you read, look closely at Figure 9.Complete the table by describing each typeof model shown.

Key ConceptsHow are atoms heldtogether in a covalentbond?

What happens whenatoms don’t shareelectrons equally?

What factors determinewhether a moleculeis polar?

How do attractionsbetween polar moleculescompare to attractionsbetween nonpolarmolecules?

Vocabulary◆ covalent bond◆ molecule◆ polar covalent

bond

Plants absorb water through their roots from soil or from a solutioncontaining nutrients, as in Figure 8. Carbon dioxide from the air entersthe plants through small openings in their leaves. The plants use theenergy from sunlight to convert water and carbon dioxide into a sugar.Energy is stored in the chemical bonds of the sugar.

The elements in sugar are carbon, oxygen, and hydrogen. All threeare nonmetals, which have relatively high ionization energies. A trans-fer of electrons does not tend to occur between nonmetal atoms. So,how are two nonmetals able to form bonds?

Covalent BondsYou and a friend are participating in a treasure hunt. The rules statethat the first person to find all eight items on a list will win a 21-speedbicycle. After about an hour, you have found six of the items on thelist and your friend has found the other two. You and yourfriend have incomplete sets of items. But if youare willing to share your items with your friend,together you will have a complete set of itemsand qualify for the prize. Of course, you willhave to be willing to share the bicycle, too.When nonmetals join together, theydisplay a similar sharing strategy.

Model Description

Electron dot

Structural formula

Space-filling

Electron cloud

a. ?

b. ?

c. ?

d. ?

Chemical Bonds 165

Figure 8 When plants are grownin water instead of soil, you cansee their roots. Plants absorbwater through their roots andcarbon dioxide through smallopenings in their leaves.

FOCUS

Objectives6.2.1 Describe how covalent bonds

form and the attractions thatkeep atoms together inmolecules.

6.2.2 Compare polar and nonpolarbonds, and demonstrate howpolar bonds affect the polarityof a molecule.

6.2.3 Compare the attractionsbetween polar and nonpolarmolecules.

Build VocabularyConcept Map Have students constructa concept map using the terms atoms,molecules, ions, covalent bonds, ionicbonds, polar, nonpolar, and electrons.Instruct students to place the terms inovals and connect the ovals with lineson which linking words are placed.Students should place the main concept(Chemical Bonding) at the top or thecenter. As the distance from the mainconcept increases, the content shouldbecome more specific.

Reading Strategya. Dots represent valence electrons.b. A line represents a pair of sharedvalence electrons. c. Three-dimensionalspheres represent atoms. d. Electronclouds represent atoms.

INSTRUCT

Covalent Bonds

Many students do not differentiateamong atoms, ions, and molecules in their sketches of particle models.Challenge this misconception by asking students to make drawings torepresent an atom, a molecule, and an ion. Students should draw a singlesphere for an atom, at least two spheresjoined in some way for a molecule, andone sphere with either a plus or a minuscharge for an ion. Visual

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Chemical Bonds 165

Print• Laboratory Manual, Investigation 6B• Reading and Study Workbook With

Math Support, Section 6.2 • Transparencies, Section 6.2

Technology• Interactive Textbook, Section 6.2• Presentation Pro CD-ROM, Section 6.2• Go Online, NSTA SciLinks, Covalent

bonding

Section Resources

Section 6.2

PPLS


166 Chapter 6

Sharing Electrons A hydrogen atom has one electron. If it hadtwo electrons, it would have the same electron configuration as ahelium atom. Two hydrogen atoms can achieve a stable electron con-figuration by sharing their electrons and forming a covalent bond. Acovalent bond is a chemical bond in which two atoms share a pair ofvalence electrons. When two atoms share one pair of electrons, thebond is called a single bond.

Figure 9 shows four different ways to represent a covalent bond.In the electron dot model, the bond is shown by a pair of dots in thespace between the symbols for the hydrogen atoms. In the structuralformula, the pair of dots is replaced by a line. The electron cloud modeland the space-filling model show that orbitals of atoms overlap whena covalent bond forms.

Molecules of Elements Two hydrogen atoms bonded togetherform a unit called a molecule. A molecule is a neutral group of atomsthat are joined together by one or more covalent bonds. The hydrogenmolecule is neutral because it contains two protons (one from eachatom) and two electrons (one from each atom). What keeps the hydro-gen atoms together in the molecule? The attractions between theshared electrons and the protons in each nucleus hold the atomstogether in a covalent bond.

A chemical formula can be used to describe the molecules of anelement as well as a compound. The element hydrogen has the chem-ical formula H2. The subscript 2 indicates that there are two atoms ina molecule of hydrogen.

Many nonmetal elements exist as diatomic molecules. Diatomicmeans “two atoms.” Four of the models in Figure 10 are of halogens. Ahalogen atom has seven valence electrons. If two halogen atoms share avalence electron from each atom, both atoms have eight valence electrons.

H H H H

Electron dot diagram Structural formula

Space-filling model Electron cloud model

H H

Molecular Models

Fluorine (F2)

Nitrogen (N2)

Chlorine (Cl2)

Bromine (Br2)

Iodine (I2)

Figure 9 As a space shuttle liftsoff, it leaves a water vapor trail. Areaction of hydrogen and oxygenproduces the water. Using Models How is the bondbetween hydrogen atomsrepresented in each model of ahydrogen molecule?

Figure 10 These space-fillingmodels represent diatomicmolecules of five elements. Using Models How many atomsare in a diatomic molecule?

166 Chapter 6

Use VisualsFigure 10 Have students examineFigure 10. Ask, Why are the atoms inthe models of diatomic molecules notcomplete spheres? (The space-fillingmodels show that orbitals of atomsoverlap when they form covalent bonds.)Have students compare the space-fillingmodels in Figure 10 to the data onatomic radii in the Data Analysis on p. 160. Ask, Why are the spheres inthe models of fluorine, chlorine, and bromine different sizes? (Thedifferent sizes of spheres model thedifferent atomic radii of the atoms.)Logical

Build Reading LiteracyVisualize Refer to page 354D inChapter 12, which provides theguidelines for visualizing.

After students have read about ionic and covalent bonds and the differencebetween polar and nonpolar bonds,encourage students to draw diagramsthat demonstrate the differences betweenthree types of bonding: nonpolarcovalent, polar covalent, and ionic. Visual, Portfolio

FYINot all nonmetal elements exist asmolecules. Of those that do, not all form diatomic molecules. Molecules of crystalline sulfur contain eight sulfur atoms, S8. Molecules of whitephosphorus contain four phosphorusatoms, P4.

Based on the octet rule, a molecule ofozone, O3, should contain one doublebond and one single coordinatecovalent bond (a bond in which one of the atoms donates an unshared pairof electrons). However, the measuredbond lengths for the two bonds in anozone molecule are identical. The bondsare hybrids—not strictly single bonds ordouble bonds.

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Customize for English Language Learners

Think-Pair-Share Have students work in pairs to think ofsituations that serve as analogies for ionic andcovalent bonding. For example, if a jewelerlends an expensive piece of jewelry to apresenter at an awards show, a guard willaccompany the presenter and stay as close as

possible. This is similar to an ionic bond, inwhich a cation stays close to an anion to whichit donates an electron. By contrast, studentsreading from one copy of the same book arelike two atoms sharing a pair of electrons in acovalent bond. Strengthen discussion skills byhaving students share analogies with the class.

Chemical Bonds 167

Multiple Covalent Bonds Nitrogen has five valence elec-trons. If two nitrogen atoms shared a pair of electrons, each one wouldhave only six valence electrons. If they shared two pairs of electrons,each atom would have only seven valence electrons. When the atomsin a nitrogen molecule (N2) share three pairs of electrons, each atomhas eight valence electrons. Each pair of shared electrons is representedby a long dash in the structural formula NKN. When two atoms sharethree pairs of electrons, the bond is called a triple bond. When twoatoms share two pairs of electrons, the bond is called a double bond.

Unequal Sharing of ElectronsIn general, elements on the right of the periodic table have a greaterattraction for electrons than elements on the left have (except for noblegases). In general, elements at the top of a group have a greater attrac-tion for electrons than elements at the bottom of a group have.Fluorine is on the far right and is at the top of its group. It has thestrongest attraction for electrons and is the most reactive nonmetal.

What does the subscript 2 in the formula for ahydrogen molecule indicate?

Analyzing Inks

Materialstest paper, metric ruler, felt-tip markers, stapler,beaker, alcohol-water mixture, Petri dish

Procedure1. Place the test paper on a clean surface. Use

the ruler to draw the pencil line shown in thedrawing. Use your markers to place color dotsat the locations shown in the drawing.

2. With the ink marks on the outside, staple thetwo ends of the paper together to form a tube.

3. Pour the alcohol-water mixture into the beakerto a depth of 0.5 cm. Stand the paper in thebeaker so that the dots are at the bottom. Thepaper should not touch the sides of the beaker.Invert the Petri dish over the beaker.

4. When the mixture reaches the top of the paper,remove the paper from the beaker. Unstaplethe paper and lay it flat. Make a drawing of theresults with each colored area labeled.

Analyze and Conclude1. Observing Which markers contained inks

that were mixtures of colored substances?

2. Formulating Hypotheses How did somemolecules in the ink move up the paper?

3. Predicting Assume that molecules in the testpaper are more polar than molecules in thealcohol-water mixture. Would you expect themost polar molecules in ink to stick tightly tothe paper or to move with the liquid? Explain.

4. Designing Experiments How could theprocedure from this lab be used to identify ablack ink whose composition is unknown?

For: Links on covalentbonding


Web Code: ccn-1062

2 cm 5 cm 8 cm 11 cm 1.5 cm

Pencil line

Unequal Sharing of Electrons

Analyzing Inks

ObjectiveAfter completing this activity, studentswill be able to• explain how differences in polarity can

be used to separate the componentsof a solution.

Skills Focus Observing, Inferring

Prep Time 10 minutes

Advance Prep Provide each groupwith an 8-cm � 16-cm piece of test(chromatography) paper. The alcohol-water mixture is 70% isopropyl alcoholin water.

Class Time 20 minutes

Safety Provide only markers containingwater-based inks or inks that are low involatile organic compounds (VOCs).Make sure that there are no flames inthe laboratory when alcohol is in use.

Teaching Tips• Have students do this lab after they

study polar molecules and attractionsbetween molecules.

• Tell students not to touch the testpaper with their bare hands becauseoil naturally present on their fingerswill interfere with the results. Theyshould wear plastic gloves.

Expected Outcome Most inks willcontain more than one pigment.

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Chemical Bonds 167

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

Answer to . . .

Figure 9 A pair of dots, a line,overlapping spheres, and overlappingelectron clouds

Figure 10 Two

The subscript 2 showsthat there are two

atoms in a hydrogen molecule.

Analyze and Conclude1. In general, darker inks (such as brown orblack) contain two or more red, blue, and yellow pigments.2. Molecules that dissolved easily in the alcohol-water mixture moved up the paper with the mixture.

3. The most polar molecules would be foundnear the bottom of the paper because they aremore strongly attracted to the paper than to thealcohol-water mixture. 4. If an unknown black ink produces a colorpattern similar to one of the known inks, they are likely to be identical. Visual, Logical


168 Chapter 6

Polar Covalent Bonds In a molecule of an element, the atomsthat form covalent bonds have the same ability to attract an electron.Shared electrons are attracted equally to the nuclei of both atoms. Ina molecule of a compound, electrons may not be shared equally.

Figure 11 shows models of the molecule that forms when hydro-gen reacts with chlorine. A chlorine atom has a greater attraction forelectrons than a hydrogen atom does. In a hydrogen chloride molecule,the shared electrons spend more time near the chlorine atom than nearthe hydrogen atom. A covalent bond in which electrons are not sharedequally is called a polar covalent bond. (One meaning of the termpolar is “opposite in character, nature, or direction.”)

When atoms form a polar covalent bond, the atom with thegreater attraction for electrons has a partial negative charge. Theother atom has a partial positive charge. The symbols �– and �+ areused to show which atom has which charge. (� is the lowercase ver-sion of the Greek letter delta.)

Polar and Nonpolar Molecules Can you assume that a mol-ecule that contains a polar covalent bond is polar? If a molecule hasonly two atoms, it will be polar. But, when molecules have more thantwo atoms, the answer is not as obvious. The type of atoms in amolecule and its shape are factors that determine whether a mol-ecule is polar or nonpolar.

Compare the models of carbon dioxide and water in Figure 12. Incarbon dioxide, there are double bonds between each oxygen atom andthe central carbon atom. Because oxygen has a greater attraction forelectrons than carbon does, each double bond is polar. However, themolecule is linear: all three atoms are lined up in a row. The carbon-oxygen double bonds are directly opposite each other. There is an equalpull on the electrons from opposite directions. The pulls cancel outand the molecule as a whole is nonpolar.

There are two single bonds in a water molecule. The bonds arepolar because oxygen has a greater attraction for electrons than hydro-gen does. Because the water molecule has a bent shape rather than alinear shape, the polar bonds do not cancel out. The two hydrogenatoms are located on the same side of the molecule, opposite theoxygen atom. The oxygen side of the molecule has a partial negativecharge. The hydrogen side of the molecule has a partial positive charge.

HCl

CO2

H2O

Figure 11 Shared electrons in a hydrogenchloride molecule spend less time near thehydrogen atom than near the chlorine atom. Inferring Which element has a greaterattraction for electrons—hydrogen orchlorine?

Figure 12 In a carbon dioxide(CO2) molecule, the polar bondsbetween the carbon atom andthe oxygen atoms cancel outbecause the molecule is linear. Ina water (H2O) molecule, the polarbonds between the oxygen atomand the hydrogen atoms do notcancel out because the moleculeis bent.

168 Chapter 6

Modeling Overall PolarityPurpose Students examine a model for molecular polarity.

Materials molecular model kit, 4 12-inch pieces of string or yarn, tape,overhead projector

Procedure Show students ball-and-stick models of a carbon dioxidemolecule and a water molecule. Havethem compare the linear shape of CO2with the bent shape of H2O. Tie or tapeone piece of string onto each of theoxygen atoms of the CO2 model andonto each of the hydrogen atoms of theH2O model. Place the CO2 model onthe overhead and demonstrate theeffect of pulling the strings gently inopposite directions. Explain that thisrepresents the canceling effect ofopposing polar bonds (dipoles). Placethe H2O model on the overhead anddemonstrate the effect of pulling thestrings gently away from the oxygenatom in the direction of the bondangles. Explain that this represents theadditive effect of polar bonds that are at an angle.

Expected Outcome The CO2 modelwill stay in one place. The H2O modelwill move in the direction of thehydrogen atoms. Visual

Build Science SkillsUsing Models Show students a ball-and-stick molecular model of ammonia,NH3. Ask students to predict whetherthe molecule is polar or nonpolar and togive a reason for their choice. (Ammoniais a polar molecule because its three bondsare oriented to one side of the centralnitrogen atom. The polar bonds do notcancel out.) Now show students amolecular model of sulfur trioxide, SO3.Ask students to predict whether thismolecule is polar or nonpolar and toexplain their reasoning. (Sulfur trioxide is a nonpolar molecule because its threebonds are oriented symmetrically in aplane around the central sulfur atom. The polar bonds cancel each other out.)Logical, Visual

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Surface Tension Refer back to the discussionin Section 3.3 about the effect of intermolecularattractions on the evaporation of water.Hydrogen bonds explain other properties ofwater, including its high surface tension.

Because molecules on the surface are drawninward by attractions from molecules beneaththe surface, surface area is reduced. Surfacetension explains why water drops bead up on a clean, waxed surface.

Facts and Figures


Reviewing Concepts1. What attractions hold atoms together in

a covalent bond?

2. What happens to the charge on atomswhen they form a polar covalent bond?

3. Name the two factors that determinewhether a molecule is polar.

4. Compare the strength of attractionsbetween polar molecules to the strength ofattractions between nonpolar molecules.

5. What is a molecule?

Critical Thinking6. Applying Concepts Which of these

elements does not bond to form molecules:oxygen, chlorine, neon, or sulfur?

7. Inferring Why is the boiling point of waterhigher than the boiling point of chlorine?

8. Using Diagrams Based on their electrondot diagrams, what is the formula for thecovalently bonded compound of nitrogenand hydrogen?

Attraction Between MoleculesIn a molecular compound, there are forces of attraction between mol-ecules. These attractions are not as strong as ionic or covalent bonds,but they are strong enough to hold molecules together in a liquid orsolid. Attractions between polar molecules are stronger thanattractions between nonpolar molecules.

Water molecules are similar in mass to methane (CH4) molecules.Yet, methane boils at �161.5°C and water boils at 100°C becausemethane molecules are nonpolar and water molecules are polar. Eachdashed line in Figure 13 represents an attraction between a partiallypositive hydrogen atom in one water molecule and a partially negativeoxygen atom in another. Molecules on the surface of a water sample areattracted to molecules that lie below the surface and are pulled towardthe center of the sample. These attractions increase the energy requiredfor water molecules to evaporate. They raise the temperature at whichvapor pressure equals atmospheric pressure—the boiling point.

Attractions among nonpolar molecules are weaker than attractionsamong polar molecules, but they do exist. After all, carbon dioxide canexist as solid dry ice. Attractions among nonpolar molecules explainwhy nitrogen can be stored as a liquid at low temperatures and highpressures. Because electrons are constantly in motion, there are timeswhen one part of a nitrogen molecule has a small positive charge andone part has a small negative charge. At those times, one nitrogen mol-ecule can be weakly attracted to another nitrogen molecule.

Chemical Bonds 169

2 -++

δδδ

Figure 13 Each dashed line inthe drawing represents anattraction between a hydrogenatom and an oxygen atom. Interpreting Diagrams In awater molecule, which atom hasa partial negative charge? Whichhas a partial positive charge?

Viscosity Review the description of thephysical property viscosity in Section 2.2.Then write a paragraph explaining howattractions between molecules might affectthe viscosity of a liquid.

Attraction BetweenMolecules

Surface TensionPurpose Students observe how surfacetension can support a needle.

Materials 200-mL beaker, water,sewing needle, tweezers, dropper pipet

Procedure Fill the beaker with water.Using tweezers, gently place the needleon the water’s surface so that surfacetension supports it. Remove the needle,and place it in the water vertically sothat it sinks. Pour the water out andcollect the needle.

Expected Outcome Surface tensionsupports the needle. Logical, Visual

ASSESSEvaluate UnderstandingHave students make a chart that com-pares and contrasts polar covalent bondsand nonpolar covalent bonds. Be surestudents discuss the bonding atoms’attractions for electrons, partial charges,and attractions between molecules.

ReteachUse Figures 11, 12, and 13 as visual aidswhile reviewing polar covalent bonds,nonpolar and polar molecules, andattractions between molecules.

A greater attraction between molecules islikely to produce an increase in viscositybecause the attractions would act inopposition to the motion of the moleculesand reduce their ability to flow.


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Chemical Bonds 169

4. Attractions between polar molecules arestronger than attractions between nonpolarmolecules.5. A neutral group of atoms that are joinedtogether by one or more covalent bonds6. Neon7. Attractions between polar water moleculesare stronger than attractions betweennonpolar chlorine molecules.8. NH3


1. The attractions between the sharedelectrons and the protons in each nucleushold the atoms together in a covalent bond. 2. When atoms form a polar covalent bond,the atom with the greater attraction forelectrons has a partial negative charge. Theother atom has a partial positive charge. 3. The type of atoms in a molecule and itsshape are factors that determine whether amolecule is polar or nonpolar.

Answer to . . .

Figure 11 Chlorine

Figure 13 The oxygen atom has apartial negative charge. The hydrogenatoms have partial positive charges.


6.3 Naming Compounds and Writing Formulas

Reading StrategyPredicting Copy the table. Before you read,predict the meaning of the term polyatomicion. After you read, if your prediction wasincorrect, revise your definition.

Key ConceptsWhat information do thename and formula of anionic compound provide?

What information do thename and formula of amolecular compoundprovide?

Vocabulary◆ polyatomic ion

Polyatomicion

VocabularyTerm

Before YouRead

After YouRead

b. ?a. ?

Figure 14 These flowerpots were coatedwith a solution of lime and water. Paintpigments were mixed with the lime washto produce the different colors. Thechemical name for lime is calcium oxide.

Thomas Drummond was a Scottish surveyor and inventor. Around1826, he discovered that a white solid called lime emits a bright lightwhen heated to a high temperature. This discovery was extremely usefulin the era before electric lighting. Limelight was used to produce a lightthat could be focused on a single spot on a stage. It also was used toproduce lighthouse beams that could be seen from a great distance.

People have used mixtures of lime and water for centuries to white-wash houses and fences. The flowerpots in Figure 14 were coated witha lime wash to which paint pigments were added. Other names for limeare quicklime and unslaked lime. Having two or more names for acompound can be confusing. Also, names like lime or quicklime don’ttell you much about the composition of a compound.

Chemists use a system for naming compounds that is based on rulesestablished by the International Union of Pure and Applied

Chemistry (IUPAC). In this system, the chemical name for limeis calcium oxide and its chemical formula is CaO. This for-

mula tells you that there is a one-to-one ratio of calciumions to oxide ions in calcium oxide. The formula of a

compound serves as a reminder of the compositionof the compound.

170 Chapter 6

170 Chapter 6

FOCUS

Objectives6.3.1 Recognize and describe

binary ionic compounds,metals with multiple ions, and polyatomic ions.

6.3.2 Name and determinechemical formulas for ionic and molecular compounds.

Build VocabularyWord-Part Analysis Ask studentswhat words they know that have theprefix poly-. (Polygon, polysyllabic,polyglot, polytechnic, and polygraph) Give a definition of the prefix. (Poly-means “many.”) Have students predictthe meaning of the term polyatomic ion.(A polyatomic ion is a covalently bondedgroup of atoms that has a positive ornegative charge and acts as a unit.)

Reading Strategya. and b. Students should assume thatany particle described as an ion has acharge. If they know the meaning ofpoly-, they may conclude that the ioncontains three or more atoms.

INSTRUCTIntegrate Language ArtsThe production of lime through thedecomposition of limestone (or anyform of calcium carbonate) has beenknown for millennia, which is why thereis a word for lime in many ancientlanguages. In Latin, this word is calx,which is the source for the namecalcium. Have students research theorigin of the phrase “in the limelight.”(Drummond developed limelight first as an aid to surveying. When lime washeated in a hydrogen-oxygen flame, itproduced a bright, white light. In 1825, a light that Drummond placed on top of a hill in Belfast could be seen in Donegalabout 105 km (66 miles) away. Limelightwas first used in a theater in 1856, whena lens was placed in front of the limelightto produce a spotlight.)Logical

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Reading Focus

1

Section 6.3

Print• Laboratory Manual, Investigation 6A• Reading and Study Workbook With

Math Support, Section 6.3 and Math Skill: Writing Formulas for IonicCompounds

• Math Skills and Problem SolvingWorkbook, Section 6.3

• Transparencies, Section 6.3

Technology• Interactive Textbook, Section 6.3• Presentation Pro CD-ROM, Section 6.3• Go Online, NSTA SciLinks, Chemical

formulas

Section Resources

Describing Ionic CompoundsBoth of the objects in Figure 15 are coated with compounds of copperand oxygen. Based on the two colors of the coatings, copper andoxygen must form at least two compounds. One name cannot describeall the compounds of copper and oxygen. There must be at least twonames to distinguish red copper oxide from black copper oxide.

The name of an ionic compound must distinguish the com-pound from other ionic compounds containing the same elements.The formula of an ionic compound describes the ratio of the ions inthe compound.

Binary Ionic Compounds A compound made from only twoelements is a binary compound. (The Latin prefix bi- means “two,” as inbicycle or bisect.) Naming binary ionic compounds, such assodium chloride and cadmium iodide, is easy. The nameshave a predictable pattern: the name of the cation followed bythe name of the anion. Remember that the name for thecation is the name of the metal without any change: sodiumatom and sodium ion. The name for the anion uses part ofthe name of the nonmetal with the suffix –ide: iodine atomand iodide ion. Figure 16 shows the names and charges foreight common anions.

Figure 15 The brass vase onthe left is coated with an oxideof copper that is red. Most ofthe surface of the plate on theright is coated with an oxide ofcopper that is black.Classifying How can you besure that the oxides of copperare different compounds?

ElementName

Fluorine

IonName

Fluoride

IonSymbol

Chlorine Chloride

IonCharge

1�

1�

Common Anions

Bromine Bromide 1�

Iodine Iodide 1�

Oxygen Oxide 2�

Sulfur Sulfide 2�

Nitrogen Nitride 3�

Phosphorus Phosphide

F�

Cl�

Br�

I�

O2�

S2�

N3�

P3� 3�

Figure 16 The table lists the element names, ionnames, symbols, and charges for eight anions. Thename of an anion is formed by adding the suffix–ide to the stem of the name of the nonmetal.

Chemical Bonds 171

Describing IonicCompounds

Some students may think that a material’sparticles possess the same properties asthe material. For example, they may thinkthat the atoms that compose the copperoxides in Figure 15 are red or black.Challenge this misconception by notingthat the black and red copper oxides bothcontain the same two elements—copperand oxygen—yet they are different colors.Logical

FYIThe copper oxides differ in more than color. Copper(I) oxide melts at1235°C and has a density of 6.0 g/cm3.Copper(II) oxide melts at 1446°C andhas a density of 6.31 g/cm3.

Build Reading LiteracyOutline Refer to page 156D in thischapter, which provides the guidelinesfor an outline.

Have students read the text on pp. 171–175 related to describing ionic and molecular compounds. Then, have students use the headings as major divisions in an outline. Havestudents refer to their outlines whenanswering the questions in the Section 6.3 Assessment.Visual

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Chemical Bonds 171

Customize for Inclusion Students

Behaviorally Disordered Have students work in groups and use indexcards to create a classroom set of flashcards.Students can use the cards to support eachother in small, noncompetitive study groups.Each card should contain information about a metal or nonmetal element that forms ions.One side of the card should list the name of an

element (for example, chlorine), the formulafor an ionic compound that contains theelement (for example, NaCl), and four cate-gories of color-keyed questions: Group inperiodic table; Name of ion; Charge on ion;Other ion in compound. Color key the answerson the other side of the card. (Halogens or 7A;Chloride; 1�; Sodium)

Answer to . . .

Figure 15 They must be differentcompounds because their colors vary,and the properties of a compoundshould be consistent.


172 Chapter 6

Ion Name

Copper(I)

Ion Symbol Ion Name

Chromium(II)

Copper(II) Chromium(III)

Ion Symbol

Some Metal Cations

Iron(II) Titanium(II)

Iron(III) Titanium(III)

Lead(II) Titanium(IV)

Lead(IV)

Cu�

Cu2�

Fe2�

Fe3�

Pb2�

Pb4� Mercury(II)

Cr2�

Cr3�

Ti2�

Ti3�

Ti4�

Hg2�

Figure 17 Many paint pigmentscontain compounds of transitionmetals. These metals often formmore than one type of ion. Theion names must contain a Romannumeral. Using Tables How isthe Roman numeral in the namerelated to the charge on the ion?

Ammonium ion(NH4

+)

NH

H +

H H

+

Metals With Multiple Ions The alkali metals, alkaline earthmetals, and aluminum form ions with positive charges equal to thegroup number. For example, the symbol for a potassium ion is K�,the symbol for a calcium ion is Ca2�, and the symbol for an aluminumion is Al3�.

Many transition metals form more than one type of ion. Notice thetwo copper ions listed in Figure 17, a copper(I) ion with a 1� chargeand a copper(II) ion with a 2� charge. When a metal forms more thanone ion, the name of the ion contains a Roman numeral to indicatethe charge on the ion. These ion names can distinguish red copper(I)oxide from black copper(II) oxide. The formula for “copper one oxide”is Cu2O because it takes two Cu1� ions to balance the charge on anO2� ion. The formula for “copper two oxide” is CuO because it takesonly one Cu2� ion to balance the charge on an O2� ion.

Polyatomic Ions The electron dot diagram in Figure 18 describesa group of atoms that includes one nitrogen and four hydrogen atoms.It is called an ammonium ion. The atoms are joined by covalent bonds.Why does the group have a positive charge? The nitrogen atom hasseven protons, and each hydrogen atom has one proton—eleven intotal. But the group has only ten electrons to balance the charge on theprotons—eight valence electrons and nitrogen’s two inner electrons.

A covalently bonded group of atoms that has a positive or negativecharge and acts as a unit is a polyatomic ion. The prefix poly- means“many.” Most simple polyatomic ions are anions. Figure 19 lists thenames and formulas for some polyatomic ions. Sometimes there areparentheses in a formula that includes polyatomic ions. For example,the formula for iron(III) hydroxide is Fe(OH)3. The subscript 3 indi-cates that there are three hydroxide ions for each iron(III) ion.

When are Roman numerals used incompound names?

Figure 18 The atoms in anammonium ion are joined bycovalent bonds. The ion loses avalence electron as it forms. Thisloss leaves only 10 electrons tobalance the charge on 11 protons.

172 Chapter 6

Use VisualsFigure 18 Have students compare thetwo models of an ammonium ion. Havethem discuss the advantages of eachtype of model. (Both models show thenumber and types of atom in the ion. Theelectron dot diagram shows the valenceelectrons. The space-filling model showsthe relative sizes of the atoms and howthey are arranged in space.) Point outthat the brackets in the electron dotdiagram indicate that the group ofatoms as a whole, not any specific atom,has a positive charge. Ask, What is thecharge on an ammonium ion? (1�)How many covalent bonds are in anammonium ion? (Four) How manyvalence electrons are involved in thebonds in the ammonium ion? (Eight)Visual

FYIPaint is a mixture of nonvolatileingredients (the pigment and thebinder) that are dispersed in a volatileliquid. A pigment is a substance thatprovides the color.

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Chemical Bonds 173

Name Formula FormulaName

Some Polyatomic Ions

Ammonium

Hydroxide

Nitrate

Sulfate

Carbonate

Phosphate

Chromate

Silicate

Acetate

Peroxide

Permanganate

Hydrogen sulfate

Hydrogen carbonate

Hydrogen phosphate

Dichromate

Hypochlorite

C2H3O2�

O22�

MnO4�

HSO4�

HCO3�

HPO42�

Cr2O72�

OCl�

NH4�

OH�

NO3�

SO42�

CO32�

PO43�

CrO42�

SiO32�

Figure 19 This table lists thenames and formulas of somepolyatomic ions. Except for theammonium ion, all the ions listedare anions. Using Tables Whichelement is found in all the anionswhose names end in -ate?

Modeling Molecules

Materialsblue plastic-foam ball, black plastic-foam ball, 7 white gumdrops, toothpicks

Procedure1. To make a model of an ammonia molecule

(NH3), insert a toothpick in each of 3 gum-drops. The gumdrops represent hydrogenatoms and the toothpicks represent bonds.

2. An ammonia molecule is like a pyramid withthe nitrogen at the top and the hydrogenatoms at the corners of the base. Insert thetoothpicks in the blue foam ball (nitrogen) sothat each gumdrop is the same distance fromthe ball.

3. The hydrogen atoms in a methane molecule(CH4) are equally spaced around the carbon.Use the black ball to make a model of methane.

Analyze and Conclude1. Comparing and Contrasting Compare the

shapes of the methane and ammonia molecules.

2. Using Models Why is carbon in the center ofthe methane molecule?

Writing Formulas for Ionic Compounds If you knowthe name of an ionic compound, you can write its formula. Place thesymbol of the cation first, followed by the symbol of the anion. Usesubscripts to show the ratio of the ions in the compound. Because allcompounds are neutral, the total charges on the cations and anionsmust add up to zero.

Suppose an atom that gains two electrons, such as sulfur, reactswith an atom that loses one electron, such as sodium. There must betwo sodium ions (Na�) for each sulfide ion (S2�). The formula forsodium sulfide is Na2S. The 2� charge on one sulfide ion is balancedby the 1� charges on two sodium ions.

Modeling Molecules

Objective After completing this activity, studentswill be able to• use physical models to compare the

shapes of molecules.

Skills Focus Using Models

Prep Time 10 minutes

Advance Prep Plastic-foam balls areavailable from craft supply stores. If blackand blue foam balls are not available,wrap other foam balls in colored foil, gluecolored tissue paper around them, orspray-paint them to match the colorsused in this book to represent atoms ofcarbon and nitrogen.

Class Time 20 minutes

Safety Tell students not to eat the gum-drops or anything else in the laboratory.

Expected Outcome Students willmake tetrahedral (pyramidal) models ofmethane molecules.

Analyze and Conclude1. The methane molecule is a tetrahe-dron (equal-sided triangular pyramid)and the ammonia molecule has atriangular shape.2. The carbon atom is in the centerbecause each of the four hydrogenatoms must bond to one carbon atom.Visual, Kinesthetic

Build Science SkillsInferring Molecules of NH3 and NF3have the same shape, but bonds in NH3are shorter than bonds in NF3. In otherwords, the hydrogen atoms are closer to the nitrogen atom in an ammoniamolecule than the fluorine atoms are to the nitrogen atom in nitrogentrifluoride. Ask, What does this dataimply about the relative sizes ofhydrogen and fluorine atoms? (Thedata confirms that a hydrogen atom has a smaller atomic radius than does a nitrogen atom.) Logical

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Chemical Bonds 173

Answer to . . .

Figure 17 The Roman numeral isequal to the charge on the ion.

Figure 19 Oxygen

When the metal in thecompound can form

more than one type of ion


174 Chapter 6

Describing Molecular CompoundsLike ionic compounds, molecular compounds have names that identifyspecific compounds, and formulas that match those names. Withmolecular compounds, the focus is on the composition of molecules.

The name and formula of a molecular compound describe thetype and number of atoms in a molecule of the compound.

Naming Molecular Compounds The general rule is that themost metallic element appears first in the name. These elements arefarther to the left in the periodic table. If both elements are in the samegroup, the more metallic element is closer to the bottom of the group.The name of the second element is changed to end in the suffix -ide,as in carbon dioxide.

Writing Formulas for Ionic CompoundsWhat is the formula for the ionic compound calcium chloride?

Read and UnderstandWhat information are you given?

The name of the compound is calcium chloride.

Plan and SolveList the symbols and charges for the cation and anion.

Ca with a charge of 2� and Cl with acharge of 1�

Determine the ratio of ions in the compound.

It takes two 1� charges to balance the 2� charge. There will be two chloride ionsfor each calcium ion.

Write the formula for calcium chloride.

CaCl2

Look Back and CheckIs your answer reasonable?

Each calcium atom loses two electrons and each chlorineatom gains one electron. So there should be a 1-to-2ratio of calcium ions to chloride ions.

1. Write the formula for thecompound calcium oxide.

2. Write the formula for thecompound copper(I) sulfide.

3. Write the formula for thecompound sodium sulfate.

4. What is the name of thecompound whose formula is NaOH?

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Web Code: ccn-1063

174 Chapter 6

Build Math SkillsPositive and Negative NumbersRemind students that the charges in anionic compound must cancel each other.Ask, How many atoms of a halogenwould combine with one atom of analkaline earth metal? Why? (Two atoms;the alkaline earth metal atom loses twoelectrons but each halogen atom needs onlyone electron to become stable.) Logical

Direct students to the Math Skills in the Skills and Reference Handbookat the end of the student text foradditional help.

Solutions1. It takes one calcium ion with a chargeof 2� to balance one oxide ion with acharge of 2�. The formula is CaO.2. Two copper(I) ions, each with a chargeof 1�, balance one sulfide ion with acharge of 2�. The formula is Cu2S.3. Two sodium ions, each with a chargeof 1�, balance one sulfate ion with acharge of 2�. The formula is Na2SO4.4. In the formula, Na represents thesodium ion and OH represents thehydroxide ion. The name of thecompound is sodium hydroxide. Logical

For Extra HelpMake sure the first step students take isto find the symbols and charges on theions. Then, check that they are able tobalance the charges.Logical

Additional Problems1. Write the formula for lithium oxide.(Li2O)2. Write the formula for iron(III) oxide.(Fe2O3)Logical

Describing MolecularCompoundsFYIThere are two exceptions to the generalrule for naming molecular compounds.Hydrogen is treated as though it werepositioned between Group 5A andGroup 6A, and oxygen is treated asthough it were positioned after chlorinebut before fluorine.

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

Chemical Bonds 175


Reviewing Concepts1. What does the formula of an ionic

compound describe?

2. What do the name and formula of amolecular compound describe?

3. What suffix is used to indicate an anion?

4. Why are Roman numerals used in the namesof compounds that contain transition metals?

5. What is a polyatomic ion?

Critical Thinking6. Applying Concepts How is it possible

for two different ionic compounds to containthe same elements?

7. Calculating How many potassium ions areneeded to bond with a phosphate ion?

Figure 20 These Greek prefixesare used to name molecularcompounds. The prefix octa-means “eight,” as in the eighttentacles of an octopus.

8. What are the names of these ioniccompounds: LiCl, BaO, Na3N, and PbSO4?

9. Name the molecular compounds withthese formulas: P2O5 and CO.

10. What is the formula for the ioniccompound formed from potassiumand sulfur?

Two compounds that contain nitrogen and oxygen have the for-mulas N2O4 and NO2. The names of these two compounds reflect theactual number of atoms of nitrogen and oxygen in a molecule of eachcompound. You can use the Greek prefixes in Figure 20 to describe thenumber of nitrogen and oxygen atoms in each molecule.

In an N2O4 molecule, there are two nitrogen atoms and fouroxygen atoms. The Greek prefixes for two and four are di- and tetra-.The name for the compound with the formula N2O4 is dinitrogentetraoxide. In an NO2 molecule, there are one nitrogen atom and twooxygen atoms. The Greek prefixes for one and two are mono- and di-.So a name for the compound with the formula NO2 is mononitrogendioxide. However, the prefix mono- often is not used for the first ele-ment in the name. A more common name for the compound with theformula NO2 is nitrogen dioxide.

Writing Molecular Formulas Writing the formula for amolecular compound is easy. Write the symbols for the elements inthe order the elements appear in the name. The prefixes indicate thenumber of atoms of each element in the molecule. The prefixes appearas subscripts in the formulas. If there is no prefix for an element in thename, there is only one atom of that element in the molecule.

What is the formula for diphosphorus tetrafluoride? Because thecompound is molecular, look for elements on the right side of the peri-odic table. Phosphorus has the symbol P. Fluorine has the symbol F.Di- indicates two phosphorus atoms and tetra- indicates four fluorineatoms. The formula for the compound is P2F4.

Numberof Atoms Prefix

1 mono-

2 di-

3 tri-

4 tetra-

5 penta-

6 hexa-

7 hepta-

8 octa-

9 nona-

10 deca-

Prefixes ForNaming Compounds

Integrate Language ArtsTo help students learn the prefixes used inmolecular compounds, have them thinkof words they know that contain theGreek prefixes listed in Figure 20. If theyare having trouble, encourage them touse a dictionary to find words. Examplesinclude monochrome, dichotomy, tricycle,tetrahedron, pentagon, hexadecimal,heptad, octave, nonagenarian, anddecathlon.Verbal

ASSESSEvaluate UnderstandingNote that the process of writingmolecular formulas is the reverse of the process for naming them. Havestudents write chemical formulas forthree substances and chemical namesfor another three substances. Havestudents exchange the formulas andnames with a partner to check andreview their work.

ReteachUse the tables on pp. 171–173 and theMath Skills on p. 174 to review namingand writing formulas for ionic com-pounds. Use the table on p. 175 toreview naming and writing formulas for molecular compounds.

Solutions8. Lithium chloride, barium oxide, sodium nitride, lead sulfate9. Diphosphorus pentaoxide (pentoxide) and carbon monoxide10. K2S because it takes two potassiumions, each with a charge of 1�, to balance one sulfide ion with a charge of 2�


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Chemical Bonds 175

5. A covalently bonded group of atoms that hasa positive or negative charge and acts as a unit6. The explanation for binary compounds is thatmetals can form more than one type of cation.(Unless students have seen a more completetable of polyatomic ions, they are unlikely toknow a second possible explanation: Somepolyatomic ions contain the same elements,e.g., sulfate and sulfite ions.)7. Three


1. The formula of an ionic compound describesthe ratio of the ions in the compound.2. The name and the formula of a molecularcompound describe the type and number ofatoms in a molecule of the compound.3. -ide4. The Roman numerals help distinguish themultiple ions of transition metals.


176 Chapter 6

6.4 The Structure of Metals

Reading StrategyRelating Cause and Effect Copy theconcept map. As you read, complete the map to relate the structure of metals to their properties.

Key ConceptsWhat are the forces thatgive a metal its structureas a solid?

How do metallic bondsproduce some of thetypical propertiesof metals?

How are the properties ofalloys controlled?

Vocabulary◆ metallic bond◆ alloy

a. ?

b. ?

Mobileelectrons

produce

Figure 21 This photograph ofthe tungsten filament from alight bulb was taken with ascanning electron microscope.Color was added to the photo.The filament is magnified morethan 100 times. The diameterof the wire is about 15 μm, or 0.0015 cm.

Light bulbs are easy to ignore unless a bulb burns out and you aresearching for a replacement in the dark. But in the decades just beforethe year 1900, light bulbs were an exciting new technology. One chal-

lenge for researchers was to find the best material for the filamentsin light bulbs. The substance had to be ductile enough to be drawninto a narrow wire. It could not melt at the temperatures produced

when an electric current passes through a narrow wire. It had to havea low vapor pressure so that particles on the surface were not easilyremoved by sublimation.

The substance the researchers found was tungsten (W), a metalwhose name means “heavy stone” in Swedish. Figure 21 shows a mag-nified view of the narrow coils in a tungsten filament. Tungsten hasthe highest melting point of any metal—3410°C—and it has the lowestvapor pressure. The properties of a metal are related to bonds withinthe metal.

Metallic BondsMetal atoms achieve stable electron configurations by losing electrons.But what happens if there are no nonmetal atoms available to acceptthe electrons? There is a way for metal atoms to lose and gain electronsat the same time. In a metal, valence electrons are free to move amongthe atoms. In effect, the metal atoms become cations surrounded by apool of shared electrons. A metallic bond is the attraction between ametal cation and the shared electrons that surround it.

176 Chapter 6

FOCUS

Objectives6.4.1 Describe the structure and

strength of bonds in metals.6.4.2 Relate the properties of metals

to their structure.6.4.3 Define an alloy and demon-

strate how the composition ofan alloy affects its properties.

Build VocabularyVocabulary Knowledge RatingChart Have students construct a chartwith four columns labeled Term, CanDefine It/Use It, Heard It/Seen It, andDon’t Know. Have students copy theterms metallic bond, metal lattice, alloy,and metallurgy into the first column andrate their term knowledge by putting acheck in one of the other columns. Askhow many students actually know eachterm. Have them share their knowledge.To provide a purpose for reading, askfocused questions to help studentspredict text content based on each term.After students have read the section,have them rate their knowledge again.

Reading Strategya. and b. Conductivity or malleability

INSTRUCT

Metallic BondsIntegrate Social StudiesIn the first light bulbs, air was removedfrom the light bulb to prevent combus-tion as the filament heated up. Thissolution was not ideal because atomscan sublime from the hot filament atvery low pressures. With almost no air,atoms of the vaporized filament haveuninterrupted paths to the inner walls ofthe bulb where they are deposited. Themodern light bulb uses an argon atmos-phere and a tungsten filament. Thisprolongs the life of the bulb becausecollisions between tungsten atoms andargon atoms can redirect tungstenatoms back toward the filament. Haveinterested students research the searchfor an effective filament. Students canpresent their findings in a poster orother visual display.Visual

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Reading Focus

1

Section 6.4

Print• Reading and Study Workbook With

Math Support, Section 6.4 • Transparencies, Section 6.4

Technology• Interactive Textbook, Section 6.4• Presentation Pro CD-ROM, Section 6.4• Go Online, Science News, Metals

Section Resources

Chemical Bonds 177

The cations in a metal form a lattice that is held in place bystrong metallic bonds between the cations and the surroundingvalence electrons. Although the electrons are moving among theatoms, the total number of electrons does not change. So, overall, themetal is neutral.

The metallic bonds in some metals are stronger than in othermetals. The more valence electrons an atom can contribute to theshared pool, the stronger the metallic bonds will be. The bonds in analkali metal are relatively weak because alkali metals contribute only asingle valence electron. The result is that alkali metals, such as sodium,are soft enough to cut with a knife and have relatively low meltingpoints. Sodium melts at 97.8°C. Transition metals, such as tungsten,have more valence electrons to contribute and, therefore, are harderand have higher melting points. Recall that tungsten melts at 3410°C.

Explaining Properties of MetalsThe structure within a metal affects the properties of metals. Themobility of electrons within a metal lattice explains some of theproperties of metals. The ability to conduct an electric current andmalleability are two important properties of metals.

Recall that a flow of charged particles is an electric current. A metalhas a built-in supply of charged particles that can flow from one loca-tion to another—the pool of shared electrons. An electric current canbe carried through a metal by the free flow of the shared electrons.

The lattice in a metal is flexible compared to the rigid lattice in anionic compound. Figure 22 is a model of what happens when someonestrikes a metal with a hammer. The metal ions shift their positions andthe shape of the metal changes. But the metal does not shatter becauseions are still held together by the metallic bonds between the ions andthe electrons. Metallic bonds also explain why metals, such as tung-sten and copper, can be drawn into thin wires without breaking.

What two important properties of metals can beexplained by their structure?

Metal changes shapebut does not break.

Hammer strikes metal.

Figure 22 In a metal, cations aresurrounded by shared valenceelectrons. If a metal is struck, theions move to new positions, butthe ions are still surrounded byelectrons. Classifying Whatproperty of metals is displayedwhen a hammer strikes a metal?

Comparing Bond TypesPurpose Students observe differencesin the properties of substances withionic bonds and metallic bonds.

Materials salt lick (or rock salt), copperwire, hammer, goggles

Procedure Take students outside to anopen area. Allow them to examine thesamples of rock salt and copper. Havestudents stand back a safe distance.While wearing goggles, hit each samplewith a hammer against a hard surface,such as concrete. Allow students toobserve how each sample looks afterbeing pounded with a hammer.

Expected Outcome The rock saltshatters because sodium chloride is anionic substance. The end of the copperwire can be pounded flat with a hammerbecause metals are malleable. Visual

Explaining Propertiesof MetalsUse VisualsFigure 22 Have students examineFigure 22. Ask, How are the cationsaffected when the hammer strikes themetal? (The cations shift positions.) Howare the metallic bonds between thecations and electrons affected whenthe hammer strikes the metal? (Themetallic bonds are unaffected. Each cationis still surrounded by electrons.)Visual

Many students think that particles insolids cannot move. Challenge thismisconception by reminding studentsthat the kinetic theory of matter saysthat all particles of matter are in constantmotion. Ask, Describe the motion ofcations in a metal when the metal isnot being struck by a hammer. (Thecations vibrate, or move repeatedly backand forth, around fixed locations.) Logical

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Chemical Bonds 177

Customize for English Language Learners

Use a Cloze StrategyUse a Cloze strategy for students with verylimited English proficiency. Have students fill inthe blanks in the following sentences whilereading Explaining Properties of Metals. The

of

within a metal lattice explains some of theproperties of metal. The lattice in a metal is compared to the

lattice in an ioniccompound. (mobility; electrons; flexible; rigid) Answer to . . .

Figure 22 Malleability

The ability to conductan electric current and

malleability


AlloysA friend shows you a beautiful ring that she says is made from puregold. Your friend is lucky to have such a valuable object. The purity ofgold is expressed in units called karats. Gold that is 100 percent pureis labeled 24-karat gold. Gold jewelry that has a 12-karat label is only 50 percent gold. Jewelry that has an 18-karat label is 75 percent gold.

The surface of an object made from pure gold can easily be wornaway by contact with other objects or dented because gold is a softmetal. When silver, copper, nickel, or zinc is mixed with gold, the goldis harder and more resistant to wear. These gold mixtures are alloys. Analloy is a mixture of two or more elements, at least one of which is ametal. Alloys have the characteristic properties of metals.

1886 Charles Halland Paul Héroultindependentlydevelop a method forusing electricity toobtain aluminumfrom aluminum oxide.

1914 The start of WorldWar I leads to the wide-scale use of weldingtechniques, such as gaswelding with acetylene,for ship building.

BESSEMER CONVERTER

MODEL T FORD

GASWELDINGGas torch

Vanadium steel This alloy becomespopular in car manufacturing

because it is lightweight and strong.

Hot gas flame The flame from aburning gas melts the surfaces

where two metal parts will join.

Milestones in MetallurgyThe science of metallurgy includes ways toextract metals from ores, refine metals,and use metals. Described here are someadvances in metallurgy since 1850.

1856 Henry Bessemerdevelops an efficientprocess for producingsteel by blowing airthrough molten iron.

178 Chapter 6

1850 1880 1910

1908 Henry Ford usesvanadium steel (an alloyof iron with carbon andvanadium) extensivelyin his Model T Fords.

178 Chapter 6

AlloysUse Community ResourcesArrange for your class to visit theworkshop of a jewelry maker,metalworker, blacksmith, or welder.Have students observe the types ofequipment used to work with differentmetals. Ask questions regarding theartisan’s choice of materials for differentprojects. Ask about the use of differentalloys in metallurgy.Interpersonal

FYIThere are different categories of alloys:true solutions, heterogeneous mixtureswith two phases (a pure element and acompound), or intermetallic compoundswith definite compositions.

Milestones inMetallurgyAsk students to choose one of thesignificant events in metallurgy historydescribed in the time line. Have them doresearch in the library or on the Internetto find out more about the materials,applications, and people involved intheir chosen topic. Have them presenttheir findings to the class. Be sure thatpart of their presentation focuses on thespecific properties of metals and alloys.Verbal

Henry Ford saw an automobile partmade from vanadium steel when aFrench racer crashed at a European racemeeting in 1905. He was impressedwith the strength and low weight of the steel. American foundries did nothave furnaces that could achieve thetemperature required to produce thealloy. Ford found a small steel companyin Ohio that was willing to experimentwith the process and produce the alloyexclusively for Ford.Verbal

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Copper Alloys The first important alloy was bronze, whose nameis associated with an important era in history—the Bronze Age.Metalworkers in Thailand may have been the first to make bronze. Butpeople in other locations probably thought they were the first to makebronze. News didn’t travel quickly in that era.

Metalworkers might have noticed that the metal they extracted byheating deposits of copper was not always the same. The difference inproperties could be traced to the presence of tin. In its simplest form,bronze contains only copper and tin, which are relatively soft metals.Mixed together in bronze, the metals are much harder and strongerthan either metal alone. Scientists can design alloys with specificproperties by varying the types and amounts of elements in an alloy.

EMPIRE STATEBUILDING

METAL PARTSFROM POWDER

Superalloyscontainingrheniumare used injet engines.

1931 The 102- storyEmpire State Building inNew York City is completed.Skyscrapers would beimpossible without steel-framed construction.

Steel-framed structure The Empire StateBuilding is supported by a framework of steelcolumns and beams that weigh 60,000 tons.

1940 1970 2000

Metal parts, suchas the gears inthis gold watch,are made byapplying heatand pressure topowdered metal

in a mold.

1942 Making small, complexparts from metal powders is lesswasteful than machining. WorldWar II spurs advances in ironpowder metallurgy.

1991 New alloyscontaining rhenium areintroduced. Thesesuperalloys are capableof retaining theirstrength at very hightemperatures.

Cause-Effect ParagraphWrite a paragraph aboutHenry Ford’s decision to usevanadium steel for automobileparts. Where did Ford first seeparts made from vanadiumsteel? What properties of thistype of steel impressed Ford?Did Ford need to overcomeany problems before goingahead with his plan?

Chemical Bonds 179

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

Have students read the passage oncopper alloys. Then, have studentsconstruct a chart that does the following:1. Identify two copper alloys describedin the text. 2. Make a list of propertiesthe alloys have in common. 3. Make alist of properties that differ between thetwo alloys.Visual

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Chemical Bonds 179

Bronze Bronze jewelry found in gravesbeneath the town of Ban Chiang in northeastThailand has been dated to 3600 B.C., but this date is controversial. If correct, the sitepredates sites in Mesopotamia by several

hundred years. Some copper alloys thatcontain little tin but are similar in color tobronze are labeled as bronzes to takeadvantage of the reputation of bronze as ahard, durable material.

Facts and Figures


180 Chapter 6

Bronze is hard and durable enough to be used for propellers onships and for statues, such as the statue of horses in Figure 23. A bronzebell has a clear, loud tone that lasts for several seconds.

A brass bell has a duller tone that dies away quickly. Brass isanother alloy of copper that has been known for centuries. In its sim-plest form, brass contains only copper and zinc. Although both bronzeand brass are alloys of copper, they have distinctly different proper-ties. Brass is softer than bronze and is easier to shape into forms suchas the French horn in Figure 23. Brass is shinier than bronze but islikely to weather more quickly.

Steel Alloys The 1900s could be called the Age of Steel because ofthe skyscrapers, automobiles, and ships that were built from steelduring the 1900s. Steel is an alloy of iron that contains small quanti-ties of carbon, ranging from less than 0.2 percent to about 3 percent bymass. The smaller carbon atoms fit in the spaces between the largeriron atoms in the lattice. The carbon atoms form bonds with neigh-boring iron atoms. These bonds make the lattice harder and strongerthan a lattice that contains only iron.

The properties of any particular type of steel depend on whichelements other than iron and carbon are used and how much of thoseelements are included. Stainless steels contain more than 10 percentchromium by mass, but almost no carbon. Stainless steels are durablebecause chromium forms an oxide that protects the steel from rust-ing. But stainless steel is more brittle than steels that contain morecarbon. The steel cables in the bridge in Figure 24 have to be strongenough to resist forces that might stretch the cables or cause them tobreak. The steel used contains sulfur, manganese, phosphorus, sili-con, and 0.81 percent carbon.

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Web Code: cce-1064

Figure 23 This ancient statue ofhorses from Venice, Italy, and thismodern French horn are bothmade from copper alloys. Thestatue is made from bronze, analloy of copper and tin. TheFrench horn is made from brass,an alloy of copper and zinc.

180 Chapter 6

Bronze and Brass TonesPurpose Students observe thedifference in tone between a brass bell and a bronze bell.

Materials brass bell, bronze bell

Procedure Allow students to examineeach bell to see if they appear differentin color or shininess. Ring each bell andallow students to note the difference in tone.

Expected Outcome In general, brassis shinier than bronze. A brass bell willhave a duller tone that does not last aslong as the clear, loud tone of a bronzebell. (Bell bronze contains about 80% copper and 20% tin, which is just the right composition to produce a hard, resonant material.) Visual, Musical

Build Science SkillsApplying Concepts After studentsread about copper alloys, present thefollowing problem to the class. Imaginethat you are going to make a windchime in shop class as a gift. You havetwo choices of metals to use, brass orbronze. Which do you choose?Explain your answer. (Some studentsmay choose brass because it is shinier andeasier to shape into forms than bronze.Others may choose bronze because it hasa clearer, louder tone and weathers betterthan brass.)Intrapersonal

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Bronze Horses The origin of the gildedbronze horses of St. Mark’s is unclear. Thesculptures are not similar in style to otherGreek or Roman sculptures of horses. Nor arethey similar in composition. From chemicalanalyses, art historians know that the bronze is 98% copper, 1% tin, and 1% lead, which is an unusually high percent of copper. Arthistorians do know that the horses were cast inpieces that were welded together. The pieces

were made using the lost wax methoddescribed in Chapter 2.

The horses can be traced from a triumphalarch in Rome to one in Constantinople. Theywere taken from Constantinople to Venice in1204 (where they were placed on a balcony ofthe cathedral), then to Paris in 1797, and thenback to Venice in 1815. For protection from airpollution, the horses are now stored inside thecathedral. A copy appears on the balcony.

Facts and Figures

Science News provides studentswith current information on metals.


Reviewing Concepts1. What holds metal ions together in a

metal lattice?

2. What characteristic of a metallic bondexplains some of the properties of metals?

3. How can scientists design alloys withspecific properties?

4. Explain why the metallic bonds in some metalsare stronger than the bonds in other metals.

5. Why are metals good conductors of electric current?

6. How does adding carbon to steel make thesteel harder and stronger?

Critical Thinking7. Predicting Which element has a higher

melting point, potassium in Group 1A orcalcium in Group 1B? Give a reason for your answer.

8. Applying Concepts Can two differentelements form a metallic bond together?

Other Alloys Airplane parts are made of many different alloysthat are suited to particular purposes. The body of a plane is large andneeds to be made from a lightweight material. Pure aluminum islighter than most metals, but it bends and dents too easily. If a smallamount of copper or manganese is added to aluminum, the result is astronger material that is still lighter than steel.

For certain aircraft parts, even lighter materials are needed. Alloysof aluminum and magnesium are used for these parts. Magnesium ismuch less dense than most metals used to build structures. However,pure magnesium is soft enough to cut with a knife, and it burns in air.An aluminum-magnesium alloy keeps the advantages of magnesiumwithout the disadvantages.

Chemical Bonds 181

Compare-Contrast Paragraph Write aparagraph comparing the properties of ioniccompounds and alloys. Relate their propertiesto the structure of their lattices.

Figure 24 The Golden Gate Bridge is a landmark in San Francisco,California. Its cables, towers, anddeck contain steel. The steel in thecables needs to resist forces that pullon the cables. The steel in the towersneeds to resist the compression forcescaused by the weight of the cables,the deck, and the vehicles that travelacross the bridge. Drawing Conclusions Would thesteel used for the cables and thesteel used for the towers have thesame composition? Give a reason foryour answer.

Integrate Materials ScienceThe properties of an alloy that aredetermined by its composition includemalleability, ductility, hardness, corrosionresistance, tensile strength (the ability toresist being pulled apart), shear strength(the ability to resist opposing forces that are not acting in a straight line),compressive strength (the ability towithstand pressures acting on a givenplane), and elasticity (the ability to returnto its original size and shape). Ask, Whatproperties of steel make it useful forthe cables and towers of the GoldenGate Bridge? (Its high tensile strengthmakes steel a useful material for the cables,and its high compressive strength makes ita useful material for the towers.) Logical

ASSESSEvaluate UnderstandingHave students write the names of thefollowing elements on index cards:copper, gold, silver, nickel, zinc, tin,iron, carbon, and aluminum. On theother side of the cards, have studentswrite common uses for these elements.(For example, alloys of aluminum andcopper are used to make airplane parts.)

ReteachUse Figure 22 to review how thestructure within a metal affects theproperties of a metal.

In the rigid network of a crystal lattice,the charged particles are not free to flowunless the solid melts. When struck, thecrystal shatters because ions with thesame charge shift position in the latticeand repel. Thus, ionic solids are brittleand are poor conductors. In a metallattice, the electrons are free to moveand conduct a current. The lattice ismalleable when struck because if acation moves it is still surrounded byparticles with an opposite charge.


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Chemical Bonds 181

5. The valence electrons are free to movebecause they are not attached to a specificmetal ion. 6. The smaller carbon atoms fit into spacesbetween the iron atoms and form bonds withneighboring iron atoms, which makes thelattice harder and stronger.7. Calcium has a higher melting point becauseit contributes twice as many electrons to themetallic bonds.8. Yes, alloys usually are mixtures of elementswith metallic bonds.


1. Metal ions are held together by the strongmetallic bonds between the cations and thesurrounding valence electrons.2. The mobility of electrons within a metallattice explains some of the properties of metals. 3. Scientists can design alloys with specificproperties by varying the types and amountsof elements in an alloy. 4. The more valence electrons a metal cancontribute, the stronger the bonds will be.

Answer to . . .

Figure 24 The compositions wouldprobably be different because theforces acting on the cables and towersare different—tensile versus compres-sive, respectively.


Chipping InTasks done by computers that filled an entireroom in the 1950s are now done by devices thesize of a credit card. This miniaturization in theelectronics industry is due to semiconductors.

Wafers and chipsSilicon crystals are cut intowafer-thin slices. Each wafercan be made into hundredsof silicon chips.

Siliconcrystal

Silicon wafer

182

DopingAn element with five valence electrons, such asphosphorus, can be added to silicon. After aphosphorus atom bonds with four atoms, there isan extra electron that is free to move. Silicon dopedwith phosphorus is called n-type silicon becauseelectrons have a negative charge. An element withthree valence electrons, such as boron, can be addedto silicon. Adding boron leaves holes to which electronscan move from neighboring atoms. Because the lack ofan electron has the effect of a positive charge, siliconwith boron is called p-type silicon.

Silicon with boron

Pure Silicon

Silicon with phosphorus

Semiconductors are solid substances, such as silicon, thathave poor electrical conductivity at ordinary temperatures.Silicon has four valence electrons. In pure silicon, eachatom forms single bonds with four other atoms. Thisarrangement leaves no electrons free to move through thesilicon. The conductivity of silicon is greatly improved byadding small amounts of other elements to silicon, aprocess called doping.

182 Chapter 6

Chipping InBackgroundThe silicon crystal shown in the photo iscalled an ingot. Pure silicon ingots can be produced by the Czochralski pulled-growth method described on p. 163.Wafers cut from the ingot can be ground,polished, cleaned, etched, doped withelements such as phosphorus and boron,and sliced with a diamond saw toproduce microchips.

Build Science Skills

Applying Concepts

Purpose Studentsobserve the importance of transistors in their lives.

Materials access to a library or theInternet, notepad for data collection

Class Time 20–30 minutes of research,one day of data collection, 20 minutesof class discussion

Procedure Have students use thelibrary or the Internet to come up with a list of electronic devices that containtransistors. Have students record everytime they use a device that contains a transistor, and the type of device.Have students compare their data in a class discussion.

Expected Outcome Devices that containtransistors include alarm clocks, watches,refrigerators, microwave ovens, washers,dryers, calculators, computers, radios, CD players, televisions, VCRs, DVD players,and video game consoles. Intrapersonal, Kinesthetic

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■ Research and write about the development oftransistors. Why were researchers looking fora replacement for vacuumtubes? How did replacingvacuum tubes with transistorsaffect the size of radios andcomputers?

■ Take a Discovery Channel VideoField Trip by watching“Good Conduct.”

Going Further

Video Field Trip

Computer chipIn 1974, the first computer microchipcontained 6000 transistors. Today, morethan 40 million transistors can beplaced on a single computer chip.

Integratedcircuit on asilicon chip

Chemical Bonds 183

TransistorIn a transistor, there are three layers of dopedsilicon. There is a layer of p-type siliconsandwiched between two layers of n-typesilicon or a layer of n-type silicon betweentwo layers of p-type silicon. Transistors areused to amplify current. A small currentapplied to the central layer of a transistor canproduce a larger current.

Vacuum tubeGlass vacuum tubes usedin early computers werefragile and took up space.

DiodeIn a diode, a layer of p-typesilicon is joined to a layer of n-type silicon. When the leads on a diode are correctly connected in a circuit, electrons flow toward thejunction between the two types of silicon.Electrons from the n-type silicon fill holes inthe p-type silicon. In devices that use batteries,a diode can keep electrons from flowing if thebatteries are not inserted correctly. Some diodesemit light when the circuit is complete.

Going FurtherTransistors were developed to solve aproblem with long distance telephoneservice. Until the 1950s, AT&T usedvacuum tubes to amplify signals as theywere transferred from one switch box tothe next. Vacuum tubes were unreliable,used a lot of power, and produced a lotof heat.

After World War II, a team at BellLaboratories in New Jersey (John Bardeen,Walter Brattain, and William Shockley)developed the transistor. In 1956, theyshared the Nobel Prize in Physics. Atransistor is a thin three-layer sandwichwith a p-type semiconductor in the outerlayers and an n-type in the middle (PNP),or an n-type semiconductor on oppositesides of a p-type (NPN).

Because transistors are so muchsmaller than vacuum tubes, devices suchas radios, televisions, and computerswere reduced in size when vacuumtubes were replaced with transistors. Verbal

Chemical Bonds 183

After students have viewed the Video Field Trip,ask them the following questions: What is themain element in computer chips? (Silicon)What is the purpose of the seed crystal that is lowered into the rotating vat of moltensilicon? (The atoms in the seed crystal form alattice. As the molten silicon cools, silicon atoms

Video Field Trip

Good Conduct

follow this pattern as they form a much largersilicon crystal.) Why is pure silicon a poorconductor of electric current? (After eachsilicon atom forms four bonds with neighboringatoms, there are no free electrons to conductelectric current.) What happens when aboron atom is substituted in the lattice?(Boron has only three valence electrons, so holes[or the absence of an electron] are left to whichelectrons from neighboring atoms can move asthey conduct electric current.) What happens

when a phosphorus atom is substituted inthe lattice? (A phosphorus atom has fivevalence electrons, so there is an extra electronthat can freely move and conduct electriccurrent.) How can the ability of pure siliconto conduct current at room temperaturebe increased? (Impurities, called dopants, can be added to increase the number ofelectrons or holes available to conduct anelectric current.)


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