Glass is a brittle substancethat is made from silicondioxide, a compound with a very rigid structure. Theaddition of small amountsof other compoundschanges the color of theglass, “staining” it.
ACTIVITYACTIVITYFocusFocus
Background Suddenly, a glass object slips from your hand andcrashes to the ground. You watch it break into many tiny piecesas you hear it hit the floor. Glass is a brittle substance. Whenenough force is applied, it breaks into many sharp, jagged pieces.Glass behaves the way it does because of its composition.
A glass container and a stained glass window have somesimilar properties because both are made mainly from silicondioxide. But other compounds are responsible for the window’sbeautiful colors. Adding a compound of nickel and oxygen to theglass produces a purple tint. Adding a compound of cobalt andoxygen makes the glass deep blue, while adding a compound of copper and oxygen makes the glass dark red.
Activity 1 There are many different kinds of glass, each with its own use. List several kinds of glass that you encounter daily.Describe the ways that each kind of glass differs from other kinds of glass.
Activity 2 Research other compounds that are sometimes addedto glass. Describe how each of these compounds changes theproperties of glass. Write a report on your findings.
Pre-Reading Questions1. How does atomic structure affect the
properties of a substance?2. Can bonds between atoms be broken?
www.scilinks.orgTopic: Properties of SubstancesSciLinks code: HK4113
Compounds and Molecules> Distinguish between compounds and mixtures.> Relate the chemical formula of a compound to the rela-
tive numbers of atoms or ions present in the compound.> Use models to visualize a compound’s chemical structure.> Describe how the chemical structure of a compound
affects its properties.
I f you step on a sharp rock with your bare foot, you feel pain.That’s because rocks are hard substances; they don’t bend.
Many rocks are made of quartz. Table salt and sugar look simi-lar; both are grainy, white solids. But they taste very different. Inaddition, salt is hard and brittle and breaks into uniform cube-like granules, while sugar does not. Quartz, salt, and sugar are allcompounds. Their similarities and differences result from theway their atoms or ions are joined.
What Are Compounds?Table salt is a compound made of two elements, sodium andchlorine. When elements combine to form a compound, the com-pound has properties very different from those of the elementsthat make it. Figure 1 shows how the metal sodium combineswith chlorine gas to form sodium chloride, NaCl, or table salt.
O B J E C T I V E S
SECTION
1
144 C H A P T E R 5
K E Y T E R M S
chemical bondchemical structurebond lengthbond angle
▲
+
Figure 1The silvery metal sodium
combines with poisonous,yellowish green chlorine gasin a violent reaction toform white granules oftable salt that you can eat.
Chemical bonds distinguish compounds from mixturesThe attractive forces that hold different atoms or ions together incompounds are called Recall how compoundsand mixtures are different. Mixtures are made of different sub-stances that are just placed together. Each substance in the mix-ture keeps its own properties.
For example, mixing blue paint and yellow paint makes greenpaint. Different shades of green can be made by mixing thepaints in different proportions, but both original paints remainchemically unchanged.
Figure 2 shows that when a mixture of hydrogen gas and oxy-gen gas is heated, a violent reaction takes place and a compoundforms. Chemical bonds are broken, and atoms are rearranged.New bonds form water, a compound with properties very differ-ent from those of the original gases.
A compound always has the same chemical formulaThe chemical formula for water is H2O, and that of table sugar isC12H22O11. The salt you season your food with has the chemicalformula NaCl. A chemical formula shows the types and numbersof atoms or ions making up the simplest unit of the compound.
There is another important way that compounds and mix-tures are different. Compounds are always made of the same el-ements in the same proportion. A molecule of water, for example,is always made of two hydrogen atoms and one oxygen atom.This is true for all water. That means water frozen in a comet inouter space and water at 37°C (98.6°F) inside the cells of yourbody both have the same chemical formula—H2O.
chemical bonds.
T H E S T R U C T U R E O F M A T T E R 145
The mixed gases are ignited by thecandle flame, and water is produced. B
Figure 2Placing a lit candle under a balloon containing
hydrogen gas and oxygen gas causes the balloonto melt, releasing the mixed gases.
A
Hydrogen gas, H2
Oxygen gas, O2 Water (steam), H2O
chemical bond the attrac-tive force that holds atoms or ions together
Chemical structure shows the bonding within a compoundAlthough water’s chemical formula tells us what atoms it is madeof, it doesn’t reveal anything about the way these atoms are con-nected. You can see how a compound’s atoms or ions are con-nected by its The structure of a compoundcan be compared to that of a rope. The kinds of fibers used tomake a rope and the way the fibers are intertwined determinehow strong the rope is. Similarly, the atoms in a compound andthe way the atoms are arranged determine many of the com-pound’s properties.
Two terms are used to specify the positions of atoms relativeto one another in a compound. A gives the distancebetween the nuclei of two bonded atoms. And when a compoundhas three or more atoms, tell how these atoms areoriented in space. Figure 3 shows the chemical structure of awater molecule. You can see that the way hydrogen and oxygenatoms bond to form water looks more like a boomerang than astraight line.
Models of CompoundsFigure 3 is a ball-and-stick model of a water molecule. Ball-and-stick models, as well as other kinds of models, help you “see” acompound’s structure by showing you how the atoms or ions arearranged in the compound.
Some models give you an idea of bondlengths and anglesIn the ball-and-stick model of water shown inFigure 3, the atoms are represented by balls.The bonds that hold the atoms together arerepresented by sticks. Although bonds betweenatoms aren’t really as rigid as sticks, this modelmakes it easy to see the bonds and the anglesthey form in a compound.
Structural formulas can also show the struc-tures of compounds. Notice how water’s struc-tural formula, which is shown below, is a lotlike its ball-and-stick model. The difference isthat only chemical symbols are used to repre-sent the atoms.
bond angles
bond length
chemical structure.
95.8 pm
104.45º
Figure 3 The ball-and-stick model in thisfigure is a giant representation of one molecule of water. Apicometer (pm) is equal to 1 � 10–12 m.
O
HH
Connection toFINE ARTSFINE ARTS
C lay has a layered structure of silicon, oxygen, alu-minum, and hydrogen atoms. Artists can mold
wet clay into any shape because water molecules letthe layers slide over one another. When clay dries,water evaporates and the layers can no longer slide.To keep the dry, crumbly clay from breaking apart,artists change the structure of the clay by heating it.The atoms in one layer bond to atoms in the layersabove and below. When this happens, the clayhardens, and the artist’s work is permanently set.
Making the Connection1. Think of other substances that can be shaped
when they are wet and that “set” when they are dried or heated.
2. Write a paragraph about one of these sub-stances and why it has these properties.
WRITINGS K I L L HO
H
chemical structure thearrangement of atoms in asubstance
bond length the averagedistance between the nucleiof two bonded atoms
bond angle the angleformed by two bonds to the same atom
Space-filling models show the space occupied by atomsFigure 4 shows another way chemists picture a water molecule. Itis called a space-filling model because it shows the space that isoccupied by the oxygen and hydrogen atoms. The problem withthis model is that it is harder to “see” bond lengths and angles.
How Does Structure Affect Properties?Some compounds, such as the quartz found in many rocks, existas a large network of bonded atoms. Other compounds, such astable salt, are also large networks, but of bonded positive andnegative ions. Still other compounds, such as water and sugar,are made of many separate molecules. Different structures givethese compounds different properties.
Compounds with network structures are strong solidsQuartz is sometimes found in the form of beautiful crystals, asshown in Figure 5. Quartz has the chemical formula SiO2, and sodoes the less pure form of quartz, sand. Figure 5 shows that everysilicon atom in quartz is bonded to four oxygen atoms. Thebonds that hold these atoms together are very strong. All of theSiIOISi and OISiIO bond angles are the same. That is, eachone is 109.5°. This arrangement continues throughout the sub-stance, holding the silicon and oxygen atoms together in a verystrong, rigid structure.
This is why rocks containing quartz are hard and inflexiblesolids. Silicon and oxygen atoms in sand have a similar arrange-ment. It takes a lot of energy to break the strong bonds betweensilicon and oxygen atoms in quartz and sand. That’s why themelting point and boiling point of quartz and sand is so high, asshown in Table 1.
T H E S T R U C T U R E O F M A T T E R 147
Table 1 Some Compounds with Network Structures
Silicon solid 1700 2230dioxide, SiO2
(quartz)
Magnesium solid 1261 2239fluoride, MgF2
Sodium solid 801 1413chloride, NaCl(table salt)
State Melting BoilingCompound (25ºC) point (ºC) point (ºC)
Oxygenatom Silicon
atom
Figure 4 This space-filling model of watershows that the two hydrogenatoms take up much less spacethan the oxygen atom.
Figure 5 Quartz and sand are made of sili-con and oxygen atoms bonded ina strong, rigid structure.
Figure 6 Each grain of table salt, or sodiumchloride, is composed of a tightlypacked network of Na� ions andCl� ions.
Chlorideion, Cl�
Sodiumion, Na�
Figure 7 Sugar, C12H22O11, is made ofmolecules.
Some networks are made of bonded ionsLike some quartz, table salt—sodium chloride—is found in theform of regularly shaped crystals. Crystals of sodium chloride arecube shaped. Like quartz and sand, sodium chloride is made of arepeating network connected by strong bonds. The network ismade of tightly packed, positively charged sodium ions and nega-tively charged chloride ions, as shown in Figure 6. The strongattractions between the oppositely charged ions cause table saltand other similar compounds to have high melting points andboiling points, as shown in Table 1.
Some compounds are made of molecules Salt and sugar are both white solids you can eat, but their struc-tures are very different. Unlike salt, sugar is made of molecules.A molecule of sugar, shown in Figure 7, is made of carbon, hydro-gen, and oxygen atoms joined by bonds. Molecules of sugar doattract each other to form crystals. But these attractions aremuch weaker than those that hold bonded carbon, hydrogen, andoxygen atoms together to make a sugar molecule.
We breathe nitrogen, N2, oxygen, O2, and carbon diox-ide, CO2, every day. All three substances are colorless,
odorless gases made of molecules. Within each mol-ecule, the atoms are so strongly attracted to one
another that they are bonded. But the moleculesof each gas have very little attraction for oneanother. Because the molecules of these gases arenot very attracted to one another, they spread outas much as they can. That is why gases can take
The strength of attractions between molecules variesCompare sugar, water, and dihydrogen sulfide in Table 2.Although all three compounds are made of molecules, their prop-erties are very different. Sugar is a solid, water is a liquid, anddihydrogen sulfide is a gas. That means that sugar moleculeshave the strongest attractions for each other, followed by watermolecules. Dihydrogen sulfide molecules have the weakestattractions for each other. The fact that sugar and water havesuch different properties probably doesn’t surprise you. Theirchemical structures are not at all alike. But what about water anddihydrogen sulfide, which do have similar chemical structures?
T H E S T R U C T U R E O F M A T T E R 149
Sugar, C12H22O11 Solid 185–186 –––––
Water, H2O Liquid 0 100
Dihydrogen Gas –86 –61sulfide, H2S
State Melting BoilingCompound (25ºC) point (ºC) point (ºC)
Table 2 Comparing Compounds Made of Molecules
Materials
Which melts more easily, sugar or salt?
✔ table salt ✔ stopwatch ✔ 2 test tubes
✔ Bunsen burner ✔ table sugar ✔ tongs
SAFETY CAUTION Wear safety goggles andgloves. Tie back long hair, confine loose clothing,and use tongs to handle hot glassware. When heat-ing a substance in a test tube, always point the openend of the test tube away from yourself and others.
1. Use your knowledge of structures to make ahypothesis about whether sugar or salt will meltmore easily.
2. To test your hypothesis, place about 1 cm3 ofsugar in a test tube.
3. Using tongs, position the test tube with sugarover the flame, as shown in the figure at right.Move the test tube back and forth slowly overthe flame. Use a stopwatch to measure the timeit takes for the sugar to melt.
4. Repeat steps 2 and 3 with salt. If your sampledoes not melt within 1 minute, remove it fromthe flame.
Analysis1. Which compound
is easier to melt?Was your hypoth-esis right?
2. How can yourelate your resultsto the structure ofeach compound?
Attractions between water molecules are calledhydrogen bondsThe higher melting and boiling points of water sug-gest that water molecules attract each other morethan dihydrogen sulfide molecules do. Figure 8 showshow an oxygen atom of one water molecule isattracted to a hydrogen atom of a neighboring watermolecule. This attraction is called a hydrogen bond.Water molecules attract each other, but these attrac-tions are not as strong as the bonds holding oxygenand hydrogen atoms together within a molecule.
150 C H A P T E R 5
S E C T I O N 1 R E V I E W
1. Classify the following substances as mixtures or compounds:a. air c. SnF2
b. CO d. pure water
2. Explain why silver iodide, AgI, a compound used in photog-raphy, has a much higher melting point than vanillin,C8H8O3, a sweet-smelling compound used in flavorings.
3. Draw a ball-and-stick model of a boron trifluoride, BF3,molecule. In this molecule, a boron atom is attached tothree fluorine atoms. Each FIBIF bond angle is 120°, andall BIF bonds are the same length.
4. Predict which molecules have a greater attraction for eachother, C3H8O molecules in liquid rubbing alcohol or CH4
molecules in methane gas.
5. Explain why glass, which is made mainly of SiO2, is oftenused to make cookware. (Hint: What properties does SiO2
have because of its structure?)
6. Critical Thinking A picometer (pm) is equal to 1 � 10�12 m.OIH bond lengths in water are 95.8 pm, while SIH bondlengths in dihydrogen sulfide are 135 pm. Why are SIHbond lengths longer than OIH bond lengths? (Hint: Whichis larger, a sulfur atom or an oxygen atom?)
S U M M A R Y
> Atoms or ions in com-pounds are joined bychemical bonds.
> A compound’s chemicalformula shows whichatoms or ions it is made of.
> A model represents a com-pound’s structure visually.
> Substances with networkstructures are usuallystrong solids with highmelting and boiling points.
> Substances made of mole-cules have lower meltingand boiling points.
> Whether a molecular sub-stance is a solid, a liquid, ora gas at room temperaturedepends on the attractionsbetween its molecules.
Figure 8Dotted lines indicate the intermolecular attractionsthat occur between water molecules, which is oftenreferred to as “hydrogen bonding.” Water is a liquidat room temperature because of these attractions.
> Explain why atoms sometimes join to form bonds.> Explain why some atoms transfer their valence electrons
to form ionic bonds, while other atoms share valenceelectrons to form covalent bonds.
> Differentiate between ionic, covalent, and metallic bonds.> Compare the properties of substances with different types
of bonds.
Ionic and Covalent Bonding
When two atoms join, a bond forms. You have already seenhow bonded atoms form many kinds of substances. Atoms
bond in different ways to form these many substances. The typeof bonds that the atoms of a substance form affect the sub-stance’s properties.
What Holds Bonded Atoms Together?Three different kinds of bonds describe the way atoms bond inmost substances. In many of the models you have seen so far, thebonds that hold atoms together are represented by sticks. Butwhat bonds atoms in a real molecule?
Bonded atoms usually have a stable electron configurationAtoms bond when their valence electrons interact. You havelearned that atoms with full outermost s and p orbitals are morestable than atoms with only partly filled outer s and p orbitals.Generally, atoms join to form bonds so that each atom has a sta-ble electron configuration. When this happens, each atom has anelectronic structure similar to that of a noble gas.
When two hydrogen atoms bond, as shown in Figure 9, thepositive nucleus of one hydrogen atom attracts the negative elec-tron of the other hydrogen atom, and vice versa. This attractionpulls the two atoms closer together. Soon the electron clouds ofthe hydrogen atoms cross each other. The shared electron cloudof the molecule that forms has two electrons (one from eachatom). A hydrogen molecule, which consists of two hydrogenatoms bonded together, has an electronic structure similar to thenoble gas helium. The molecule will not fall apart unless enoughenergy is added to break the bond.
O B J E C T I V E S
SECTION
2
T H E S T R U C T U R E O F M A T T E R 151
K E Y T E R M S
ionic bondmetallic bondcovalent bondpolyatomic ion
▲
Hydrogen atom Hydrogen atom
�
1e� 1e�
Hydrogen molecule
2e� in shared electron cloud
Figure 9 When two hydrogen atoms arevery close together, their electronclouds overlap, and a bond forms.The two electrons of the hydrogenmolecule that forms are in theshared electron cloud.
Bonds can bend and stretch without breakingAlthough some bonds are stronger and more rigid than others, all bonds behave more like flexible springs than like sticks, asFigure 10 shows. The atoms move back and forth a little and theirnuclei do not always stay the same distance apart. In fact, mostreported bond lengths are averages of these distances. Althoughbonds are not rigid, they still hold atoms together tightly.
Ionic Bondsare formed between oppositely
charged ions. Atoms of metal elements, such assodium and calcium, form the positivelycharged ions. Atoms of nonmetal elements,such as chlorine and oxygen, form the nega-tively charged ions.
Ionic bonds are formed by the transfer ofelectronsSome atoms do not share electrons to fill theiroutermost energy levels completely. Instead,they transfer electrons. One of the atoms gainsthe electrons that the other atom loses. Bothions that form usually have stable electron con-figurations. The result is a positive ion and anegative ion, such as the Na� ion and the Cl�
ion in sodium chloride.These oppositely charged ions attract each
other and form an ionic bond. Each positivesodium ion attracts several negative chlorideions. These negative chloride ions attract morepositive sodium ions, and so on. Soon a net-work of these bonded ions forms a crystal oftable salt.
Ionic bonds
152 C H A P T E R 5
Figure 10 Chemists often use a solid bar to show a bondbetween two atoms, butreal bonds are flexible, like stiff springs.
ionic bond a bond formedby the attraction betweenoppositely charged ions
▲
Connection toSOCIAL STUDIESSOCIAL STUDIES
American scientist Linus Pauling studied howelectrons are arranged within atoms. He also
studied the ways that atoms share and exchangeelectrons. In 1954, he won the Nobel Prize inchemistry for his valuable research.
Later, Pauling fought to ban nuclear weaponstesting. Pauling was able to convince more than11 000 scientists from 49 countries to sign a peti-tion to stop nuclear weapons testing. Pauling wonthe Nobel Peace Prize in 1962 for his efforts. A yearlater, a treaty outlawing nuclear weapons testing inthe atmosphere, in outer space, and underwaterwent into effect.
Making the Connection1. Electronegativity is an idea first thought of by
Pauling. It tells how easily an atom accepts elec-trons. Which is more electronegative, a fluorineatom or a calcium atom? Why?
2. Nuclear weapons testing can harmliving things because of the result-ing radiation. Write a paragraph explaining howhigh levels of radiation can affect your body.
Ionic compounds are in the form of networks,not moleculesBecause sodium chloride is a network of ions, itdoes not make sense to talk about “a molecule ofNaCl.” In fact, every sodium ion is next to six chlo-ride ions, as shown in Figure 6. Instead, chemiststalk about the smallest ratio of ions in ionic com-pounds. Sodium chloride’s chemical formula, NaCl,tells us that there is one Na� ion for every Cl� ion,or a 1:1 ratio of ions. This means the compound hasa total charge of zero. One Na� ion and one Cl� ionmake up a formula unit of NaCl.
Not every ionic compound has the same ratio ofions as sodium chloride. An example is calcium fluo-ride, which is shown in Figure 11. The ratio of Ca2� ions to F�
ions in calcium fluoride must be 1:2 to make a neutral compound.That is why the chemical formula for calcium fluoride is CaF2.
When melted or dissolved in water, ionic compounds conduct electricityElectric current is moving charges. Solid ionic compounds donot conduct electricity because the charged ions are locked intoplace, causing the melting points of ionic compounds to be veryhigh—often well above 300°C. But if you dissolve an ionic com-pound in water or melt it, it can conduct electricity. That’sbecause the ions are then free to move, as shown in Figure 12.
T H E S T R U C T U R E O F M A T T E R 153
Figure 11There are twice as many fluorideions as calcium ions in a crystal of calcium fluoride, CaF2. So oneCa2� ion and two F� ions makeup one formula unit of thecompound.
Calcium ion, Ca2� Fluoride ion, F�One formulaunit
Figure 12Like other ionic compounds,sodium chloride conducts electric-ity when it is dissolved in water.
Metallic BondsMetals, like copper, shown in Figure 13, can conduct electricitywhen they are solid. Metals are also flexible, so they can bendand stretch without breaking. Copper, for example, can be ham-mered flat into sheets or stretched into very thin wire. What kindof bonds give copper these properties?
Electrons move freely between metal atomsThe atoms in metals like copper form Theattraction between one atom’s nucleus and a neighboring atom’selectrons packs the atoms closely together. This close packingcauses the outermost energy levels of the atoms to overlap, asshown in Figure 13. Therefore, electrons are free to move fromatom to atom. This model explains why metals conduct electric-ity so well. Metals are flexible because the atoms can slide pasteach other without their bonds breaking.
metallic bonds.
154 C H A P T E R 5
metallic bond a bondformed by the attractionbetween positively chargedmetal ions and the electronsaround them
▲
Figure 13 Copper is a flexible metalthat melts at 1083°Cand boils at 2567°C.Copper conductselectricity becauseelectrons can movefreely between atoms.
Copper
QuickQuickQuick ACTIVITYACTIVITY
Copper and other metals have close-packedstructures. This means their atoms are packedvery tightly together. In this activity, you will builda close-packed structure using ping pong balls.
1. Place three books flat on a table so that theiredges form a triangle.
2. Fill the triangular space between the bookswith the spherical “atoms.” Adjust the booksso that the atoms make a one-layer, close-packed pattern, as shown at right.
3. Build additional layers on top of the firstlayer. How many other atoms does eachatom touch? Where have you seen otherarrangements that are similar to this one?
Covalent BondsCompounds that are made of molecules, like water and sugar,have Compounds existing as networks ofbonded atoms, such as silicon dioxide, are also held together bycovalent bonds. Covalent bonds are often formed between non-metal atoms.
Covalent compounds can be solids, liquids, or gases. Exceptfor silicon dioxide and other compounds with network struc-tures, most covalent compounds have low melting points—usually below 300°C. In compounds that are made of molecules,the molecules are free to move when the compound is dissolvedor melted. But most of these molecules remain intact and do notconduct electricity because they are not charged.
Atoms joined by covalent bonds share electronsSome atoms, like the hydrogen atoms in Figure 9, bond to formmolecules. Figure 14A shows how two chlorine atoms bond toform a chlorine molecule, Cl2. Before bonding, each atom hasseven electrons in its outermost energy level. The atoms don’ttransfer electrons to one another because each needs to gain anelectron. If each atom shares one electron with the other atom,then both atoms together have a full outermost energy level. Thatis, both atoms together have eight valence electrons. The wayelectrons are shared depends on which atoms are sharing theelectrons. Two chlorine atoms are exactly alike. When they bond,electrons are equally attracted to the positive nucleus of eachatom. Bonds like this one, in which electrons are shared equally,are called nonpolar covalent bonds.
The structural formula in Figure 14B shows how the chlorineatoms are connected in the molecule that forms. A single linedrawn between two atoms indicates that the atoms share twoelectrons and are joined by one covalent bond.
covalent bonds.
T H E S T R U C T U R E O F M A T T E R 155
Two chlorine atoms share electrons equally to forma nonpolar covalent bond.A
A single line drawn betweentwo chlorine atoms shows thatthe atoms share two electrons.Dots represent electrons that arenot involved in bonding.
B
Cl–ClOne covalent bond (two shared electrons)
Each chlorine atom has sixelectrons that are not shared.
Covalent bonds form whenatoms share pairs of valenceelectrons.
V
Figure 14
covalent bond a bondformed when atoms shareone or more pairs of electrons
▲
www.scilinks.orgTopic: Chemical BondingSciLinks code: HK4021
Atoms may share more than one pair ofelectronsFigure 15 shows covalent bonding in oxygengas, O2 and nitrogen gas, N2. Notice that thebond joining two oxygen atoms is repre-sented by two lines. This means that twopairs of electrons (a total of four electrons)are shared to form a double covalent bond.
The bond joining two nitrogen atoms isrepresented by three lines. Two nitrogenatoms form a triple covalent bond by shar-ing three pairs of electrons (a total of sixelectrons).
The bond between two nitrogen atoms isstronger than the bond between two oxygenatoms. That’s because more energy isneeded to break a triple bond than to breaka double bond. Triple and double bonds arealso shorter than single bonds.
Atoms do not always share electrons equallyWhen two different atoms share electrons, the electrons are notshared equally. The shared electrons are attracted to the nucleusof one atom more than the other. An unequal sharing of electronsforms a polar covalent bond.
Usually, electrons are more attracted to atoms of elements thatare located farther to the right and closer to the top of the periodictable. The shading in Figure 16 shows that the shared electrons inthe ammonia gas, NH3, in the headspace of this container, arecloser to the nitrogen atom than they are to the hydrogen atoms.
Polyatomic IonsUntil now, we have talked about compounds that have eitherionic or covalent bonds. But some compounds have both ionicand covalent bonds. Such compounds are made of
which are groups of covalently bonded atoms that haveeither lost or gained electrons. A polyatomic ion acts the same asthe ions you have already encountered.
ions,polyatomic
156 C H A P T E R 5
Figure 16The darker shading around the nitrogen atom as com-pared to the hydrogen atoms shows that electrons aremore attracted to nitrogen atoms than to hydrogen atoms.So the bonds in ammonia are polar covalent bonds.
2e–6e–
2e–6e–
2e–5e–
2e–5e–
:O O:
: :
:N N:
Double covalent bond Triple covalent bond
Four electrons are in the shared electron cloud.
Six electrons are in the shared electron cloud.
Oxygen Nitrogen
—— ———
Ammonia
Disc One, Module 4:Chemical BondingUse the Interactive Tutor to learn moreabout this topic.
Figure 15The elements oxygen and nitrogen have covalentbonds. Electrons not involved in bonding are represented by dots.
There are many common polyatomic ionsMany compounds you use either contain or are made from poly-atomic ions. For example, your toothpaste may contain bakingsoda. Another name for baking soda is sodium hydrogen carbon-ate, NaHCO3. Hydrogen carbonate, HCO3
�, is a polyatomic ion.Sodium carbonate, Na2CO3, is often used to make soaps and othercleaners and contains the carbonate ion, CO3
2�. Sodium hydroxide,NaOH, has hydroxide ions, OH�, and is also used to make soaps.A few of these polyatomic ions are shown in Figure 17.
Oppositely charged polyatomic ions, like other ions, can bondto form compounds. Ammonium nitrate, NH4NO3, and ammo-nium sulfate, (NH4)2SO4, both contain positively chargedammonium ions, NH4
�. Nitrate, NO3�, and sulfate, SO4
2�, are bothnegatively charged polyatomic ions.
Parentheses group the atoms of a polyatomic ionYou might be wondering why the chemical formula for ammo-nium sulfate is written as (NH4)2SO4 instead of as N2H8SO4. Theparentheses around the ammonium ion are there to remind youthat it acts like a single ion. Parentheses group the atoms of theammonium ion together to show that the subscript 2 applies tothe whole ion. There are two ammonium ions for every sulfateion. Parentheses are not needed in compounds like ammoniumnitrate, NH4NO3, because there is a 1:1 ratio of ions.
Always keep in mind that a polyatomic ion’s charge appliesnot only to the last atom in the formula but to the entire ion. Thecarbonate ion, CO3
2�, has a 2� charge. This means that CO3, notjust the oxygen atom, has the negative charge.
Some polyatomic anion names relate to their oxygen contentYou may have noticed that many polyatomic anions are made ofoxygen. Most of their names end with -ite or -ate. These endingsdo not tell you exactly how many oxygen atoms are in the ion, butthey do follow a pattern. Think about sulfate (SO4
2�) and sulfite(SO3
2�), nitrate (NO3�) and nitrite (NO2
�), and chlorate (ClO3�) and
chlorite (ClO2�). The charge of each ion pair is the same. But
notice how the ions have different numbers of oxygen atoms.Their names also have different endings.
An -ate ending is used to name the ion with one more oxygenatom. The name of the ion with one less oxygen ends in -ite.Table 3, on the next page, lists several common polyatomicanions. As you look at this table, you’ll notice that not all of theanions listed have names that end in -ite or -ate. That’s becausesome polyatomic anions, like hydroxide (OH�) and cyanide(CN�), are not named according to any general rules.
T H E S T R U C T U R E O F M A T T E R 157
–
2–
+
Carbonate ion, CO
Hydroxide ion, OH–
Ammonium ion, NH4+
32–
Figure 17The hydroxide ion (OH�),carbonate ion (CO3
2�), andammonium ion (NH4
�) areall polyatomic ions.
SPACE SCIENCEMost of the ions andmolecules in spaceare not the same asthose that are found
on Earth or in Earth’s atmos-phere. C3H, C6H2, and HCO�
have all been found in space.So far, no one has been able tofigure out how these unusualmolecules and ions form inspace.
1. Determine if the following compounds are likely to haveionic or covalent bonds.a. magnesium oxide, MgO c. ozone, O3
b. strontium chloride, SrCl2 d. methanol, CH3OH
2. Identify which two of the following substances will conductelectricity, and explain why.a. aluminum foilb. sugar, C12H22O11, dissolved in waterc. potassium hydroxide, KOH, dissolved in water
3. Draw the structural formula for acetylene. Atoms bond inthe order HCCH. Carbon and hydrogen atoms share twoelectrons, and each carbon atom must have a total of fourbonds. How many electrons do the carbon atoms share?
4. Predict whether a silver coin can conduct electricity. Whatkind of bonds does silver have?
5. Describe how it is possible for calcium hydroxide, Ca(OH)2,to have both ionic and covalent bonds.
6. Explain why electrons are shared more equally in ozone, O3,than in carbon dioxide, CO2.
7. Analyze whether dinitrogen tetroxide, N2O4, has covalent orionic bonds. Describe how you reached this conclusion.
8. Critical Thinking Bond energy measures the energy per moleof a substance needed to break a bond. Which element hasthe greater bond energy, oxygen or nitrogen? (Hint: Whichelement has more bonds?)
S U M M A R Y
> Atoms bond when theirvalence electrons interact.
> Cations and anions attracteach other to form ionicbonds.
> When ionic compounds are melted or dissolved in water, moving ions canconduct electricity.
> Atoms in metals are joinedby metallic bonds.
> Metals conduct electricitybecause electrons canmove from atom to atom.
> Covalent bonds form whenatoms share electron pairs.Electrons may be sharedequally or unequally.
> Polyatomic ions are cova-lently bonded atoms that have either lost orgained electrons. Theirbehavior resembles that of simple ions.
> Name simple ionic and covalent compounds.> Predict the charge of a transition metal cation in an ionic
compound.> Write chemical formulas for simple ionic compounds.> Distinguish a covalent compound’s empirical formula
from its molecular formula.
Compound Names and Formulas
Just like elements, compounds have names that distinguishthem from other compounds. Although the compounds BaF2
and BF3 may appear to have similar chemical formulas, theyhave very different names. BaF2 is barium fluoride, and BF3 isboron trifluoride. When talking about these compounds, you havelittle chance for confusing their names. You can see that thenames of these compounds reflect the elements from which thecompounds are formed.
Naming Ionic CompoundsIonic compounds are formed by the strong attractionsbetween cations and anions. Both ions are important tothe compound’s structure, so it makes sense that bothions are included in the name.
Names of cations include the elements of which theyare composedIn many cases, the name of the cation is just like the nameof the element from which it is made. You have alreadyseen this for many cations. For example, when an atom ofthe element sodium loses an electron, a sodium ion, Na�,forms. Similarly, when a calcium atom loses two elec-trons, a calcium ion, Ca2�, forms. And when an alu-minum atom loses three electrons, an aluminum ion,Al3�, forms. These and other common cations are listedin Table 4. Notice how ions of Group 1 elements have a 1�
charge and ions of Group 2 elements have a 2� charge.
Names of anions are altered names of elementsAn anion that is made of one element has aname similar to the element. The differenceis the name’s ending. Table 5 lists some com-mon anions and shows how they are named.Just like most cations, anions of elements inthe same group of the periodic table havethe same charge.
NaF is made of sodium ions, Na�, andfluoride ions, F�. Therefore, its name issodium fluoride. Figure 18 shows how cal-cium chloride gets its name.
Some cation names must show their chargeThink about the compounds FeO and Fe2O3. According to therules you have learned so far, both of these compounds would benamed iron oxide, even though they are not the same compound.Fe2O3, a component of rust, is a reddish brown solid that meltsat 1565°C. FeO, on the other hand, is a black powder that meltsat 1420°C. These different properties tell us that they are differ-ent compounds and should have different names.
Iron is a transition metal. Transition metals may form severalcations—each with a different charge. A few of these cations arelisted in Table 6. The charge of the iron cation in Fe2O3 is differ-ent from the charge of the iron cation in FeO. In cases like this,the cation name must be followed by a Roman numeral in paren-theses. The Roman numeral shows the cation’s charge. Fe2O3 ismade of Fe3� ions, so it is named iron(III) oxide. FeO is made ofFe2� ions, so it is named iron(II) oxide.
160 C H A P T E R 5
Fluorine, F Fluoride ion, F� 1�
Chlorine, Cl Chloride ion, Cl�
Bromine, Br Bromide ion, Br�
Iodine, I Iodide ion, I�
Oxygen, O Oxide ion, O2� 2�
Sulfur, S Sulfide ion, S2�
Nitrogen, N Nitride ion, N3� 3�
Element name Ion name Ion and symbol and symbol charge
Table 5 Some Common Anions
CaCl2Ca2+ Cl−
Figure 18Ionic compounds are named fortheir positive and negative ions.
Determining the charge of a transition metal cationHow can you tell that the iron ion in Fe2O3 has a charge of 3�?Like all compounds, ionic compounds have a total charge ofzero. This means that the total positive charges must equal thetotal negative charges. An oxide ion, O2�, has a charge of 2�.Three of them have a total charge of 6�. That means the totalpositive charge in the formula must be 6�. For two iron ions tohave a total charge of 6�, each ion must have a charge of 3�.
Writing Formulas for Ionic CompoundsYou have seen how to determine the charge of each ion in a com-pound if you are given the compound’s formula. Following asimilar process, you can determine the chemical formula for acompound if you are given its name.
T H E S T R U C T U R E O F M A T T E R 161
Math SkillsMath Skills
Writing Ionic Formulas What is the chemical formula for aluminum fluoride?
List the symbols for each ion.Symbol for an aluminum ion from Table 4: Al3�
Symbol for a fluoride ion from Table 5: F�
Write the symbols for the ions with the cation first.Al3�F�
Find the least common multiple of the ions’ charges.The least common multiple of 3 and 1 is 3. To make aneutral compound, you need a total of three positivecharges and three negative charges.
To get three positive charges: you need only one Al3�
ion because 1 � 3� � 3�.To get three negative charges: you need three F� ionsbecause 3 � 1� � 3�.
Write the chemical formula, indicating with subscripts howmany of each ion are needed to make a neutral compound.
AlF3
4
3
2
1Practice
HINTOnce you have determined achemical formula, always checkthe formula to see if it makesa neutral compound. For thisexample, the aluminum ion hasa charge of 3�. The fluoride ionhas a charge of only 1�, butthere are three of them for atotal of 3�.
(3�) � (3�) � 0, so the chargesbalance, and the formula isneutral.
Writing Ionic Formulas Write formulas for the following ionic compounds.1. lithium oxide 3. titanium(III) nitride2. beryllium chloride 4. cobalt(III) hydroxide
Naming Covalent CompoundsCovalent compounds, like SiO2 (silicon dioxide) and CO2 (carbondioxide), are named using different rules than those used toname ionic compounds.
Numerical prefixes are used to name covalent compounds oftwo elementsFor two-element covalent compounds, numerical prefixes tellhow many atoms of each element are in the molecule. Table 7lists some of these prefixes. If there is only one atom of the firstelement, it does not get a prefix. Whichever element is farther tothe right in the periodic table is named second and ends in -ide.
There are one boron atom and three fluorine atoms in borontrifluoride, BF3. Dinitrogen tetroxide, N2O4, is made of two nitro-gen atoms and four oxygen atoms, as shown in Figure 19. Noticehow the a in tetra is dropped to make the name easier to say.
Chemical Formulas for Covalent CompoundsEmeralds, shown in Figure 20, are made of a mineral called beryl.The chemical formula for beryl is Be3Al2Si6O18. But how did peo-ple determine this formula? It took some experiments. Chemicalformulas like this one were determined by first measuring themass of each element in the compound.
A compound’s simplest formula is its empirical formulaOnce the mass of each element in a sample of the compound isknown, scientists can calculate the compound’s
or simplest formula. An empirical formula tells us thesmallest whole-number ratio of atoms that are in a compound.Formulas for most ionic compounds are empirical formulas.
Covalent compounds have empirical formulas, too. Theempirical formula for water is H2O. It tells you that the ratio ofhydrogen atoms to oxygen atoms is 2:1. Scientists have to ana-lyze unknown compounds to determine their empirical formulas.
formula,empirical
162 C H A P T E R 5
empirical formula thecomposition of a compoundin terms of the relative num-bers and kinds of atoms in thesimplest ratio
▲
1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-
Numberof atoms Prefix
Table 7 Prefixes Used toName Covalent Compounds
Figure 20 Emerald gemstones are cut from themineral beryl. Very tiny amounts ofchromium(III) oxide impurity in thegemstones gives them their beautifulgreen color.
N2O4Dinitrogen tetroxide
Figure 19One molecule of dinitrogen tetroxide has two nitrogen atomsand four oxygen atoms.
Determining empirical formulasIf a 142 g sample of an unknowncompound contains only the elementsphosphorus and oxygen and isfound to contain 62 g of P and 80 gof O, its empirical formula is easy tocalculate. This process is shown inFigure 21.
Different compounds can have thesame empirical formulaIt’s possible for several compounds tohave the same empirical formulabecause empirical formulas only rep-resent a ratio of atoms. Formaldehyde, acetic acid, and glucose allhave the empirical formula CH2O, as shown in Table 8. These threecompounds are not at all alike, though. Formaldehyde is some-times used to keep dead organisms from decaying so that they canbe studied. Acetic acid gives vinegar its sour taste and strong smell.And glucose is a sugar that plays a very important role in yourbody chemistry. Some other formula must be used to distinguishthese three very different compounds.
T H E S T R U C T U R E O F M A T T E R 163
Empirical Molar MolecularCompound formula mass formula Structure
Table 8 Empirical and Molecular Formulas for Some Compounds
Formaldehyde CH2O 30.03 g/mol CH2O
Acetic acid CH2O 60.06 g/mol 2 � CH2O � C2H4O2
Glucose CH2O 180.18 g/mol 6 � CH2O � C6H12O6
OxygenPhosphorus
80 g O x 1 mol O 16.00 g O
= 5.0 mol O
62 g P x 1 mol P 30.97 g P
= 2.0 mol P
Exactly 142 g of Unknown Compound
= P2O5Empirical formula
Figure 21 Once you determine the mass ofeach element in a compound, youcan calculate the amount of eachelement in moles. The empiricalformula for the compound is theratio of these amounts.
Molecular formulas are determined from empirical formulasFormaldehyde, acetic acid, and glucose are all covalent com-pounds made of molecules. They all have the same empiricalformula, but each compound has its own Acompound’s molecular formula tells you how many atoms are inone molecule of the compound.
In some cases, a compound’s molecular formula is the sameas its empirical formula. The empirical and molecular formulasfor water are both H2O. You can see from Table 8 on the previouspage that this is also true for formaldehyde. In other cases, acompound’s molecular formula is a small whole-number multi-ple of its empirical formula. The molecular formula for aceticacid is two times its empirical formula, and that of glucose is sixtimes its empirical formula.
molecular formula.
164 C H A P T E R 5
S E C T I O N 3 R E V I E W
1. Name the following ionic compounds, specifying the chargeof any transition metal cations.a. FeI2 c. CrCl2b. MnF3 d. CuS
2. Name the following covalent compounds:a. As2O5 c. P4S3 e. SeO2
b. SiI4 d. P4O10 f. PCl3
3. Explain why Roman numerals must be included in the namesof MnO2 and Mn2O7. Name both of these compounds.
4. Identify how many fluorine atoms are in one molecule ofsulfur hexafluoride.
5. Critical Thinking An unknown compound contains 49.47% C, 5.20% H, 28.85% N, and a certain percentage ofoxygen. What percentage of the compound must be oxygen?(Hint: The sum of the percentages should equal 100%.)
6. What is the charge of the cadmium cation in cadmiumcyanide, Cd(CN)2, a compound used in electroplating?Explain your reasoning.
7. Determine the chemical formulas for the following ioniccompounds:a. magnesium sulfate c. chromium(II) fluorideb. rubidium bromide d. nickel(I) carbonate
S U M M A R Y
> To name an ionic com-pound, first name thecation and then the anion.
> If an element can formcations with differentcharges, the cation namemust include the ion’scharge. The charge is writ-ten as a Roman numeral inparentheses.
> Prefixes are used to namecovalent compounds madeof two different elements.
> An empirical formula tellsthe relative numbers ofatoms of each element in a compound.
> A molecular formula tellsthe actual numbers ofatoms in one molecule of a compound.
> Covalent compounds haveboth empirical and molecu-lar formulas.
molecular formulaa chemical formula thatshows the number andkinds of atoms in a mol-ecule, but not the arrange-ment of atoms
The word organic has many different meanings. Most peopleassociate the word organic with living organisms. Perhaps
you have heard of or eaten organically grown fruits or vegetables.What this means is that they were grown using fertilizers andpesticides that come from plant and animal matter. In chemistry,the word organic is used to describe certain compounds.
Organic CompoundsAn is a covalently bonded compound madeof molecules. Organic compounds contain carbon and, almostalways, hydrogen. Other atoms, such as oxygen, nitrogen, sulfur,and phosphorus, are also found in some organic compounds.
Many ingredients of familiar substances are organic com-pounds. The effective ingredient in aspirin is a form of theorganic compound acetylsalicylic acid, C9H8O4. Sugarless chew-ing gum also has organic compounds as ingredients. Two ingre-dients are the sweeteners sorbitol, C6H14O6, and aspartame,C14H18N2O5, both of which are shown in Figure 22.
Carbon atoms form four covalent bonds in organic compoundsWhen a compound is made of only carbon and hydrogen atoms,it is called a hydrocarbon. Methane, CH4, is the simplest hydro-carbon. Its structure is shown in Figure 23. Methane gas isformed when living matter, such as plants, decay, so it is oftenfound in swamps and marshes. The natural gas used in Bunsenburners is also mostly methane. Carbon atoms have four valenceelectrons to use for bonding. In methane, each of these electronsforms a different CIH single bond.
A carbon atom may also share two of its electrons with twofrom another atom to form a double bond. Or a carbon atommay share three electrons to form a triple bond. However, a car-bon atom can never form more than a total of four bonds.
Alkanes have single covalent bondsAlkanes are hydrocarbons that have only single covalent bonds.Figure 23 shows that methane, the simplest alkane, has only CIHbonds. But alkanes can also have CIC bonds. You can see fromFigure 24 that ethane, C2H6, has a CIC bond in addition to sixCIH bonds. Notice how each carbon atom in both of these com-pounds bonds to four other atoms.
Many gas grills are fueled by another alkane, propane, C3H8.Propane is made of three bonded carbon atoms. Each carbonatom on the end of the molecule forms three bonds with threehydrogen atoms, as shown in Figure 25. Each of these end carbonatoms forms its fourth bond with the central carbon atom. Thecentral carbon atom shares its two remaining electrons with twohydrogen atoms. You can see only one hydrogen atom bonded tothe central carbon atom in Figure 25 because the second hydro-gen atom is on the other side.
166 C H A P T E R 5
Figure 25This camper is preparinghis dinner on a gas grillfueled by propane. Pro-pane is an alkane thathas two CIC bonds andeight CIH bonds.
Methane
Ethane
Propane
Figure 24Ethane, another alkane, has oneCIC bond and six CIH bonds.
Figure 23Methane is an alkane that hasfour CIH bonds.
Arrangements of carbon atoms in alkanesThe carbon atoms in methane, ethane, andpropane all line up in a row because that istheir only possible arrangement. When thereare more than three bonded carbon atoms, thecarbon atoms do not always line up in a row.When they do line up, the alkane is called anormal alkane, or n-alkane for short. Table 9shows chemical formulas for the n-alkanesthat have up to 10 carbon atoms. Condensedstructural formulas are also included in thetable to show how the atoms bond.
The carbon atoms in any alkane with morethan three carbon atoms can have more thanone possible arrangement. Carbon atom chainsmay be branched or unbranched, and they caneven form rings. Figure 26 shows some of thepossible ways six carbon atoms can bearranged when they form hydrocarbons withonly single covalent bonds.
Alkane chemical formulas usually follow a patternExcept for cyclic alkanes like cyclohexane, the chemical for-mulas for alkanes follow a special pattern. The number of hydro-gen atoms is always two more than twice the number of carbonatoms. This pattern is shown by the chemical formula CnH2n�2.
T H E S T R U C T U R E O F M A T T E R 167
Figure 26Hexane, 2-methylpentane,2,3-dimethylbutane, andcyclohexane are some ofthe forms six carbon atomswith single covalent bondsmay take.
Methane CH4 CH4
Ethane C2H6 CH3CH3
Propane C3H8 CH3CH2CH3
Butane C4H10 CH3(CH2)2CH3
Pentane C5H12 CH3(CH2)3CH3
Hexane C6H14 CH3(CH2)4CH3
Heptane C7H16 CH3(CH2)5CH3
Octane C8H18 CH3(CH2)6CH3
Nonane C9H20 CH3(CH2)7CH3
Decane C10H22 CH3(CH2)8CH3
Molecular Condensed n-Alkane formula structural formula
Alkenes have double carbon-carbon bondsAlkenes are also hydrocarbons. Alkenes are different from alkanesbecause they have at least one double covalent bond between car-bon atoms. This is shown by CJC. Alkenes are named like al-kanes but with the -ane ending replaced by -ene.
The simplest alkene is ethene (or ethylene), C2H4. Ethene isformed when fruit ripens. Propene (or propylene), C3H6, is usedto make rubbing alcohol and some plastics. The structures ofboth compounds are shown in Figure 27.
Alcohols have IOH groupsAlcohols are organic compounds that are made of oxygen as wellas carbon and hydrogen. Alcohols have hydroxyl, or IOH,groups. The alcohol methanol, CH3OH, is sometimes added toanother alcohol ethanol, CH3CH2OH, to make denatured alco-hol. Denatured alcohol is found in many familiar products, asshown in Figure 28. Isopropanol, which is found in rubbing alco-hol, has the chemical formula C3H8O, or (CH3)2CHOH. You mayhave noticed how the names of these three alcohols all end in -ol.This is true for most alcohols.
Alcohol molecules behave similarly to water moleculesA methanol molecule is like a water molecule except that one ofthe hydrogen atoms is replaced by a methyl, or ICH3, group. Justlike water molecules, neighboring alcohol molecules areattracted to one another. That’s why many alcohols are liquids atroom temperature. Alcohols have much higher boiling pointsthan alkanes of similar size.
168 C H A P T E R 5
Figure 27The peaches in this plastic container,which is made by joining propenemolecules, release ethene gas asthey ripen.
Methanol
Ethene
Figure 28Many products contain a mixtureof the alcohols methanol andethanol. This mixture is called“denatured alcohol.”
PolymersWhat do the DNA inside the cells of your body, rubber, wood, andplastic milk jugs have in common? They are all made of largemolecules called
Many polymers have repeating subunitsSome small organic molecules bond to form long chains calledpolymers. Polyethene, which is also known as polyethylene orpolythene, is the polymer plastic milk jugs are made of. Thename polyethene tells its structure. Poly means “many.” Ethene isan alkene whose chemical formula is C2H4. Therefore, poly-ethene is “many ethenes,” as shown in Figure 29. The originalmolecule, in this case C2H4, is called a monomer.
Some polymers are natural; others are man-madeRubber, wood, cotton, wool, starch, protein, and DNA are all nat-ural polymers. Man-made polymers are usually either plastics orfibers. Most plastics are flexible and easily molded, whereasfibers form long, thin strands.
Some polymers can be used as both plastics and fibers. Forexample, polypropene (polypropylene) is molded to make plasticcontainers, like the one shown in Figure 27, as well as some partsfor cars and appliances. It is also used to make ropes, carpet, andartificial turf for athletic fields.
The elasticity of a polymer is determined by its structureAs with all substances, the properties of a polymer are deter-mined by its structure. Polymer molecules are like long, thinchains. A small piece of plastic or a single fiber is made of bil-lions of these chains. Polymer molecules can be likened tospaghetti. Like a bowl of spaghetti, the chains are tangled but canslide over each other. Milk jugs are made of polyethene, a plasticmade of such noodlelike chains. You can crush or dent a milk jugbecause the plastic is flexible. Once the jug has been crushed,though, it does not return to its original shape. That’s becausepolyethene is not elastic.
When the chains are connected to each other, or cross-linked,the polymer’s properties change. Some become more elastic andcan be likened to a volleyball net. Like a volleyball net, an elasticpolymer can stretch. When the polymer is released, it returns toits original shape. Rubber bands are elastic polymers. As long asa rubber band is not stretched too far, it can shrink back to itsoriginal form.
polymers.
T H E S T R U C T U R E O F M A T T E R 169
polymer a large moleculethat is formed by more thanfive monomers, or small units
▲
Figure 29 Polyethene is a polymer made ofmany repeating ethene units. Asthe polymer forms, ethene’s dou-ble bonds are replaced by singlebonds.
Quick
ACTIVITYACTIVITYQuickQuickQuick
Polymer MemoryPolymers that return to theiroriginal shape after stretchingcan be thought of as having a“memory.” In this activity, youwill compare the memory ofa rubber band with that ofthe plastic rings that hold asix-pack of cans together.
1. Which polymer stretchesbetter without breaking?
2. Which one has bettermemory?
3. Warm the stretched six-pack holder over a hotplate, being careful notto melt it. Does it retainits memory?
Biochemical CompoundsBiochemical compounds are naturally occurring organic com-pounds that are very important to living things. Carbohydratesgive you energy. Proteins form important parts of your body, likemuscles, tendons, fingernails, and hair. The DNA inside yourcells gives your body information about what proteins you need.Each of these biochemical compounds is a polymer.
Many carbohydrates are made of glucoseThe sugar glucose is a Glucose provides energy toliving things. Starch, also a carbohydrate, is made of many bondedglucose molecules. Plants store their energy as chains of starch.
Starch chains pack closely together in a potato or a pasta noo-dle. When you eat such foods, enzymes in your body break downthe starch, making glucose available as a nutrient for your cells.Glucose that is not needed right away is stored as glycogen. Whenyou become active, glycogen breaks apart and glucose moleculesgive you energy. Athletes often prepare themselves for their eventby eating starchy foods. They do this so they will have moreenergy when they exert themselves later on, as shown in Figure 30.
Proteins are polymers of amino acidsMany polymers are made of only one kind of molecule. Starch, forexample, is made of only glucose. on the other hand,are made of many different molecules that are called
Amino acids are made of carbon, hydrogen, oxygen, andnitrogen. Some amino acids also contain sulfur. There are 20 amino acids found in naturally occurring proteins. The waythese amino acids combine determines which protein is made.
acids.amino
Proteins,
carbohydrate.
170 C H A P T E R 5
Figure 30 Athletes often eat lots of foodsthat are high in carbohydrates theday before a big event. This pro-vides them with a ready supply of stored energy.
carbohydrate any organiccompound that is made ofcarbon, hydrogen, and oxygenand that provides nutrients tothe cells of living things
protein an organic com-pound that is made of one ormore chains of amino acidsand that is a principal com-ponent of all cells
amino acid any one of 20different organic moleculesthat contain a carboxyl and anamino group and that com-bine to form proteins
Proteins are long chains made of amino acids. A smallprotein, insulin, is shown in Figure 31. Many proteins aremade of thousands of bonded amino acid molecules. Thismeans that millions of different proteins can be madewith very different properties. When you eat foods thatcontain proteins, such as cheese, your digestive systembreaks down the proteins into individual amino acids.Later, your cells bond the amino acids in a different orderto form whatever protein your body needs.
DNA is a polymer with a complex structureYour DNA determines your entire genetic makeup. It ismade of organic molecules containing carbon, hydro-gen, oxygen, nitrogen, and phosphorus.
Figuring out the complex structure of DNA was oneof the greatest scientific challenges of the twentieth cen-tury. Instead of forming one chain, like many proteinsand polymers, DNA is in the form of paired chains, orstrands. It has the shape of a twisted ladder known as adouble helix.
T H E S T R U C T U R E O F M A T T E R 171
Materials
What properties does a polymer have?
✔ water ✔ borax ✔ plastic spoons
✔ white glue ✔ 250 mL beakers (2) ✔ plastic sandwich bags
SAFETY CAUTION Wear safety goggles, gloves, and a laboratory apron. Be sure to work in an openspace and wear clothes that can be cleaned easily.
1. In one beaker, mix 4 g borax with 100 mL water,and stir well.
2. In the second beaker, mix equal parts of glueand water. This solution will determine theamount of new material made. The volume of diluted glue should be between 100 and200 mL.
3. Pour the borax solution into the beaker contain-ing the glue, and stir well using a plastic spoon.
4. When it becomes too thick to stir, remove thematerial from the cup and knead it with your fin-gers. You can store this new material in a plasticsandwich bag.
Analysis1. What happens to the new material when it is
stretched, or rolled into a ball and bounced?
2. Compare the properties of the glue with those of the new material.
3. The properties of the new material resulted from the bonds between the borax and the glue particles. If too little borax were used, inwhat way would the properties of the newmaterial differ?
4. Does the new material have the properties of a polymer? Explain how you reached thisconclusion.
SS
S
SS
S
Figure 31 Insulin controls the use and storage ofglucose in your body. Each color in thechain represents a different amino acid.
Your body has many copies of your DNAMost cells in your body have a copy of your genetic material inthe form of chromosomes made of DNA. For new cells to havethe right amount of DNA, the DNA must be copied. Copying can-not happen unless the two DNA strands are first separated.
Proteins called helicases unwind DNA by separating thepaired strands. Proteins called DNA polymerases then pair upnew monomers with those already on the strand. At the end ofthis process, there are two strands of DNA.
DNA’s structure resembles a twisted ladderDNA’s structure can be likened to a ladder. Alternating sugar mol-ecules and phosphate units correspond to the ladder’s sides, asshown in Figure 32. Attached to each sugar molecule is one offour possible DNA monomers—adenine, thymine, cytosine, orguanine. These DNA monomers pair up with DNA monomersattached to the opposite strand in a predictable way, as shown inFigure 32. Together, the DNA monomer pairs make up the rungsof the ladder.
172 C H A P T E R 5
S E C T I O N 4 R E V I E W
1. Identify the following compounds as alkanes, alkenes, oralcohols based on their names:a. 2-methylpentane d. butanolb. 3-methyloctane e. 3-heptenec. 1-nonene f. cyclohexanol
2. Explain why the compound CBr5 does not exist. Give anacceptable chemical formula for a compound made of onlycarbon and bromine.
3. Determine how many hydrogen atoms a compound has if itis a hydrocarbon and its carbon atom skeleton is CJCICJC.
4. Compare the structures and properties of carbohydrateswith those of proteins.
5. Identify which compound is an alkane: CH2O, C6H14, orC3H4. Explain your reasoning.
6. Critical Thinking Alkynes, like alkanes and alkenes, arehydrocarbons. Alkynes have carbon-carbon triple covalentbonds, or CKC bonds. Draw the structure of the alkyne thathas the chemical formula C3H4. Can you guess the name ofthis compound?
S U M M A R Y
> Alkanes have CIC andCIH bonds.
> Alkenes have CJC andCIH bonds.
> Alcohols have one or moreIOH groups.
> Polymers form when smallorganic molecules bond toform long chains.
> Biochemical compoundsare polymers important toliving things.
> Sugars and starches arecarbohydrates that provideenergy.
> Amino acids bond to formpolymers called proteins.
> DNA is a polymer shapedlike a twisted ladder.
Figure 32 In DNA, cytosine, C, always pairswith guanine, G. Adenine, A,always pairs with thymine, T.
KWL NotesKWL stands for “what I Know—what I Want to know—what I Learned”. The KWL strategyhelps you relate your new ideas and concepts with those you have already learned.
Read the section objectives.
We’ll use the first objective, “Distinguish between compounds and mixtures,”from Section 1.
Divide a blank sheet of paper into three columns, and label the columns “What I know,” “What I want to know,” and “What I learned.”
In the first column, write what information you know about the objective.
In the second column, write the information that you want to know about theobjective.
After you have read the section, write in the third column what you havelearned.
Use the remaining objectives from Section 1 to create a table of KWL notes. Compare theideas you wrote down in the first column with the items in the third column. If some ofyour initial ideas are incorrect, cross them out.
5
4
3
2
1
Study SkillsStudy SkillsStudy Skills
What I know What I want to know What I have learned
PracticePractice
water is a compound
mixtures can beseparated
grape juice is a mixture
how to distinguish betweencompounds and mixture
Compounds are heldtogether by chemical bonds,but mixtures are not.
Compounds are alwaysmade of the same propor-tion of elements, but mix-tures are not.
5. Ionic solidsa. are formed by networks of ions that have
the same charge.b. melt at very low temperatures.c. have very regular structures.d. are sometimes found as gases at room
temperature.
6. A chemical bond can be defined asa. a force that joins atoms together.b. a force blending nuclei together.c. a force caused by electric repulsion.d. All of the above
7. Which substance has ionic bonds?a. CO c. KClb. CO2 d. O2
8. Covalent bondsa. join atoms in some solids, liquids, and
gases.b. usually join one metal atom to another.c. are always broken when a substance is
dissolved in water.d. join molecules in substances that have
molecular structures.
9. A compound has an empirical formula CH2.Its molecular formula could bea. CH2. c. C4H8.b. C2H4. d. Any of the above
10. The chemical formula for calcium chloride isa. CaCl. c. Ca2Cl.b. CaCl2. d. Ca2Cl2.
11. The empirical formula of a moleculea. can be used to identify the molecule.b. is sometimes the same as the molecular
formula for the molecule.c. is used to name the molecule.d. shows how atoms bond in the molecule.
12. All organic compoundsa. come only from living organisms.b. contain only carbon and hydrogen.c. are biochemical compounds.d. have atoms connected by covalent bonds.
Chapter HighlightsBefore you begin, review the summaries of thekey ideas of each section, found at the end ofeach section. The key vocabulary terms arelisted on the first page of each section.
1. Which of the following is not true of compounds made of molecules?a. They may exist as liquids.b. They may exist as solids.c. They may exist as gases.d. They always have very high melting
points.
2. Compounds are different from mixturesbecausea. compounds are held together by chemical
bonds.b. each substance in a compound maintains
its own properties.c. each original substance in a compound
remains chemically unchanged.d. mixtures are held together by chemical
bonds.
3. What can be learned by looking at a modelof compound?a. chemical structureb. the strength of attraction between
moleculesc. the electron configuration of the atoms
involvedd. the types of bonds formed between the
atoms
4. Crystals of salt, called sodium chloride, area. made of molecules.b. made of a network of ions.c. chemically similar to sugar crystals.d. weak solids.
13. Compare the chemical structure of oxygendifluoride with that of carbon dioxide. Whichcompound has the larger bond angle?
14. Determine whether the chemical formulaC5H5N5 is the empirical formula or molecu-lar formula for adenine.
15. Name the following covalent compounds:a. SF4 c. PCl3b. N2O d. P2O5
16. Compare the metallic bonds of copper withthe ionic bonds of copper sulfide. Why aremetals rather than ionic solids used in elec-trical wiring?
17. Explain why proteins and carbohydrates arepolymers. What is each polymer made of?
18. Discuss two ways that atoms share electronsusing the terms nonpolar covalent bonds andpolar covalent bonds.
19. Compare ionic bonds and covalent bonds,and list two differences between them.
20. What does an organic compound contain?List several organic compounds that can befound in your body or in your daily life.
21. Describe the type of bonds that alkanes andalkenes have. How are they different? Arethere any alkanes or alkenes that you arefamiliar with?
22. What is a hydroxyl group? What organiccompound contains a hydroxyl group?
23. What is a hydrocarbon made of? Name themost simple hydrocarbon.
24. Graphing Which of the graphs below showshow bond length and bond energy arerelated? Describe the flawed relationshipsshown by each of the other graphs.
25. Graphing The melting points of elements inthe same group of the periodic table follow apattern. A similar pattern is also seen amongthe melting points of ionic compounds whenthe cations are made from elements that arein the same group. To see this, plot the melt-ing point of each of the ionic compounds inthe table below on the y-axis and the averageatomic mass of the element that the cation ismade from on the x-axis.
a. What trend do you notice in the meltingpoints as you move down Group 2?
b. BeCl2 has a melting point of 405°C. Isthis likely to be an ionic compound likethe others? Explain. (Hint: Locateberyllium in the periodic table.)
c. Predict the melting point of the ioniccompound RaCl2. (Hint: Check theperiodic table, and compare radium’slocation with the location of magne-sium, calcium, strontium, and barium.)
USING VOC ABULARYUSING VOC ABULARY
T H E S T R U C T U R E O F M A T T E R 175
BUILDING MATH SKILLSBUILDING MATH SKILLS
Ener
gy (
kJ/m
ol)
Ener
gy (
kJ/m
ol)
Ener
gy (
kJ/m
ol)
Bond length (pm) Bond length (pm) Bond length (pm)
30. Critical Thinking A classmate insists thatsodium gains a positive charge when itbecomes an ion because it gains a proton.Explain this student’s error.
31. Applying Knowledge Compare the threetypes of bonds based on what happens tothe valence electrons of the atoms.
32. Applying Knowledge In addition to carbonand hydrogen atoms, list four elements thatcan bond to carbon in organic compounds.
33. Critical Thinking Describe what attractiveforce(s) must be overcome to melt ice.
34. Applying Knowledge How many pairs ofelectrons are shared in the following typesof bonds?
a. a single bondb. a double bondc. a triple bond
35. Understanding Systems Explain why mostmetals are malleable and ductile but ioniccrystals are not.
36. Working Cooperatively For one day, writedown all of the ionic compounds listed onthe labels of the foods you eat. Also writedown the approximate mass youeat of each compound. As aclass, make a master list in theform of a computer spreadsheet thatincludes all of the ionic compounds eaten bythe whole class. Identify which compoundswere eaten by the most people. Together,create a poster describing the dietary guide-lines for the ionic compound that was eatenmost often.
26. Writing Ionic Formulas Determine thechemical formula for each of the followingionic compounds:
a. strontium nitrate, an ingredient in somefireworks, signal flares, and matches
b. sodium cyanide, a compound used inelectroplating and treating metals
c. chromium(III) hydroxide, a compoundused to tan and dye substances
d. aluminum nitride, a compound used inthe computer-chip-making process
e. tin(II) fluoride, the source of fluoridefor many toothpastes
f. potassium sulfate, a compound used inthe glass-making process
27. Evaluating Data A substance is a solid atroom temperature. It is unable to conductelectricity as a solid but can conduct elec-tricity as a liquid. This compound melts at755°C. Would you expect this compound tohave ionic, metallic, or covalent bonds?
28. Creative Thinking Dodecane is a com-bustible organic compound used in jet fuelresearch. It is an n-alkane made of 12 car-bon atoms. How many hydrogen atoms doesdodecane have? Draw the structural formulafor dodecane.
29. Applying Knowledge The length of a bonddepends upon its type. Predict the relativelengths of the carbon-carbon bonds in thefollowing molecules, and explain yourreasoning.
THINKING CR ITIC ALLYTHINKING CR ITIC ALLY
176 C H A P T E R 5
DEVELPPING LI FE/WORK SKILLSDEVELOPING LI FE/WORK SKILLS
37. Making Decisions People on low-sodiumdiets must limit their intake of table salt.Luckily, there are salt substitutes that do not contain sodium. Research differentkinds of salt substitutes, and describe howeach one affects your body. Determinewhich salt substitute you would use if youwere on a low-sodium diet.
38. Locating Information Numerical recyclingcodes identify the composition of a plasticso that it can be sorted and recycled. Foreach of the recycling codes, 1–6, identify the plastic, its physical properties, and atleast one product made of this plastic.
39. Interpreting and Communicating Cova-lently bonded solids, such as silicon, an element used in computer components, areharder than some pure metals. Researchtheories that explain the hardness of cova-lently bonded solids and their usefulness in the computer industry. Present your findings to the class.
40. Connection to Health The figure belowshows how atoms are bonded in a moleculeof vitamin C. Which elements isvitamin C made of? What is itsmolecular formula? Write aparagraph explain-ing some of thehealth benefits oftaking vitamin Csupplements.
41. Concept Mapping Copy the unfinished con-cept map below onto a sheet of paper.Complete the map by writing the correctword or phrase in the lettered boxes.
Photo Credits: Chapter Opener photo of breaking bottle by John Bagley, stained glass by RobertFrerck/Getty Images/Stone; FIg. 1A, E. R. Degginger/Color-Pic, Inc.; Fig. 1B, Yoav Levy/Phototake;Fig. 1C, Charles D. Winters; Fig.1D, Sam Dudgeon/HRW; Fig. 2, Sergio Purtell/Foca/HRW; Fig. 5, E.R. Degginger/Color-Pic, Inc.; Fig. 6, Dr. Dennis Kunkel/Phototake; Fig. 7, Sergio Purtell/Foca/HRW;“Quick Lab,” Sam Dudgeon/HRW; Fig. 8, Sergio Purtell/Foca/HRW; Fig. 12, Sam Dudgeon/HRW; Fig. 13, Erin Garvey/Index Stock Imagery, Inc.; “Quick Activity,” Sam Dudgeon/HRW; Fig. 16, RandallAlhadef/HRW; Fig. 18, Peter Van Steen/HRW; Fig. 20, Getty Images/The Image Bank; Fig. 22, PeterVan Steen/HRW; Fig. 25, Marc Grimberg/Getty Images/The Image Bank; Figs. 27-29, SamDudgeon/HRW; Fig. 30, David J. Phillip/AP/Wide World Photos; “Skills Practice Lab,” SergioPurtell/Foca; “Career Feature,” (portraits), Steve Fischbach/HRW; (barrels), W. & D. McIntyre/PhotoResearchers, Inc.
www.scilinks.orgTopic: Vitamin C SciLinks code: HK4147
� Procedure1. Prepare a data table in your lab report similar to the
one shown at right.
Making Latex RubberSAFETY CAUTION If youget a chemical on yourskin or clothing, wash itoff with lukewarm water while calling to your teacher. Ifyou get a chemical in your eyes, flush it out immediatelyat the eyewash station and alert your teacher.
2. Pour 1 L of deionized water into a 2 L container.
3. Use a 25 mL graduated cylinder to pour 10 mL of liquid latex into one of the paper cups.
4. Clean the graduated cylinder thoroughly with soapand water, then rinse it with deionized water and useit to add 10 mL of deionized water to the liquid latex.
5. Use the same graduated cylinder to add 10 mL ofacetic acid solution to the liquid latex-water mixture.
6. Stir the mixture with a wooden craft stick. As you stir,a “lump” of the polymer will form around the stick.
7. Transfer the stick and the attached polymer to the 2 Lcontainer. While keeping the polymer underwater,gently pull it off the stick with your gloved hands.
8. Squeeze the polymer underwater to remove anyunreacted chemicals, shape it into a ball, and removethe ball from the water.
9. Make the ball smooth by rolling it betweenyour gloved hands. Set the ball on a papertowel to dry while youcontinue with the nextpart of the lab.
Many polymers are able to “bounceback” after they are stretched, bent, orcompressed. In this lab, you will com-pare the bounce heights of two ballsmade from different polymers.
> Synthesize two different polymers,shape each into a ball, and measurehow high each ball bounces.
> Concludewhich polymer would make a bettertoy ball.
10. Wash your gloved hands with soap and water, then remove the gloves and dis-pose of them. Wash your hands again with soap and water.
Making an Ethanol-silicate PolymerSAFETY CAUTION Put on a fresh pair of gloves. Ethanol is flammable, so make surethere are no flames or other heat sources anywhere in the laboratory.
11. Use a clean 25 mL graduated cylinder to pour 12 mL of sodium silicate solutioninto the clean paper cup.
12. Use a 10 mL graduated cylinder to add 3 mL of the ethanol solution to thesodium silicate solution.
13. Stir the mixture with the clean wooden craft stick until a solid polymer forms.
14. Remove the polymer with your gloved hands, and gently press it between yourpalms until you form a ball that does not crumble. This activity may take sometime. Occasionally dripping some tap water on the polymer might be helpful.
15. When the ball no longer crumbles, dry it very gently with a paper towel.
16. Repeat step 10, and put on a fresh pair of gloves.
17. Examine both polymers closely. Record in your lab report how the two polymersare alike and how they are different.
18. Use a meterstick to measure the highest bounce height of each ball when each isdropped from a height of 1 m. Drop each ball five times, and record the highestbounce height each time in your data table.
� Analysis1. Calculate the average bounce height for each ball by adding the five bounce
heights and dividing by 5. Record the averages in your data table.
2. Based on only their bounce heights, which polymer would make a better toy ball?
� Conclusions3. Suppose that making a latex rubber ball costs 22 cents and that making an
ethanol-silicate ball costs 25 cents. Does this fact affect your conclusion aboutwhich polymer would make a better toy ball? Besides cost, what are other important factors that should be considered?
Have you ever looked at something and wondered what chemicals it contained? That’s what analytical chemists do for a living. They use a range of tests to determine the chemical makeup of a sample. To find out more about analytical chemistry as a career, read the interview with analytical chemist Roberta Jordan, who works at the Idaho National Engineering and Environmental Laboratory, in Idaho Falls, Idaho.
Analytical Chemist
What is your work as an analytical chemist like?
We deal with radioactive waste generatedby old nuclear power plants and old sub-marines, and we try to find a safe way tostore the waste. I’m more like a consultant.A group of engineers that are working on aprocess will come to me. I tell them whatthings they need to analyze for and whythey need to do that. On the flip side, I’lltell them what techniques they need to use.
What do you like best aboutyour work?
It forces me to stay current with any newtechniques, new areas that are going on inanalytical chemistry. And I like the team approach because it allows me to work ondifferent projects.
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What do you find mostinteresting about yourwork?
Probably the most interesting thing is to observe how different industries and differ-ent labs conduct business. It gives you abroad feel for how chemistry is done.
What qualities does a goodchemist need?
I think you do need to be good at scienceand math and to like those subjects. Youneed to be fairly detail-oriented. You have to be precise. You need to be analytical ingeneral, and you need to be meticulous.
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In addition to working as an analytical chemist,Roberta Jordan mentorsstudents regularly in thelocal schools.
“Chemistry is ineverything wedo. Just to takea breath and eata meal involveschemistry.”
What part of your education doyou think was most valuable?
I think it was worthwhile spending a lot of energy on my lab work. With any science, themost important part is the laboratory experience,when you are applying those theories that youlearn. I’m really a proponent of being involved inscience-fair activities.
What advice do you have for students who are interested inanalytical chemistry?
It’s worthwhile to go to the career center or library and do a little research. Take the time tofind out what kinds of things you could do withyour degree. You need to talk to people whohave a degree in that field.
Do you think chemistry has abright future?
I think that there are a lot of things out therethat need to be discovered. My advice is to gofor it and don’t think that everything we needto know has been discovered. Twenty to thirtyyears down the road, we will have to think of anew energy source, for example.
“One of the things necessary to be agood chemist is you have to be creative.You have to be able to think above andbeyond the normal way of doing thingsto come up with new ideas, new experiments.”
