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chaprer previeuj

sections

1 Stability in BondingLab Atomic TradingCards

Types of Bonds

J Writing Formulas and NamingCompoundsLab ModelingChemical Bonding

y/0 Virtual Lab How can you tell whichelements form chemical bonds?

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fjs*

ftemrits Form Ci)&rdoa\ BondsJust as these skydivers are linked togetherto make a stable formation, the atoms inelements can link together with chemicalbonds to form a compound. You will readabout how chemical bonds form and learn

how to write chemical formulas and

equations.

Science Journal Describe how glue is similar tochemical bonds.

Start-Up Activities

fiaun^i

Chemical Bonds and MixingYou probably have noticed that some liquidssuch as oil-and-vinegar salad dressingswill notstay mixed after the bottle is shaken. However,

rubbing alcohol and water will mix. The com

pounds that make up the two liquids are different. This lab will demonstrate the influence the

types of chemical bonds have on how the

compounds mix. &&• -^ (§) ~\u

1. Complete the safetyform.

2. Pour 20 mL ofwater intoa 100-mL graduated cylinder.

3. Pour 20 mL of vegetable oil into the samecylinder. Vigorously swirl the two liquidstogether, and observe for several minutes.

4. Add two drops offood dye and observe.

5. After several minutes, slowly pour30 mL ofrubbingalcohol into the cylinder.

6. Add two more drops of food dye andobserve.

7. Think Critically In your Science Journal,write a paragraph describing how thedifferent liquids mixed. Would yourfinalresults be different if you added theliquids in a different order? Explain.

FOLDABLES|Study Organizer

Chemical Formulas Everycompound has a chemical formula that tells exactly which

elements are present in that compound andexactly how many atoms of each element arepresent in that compound. Make the followingFoldable to help identify the chemical formulasin this chapter.

Fold a sheet of notebook paper vertically from side to side.

Cut along every third line of only thetop layer to form tabs.

Read and Write Go through the chapter, findten chemical formulas, and write them on the

front of the tabs. As you read the chapter, writewhat compound each formula represents underthe appropriate tab.

SciencejrPreview thischapter's contentand activities at

gpescience.com

I Stability in BondingReading Guide f

0 Review VocabularyTi/falt You'll Learn *W6%> It's Important• Describe how a compound differs The millions of different kinds compound: substance formed from

from its component elements. of matter around us are a result two or more elements in which the

• Explain what a chemical formula of chemical bonds. exact combination and proportion ofrepresents. elements is always the same

• Explain that the electric forcesbetween electrons and protons, New Vocabularywhich are oppositely charged, are • chemical formula

essential to forming compounds. • ion

• State a reason why chemicalbonding occurs.

__ •*

Figure 1 The differencebetween the elemental copper

metal and the copper compound

formed on the Statue of Liberty

is striking.

688 CHAPTER 22 Chemical Bonds

Combined ElementsHave you noticed the color of the Statueof Liberty? It isgreen.

The Statue of Liberty is made of copper, which is an element.Uncombined, elemental copper is a bright, shiny copper color.

When copper is exposed to the weather over alongperiodof time, the metalcombines withsubstances in the atmosphere and changes color.

Compounds Some of the matter around you isin the form of uncombined elements such as

copper, sulfur, and oxygen. However, like manyother sets of elements, these three elements canunite chemically to form a compound when theconditions are right. The green coating on theStatueof Libertyand some old pennies is a resultof this chemical change. One compound in this

coating, seen in contrast with the elemental copper in Figure 1, is a compound called copper sulfate. Coppersulfate isn't shiny and copper coloredlike elemental copper. Nor is it a pale-yellow solid like sulfur or a colorless,odorless gas like oxygen. It has its ownunique properties.

_

H

•\

-> Na]+ [:c|:Sodium + Chlorine —> Sodium chloride

New Properties An observation you will make is thatthe compound formed whenelements combine often has

chemical and physical properties that aren't anything likethose of the individual ele

ments. Sodium chloride, forexample, shown in Figure 2, isa compound made from theelements sodium and chlo

rine. Sodium is a shiny, soft,silvery metal that reacts violently with water. Chlorine isa poisonous greenish yellowgas. Would you have guessedthat these elements combine

to make ordinary table salt?

Figure 2 Sodium is a soft, silvery metal thatcombines with chlorine, agreenish yellow gas, to form sodium chloride, which isa white crystalline solid.Describe how theproperties of table saltaredifferent from thoseofsodium andchlorine.

FormulasThe chemical symbols Na and CI represent the elements

sodium andchlorine. When written as NaCl, thesymbols make upaformula, or chemical shorthand, for thecompound sodium chloride. A chemical formula tells what elements a compound contains and the exact number of the atoms of

each element in a unit of that compound. Thecompound that you are probablymost familiarwith is H20, commonly known as water. Thisformula contains the symbols H for theelement hydrogen and O for the element oxygen. Notice the number 2writtenasa subscriptafter the H for hydrogen. Subscript means"written below." A subscript written after asymbol tells how many atoms of that elementare in a unit of the compound. If a symbol hasno subscript, the unit contains only one atomof that element. A unit of H20 contains twohydrogen atoms and one oxygen atom.

Lookat the formulas for each compoundlisted in Table 1. What elements combine toform each compound? How many atoms ofeach element are required to form each of thecompounds?

lartHMMUhlMM Describe what a chemicalformula tellsyou.

Table 1 Some Familiar Compound

Familiar Name Chemical Name Formula

Sand

Milk ofmagnesia

Cane sugar

Lime

Vinegar

Laughing gas

Grain alcohol

Battery acid

Stomach acid

Silicon dioxide SiO,

Magnesium hydroxide Mg(0H)2

Sucrose

Calcium oxide

Acetic acid

Dinitrogen oxide

Ethanol

Sulfuric acid

Hydrochloric acid

Ci2"zrii

CaO

CH3COOH

N20

C2H5OH

H2S04

HCI

SECTION 1 Stability in Bonding 689

1 2

Li Be

2fe

• fe St;*

* it *

« •

Na Mg

3fe ' *••

fe «. * * •" • #'•'*•• ... *

e • Jfe, -.,.,.'• «

'*• fefe •'•"•>..•

fe fe

13 14 15 16 17 18

B C N 0 F Ne

, *;*v.... *fe fe

• t fe fe* fe fe fe

fe ,frfr.. fe

• • * * « •

ft-'

•

fe•-' fe

••-. • ••

fe '•-•.<•• - fe

• •••••••••''"• fe

Al Si P S CI Ar

* fe « fe fe

fe fefe «, fe fe

fe fefe fc 4, fe

fe ' fe* # fe fe

« fe * fe« fe fe &

fe fe# fe fe fe

• i ft- *fe W fem *

fe w fe . • : * 4> * : • :s 4>

«> fe fe fefe •

fc • *. «fe # « *

fe fe fe & « -- % * fe fe

Figure 3 The number ofelectrons in each group's outer level

increases across the table until the

noble gases in Group 18, where

each has a complete outer energy

level.

Atomic StabilityWhy do atoms form compounds? The electric forces

between electrons and protons, which are oppositely charged,hold atoms and molecules together, and thus they are the forcesthat cause compounds to form. The periodic table on the insideback cover of your book lists the known elements. However, thesix noble gases in Group 18 do not form compounds, or do sowith difficulty. Atoms of noble gases are unusually stable.Compounds of these atoms rarely form because they are almostalways less stable than the original atoms.

The Unique Noble Gases To understand the stabilityof thenoble gases, it is helpful to look at electron dot diagrams acrossa period. Electron dot diagrams show only the electrons in theouter energy level of an atom. They contain the chemical symbol for the element surrounded by dots representing its outerelectrons. How do you know how many dots to make? ForGroups 1 and 2 and 13 through 18, you can use the periodictable or the portion of it shown in Figure 3. Elements in Group1 each have one outer electron. Those in Group 2 each have two.Those in Group 13 each have three, those in Group 14, four, andso on to Group 18, the noble gases, which each have eight.

Chemical Stability An atom is chemically stable when itsoutermost energy level has the maximum number of electrons.The outer energy levels of helium and hydrogen are stable withtwo electrons. The outer energylevels of all the other elementsare stable when they contain eight electrons. Figure 4 showselectron dot diagrams of some of the noble gases. Notice thateight dots surround Kr, Ne, Xe, Ar, and Rn, and two dotssurround He.

Figure 4 Electron dotdiagramsof noblegases show that they eachhave a stable outer energy level.

9 9

He9 9

•Kr 99 9

9 9

:Ne:9 9

9 9

* AG*9 9

9 9

9 §\i 99 9

9 9

•Kris9 9


Energy Levels and Other Elements How do the dot diagrams represent other elements, and how does this relate to theirability to make compounds? Hydrogen and helium, the elementsin period 1 of the periodic table, can hold a maximum of two electrons in their outer energy levels. Hydrogen contains one electronin its lone energy level. A dot diagram for hydrogen has a singledot next to its symbol. This means that hydrogen's outer energylevel is not full. It is more stable when it is part of a compound.

In contrast, helium's outer energy level contains two electrons. Its dot diagram has two dots—a pair of electrons—nextto its symbol. Helium has a full outer energy level and is chemicallystable. Helium rarely forms compounds and the element isa commonly used gas.

When you look at the elements in Groups 13 through 17, yousee that none of the elements hasa stable energy level. Each groupcontains too few electrons for a stable level of eight electrons.

Reading Check Why is it rareforhelium to form a compound?

Outer Levels—Getting Their Fill As you just learned,hydrogen is an element that does not have a full outer energylevel. How does hydrogen, or any other element, becomestable?Atoms with partially stable outer energylevels can lose, gain,orshare electrons to obtain stable outer energy levels. They do thisbycombining with other atoms that also have partially completeouter energy levels. As a result, each becomes stable. Figure 5shows electron dot diagrams for sodium and chlorine. Whenthey combine, sodium loses one electron and chlorine gains oneelectron. You can see from the electron dot diagram that chlorine now has a stable outer energy level, similar to a noble gas.But what about sodium?

ScienceSTopic: Dot DiagramsVisit gpescience.com for Web

links to information about using

dot diagrams to represent outer

energy level electrons.

Activity Draw a dot diagram ofmethane, CH..

• fe

Na*

Sodium

CI

• •

Figure 5 Sodium, as a Group 1element, will lose one electron. It

will have the same number of elec

trons as neon. Chlorine, which

belongs to Group 17,will gainoneelectron. It will have the same

number of electronsas argon.Identify thegroup number whereneon and argon can be found.

SECTION! Stability in Bonding 691

H

Figure 6 In water, hydrogenand oxygen each contribute one

electron to each hydrogen-oxygen

bond. The atoms share those elec

trons instead of giving them up.

Stability Is Reached Sodium had only one electron in itsouter energy level, which it lost when it combined with chlorineto form sodium chloride. However, look back to the next, outermost energy level of sodium. This is now the new outer energylevel, and it is stable with eight electrons. When the outer electron of sodium is removed, a complete inner energy levelbecomes the new outer energy level. Sodium and chlorine arestable now because of the exchange of an electron.

In the compound water, each hydrogen atom needs one electron to have a stable outer energy level. The oxygen atom needstwo electrons for its outer level to be stable with eight electrons.Hydrogen and oxygen become stable and form bonds in a different way from sodium and chlorine. Instead of gaining or losing electrons, they share them. Figure 6 shows how hydrogenand oxygen share electrons to achieve a more stable arrangement of electrons.

Atoms, too, lose or gain to meet a goal—a stable energylevel. They do not lose or gain an advantage. Instead, they loseor gain electrons. An atom that has lost or gained electrons iscalled an ion. An ion is a charged particle because it now haseither more or fewer electrons than protons. The positive andnegative charges are not balanced. It is the electric forcesbetween oppositely charged particles, such as ions, that holdcompounds together.

SummaryCombined Elements

• When elements combine, the new compoundhas unique properties that are different fromthe original properties of the elements.

Formulas

• Chemical symbols and numbers are shorthandfor the elements and their amounts in chemi

cal formulas.

Atomic Stability

• The elements of Group 18, the noble gases,rarely combine with other elements.

• Electron dot diagrams show the electrons inthe outer energy level of an atom.

• Mostatoms need eight electrons to completetheir outer energy levels.

Self Check

1. Compareand contrast the properties of the individual elements that combine to make salt with the compound salt.

2. Identify whatthe formula BaF2 tells you about thiscompound.

3. Explain why some elements are stable on their ownwhile others are more stable in compounds.

4. Think Critically The label on a box of cleanser statesthat it contains CH3COOH. What elements are in thiscompound? How manyatoms ofeach element can befoundinaunitofCH3COOH?


Applying Math

5. Use Percentages Given that the molecularweight ofMg(OH)2, magnesium hydroxide, is 58.32 g,what percentage of this compound is oxygen?

More Section Review gpescience.com

Atomic Trading CardsPerhaps you have seen or collected tradingcards of famous athletes. Usually, each card hasa picture of the athlete on one side with important statistics related to the sport on the back.Atomsalso can be identified by their propertiesand statistics.

© Real-World ProblemHow can a visible model show how energy levels fill when atoms combine?

Goals

• Display the electrons ofelements accordingto their energy levels.

• Compare and classify elements accordingto their outer energy levels.

Materials

4-in X 6-in index cards

periodic table

© Procedure1. You will get an assigned element from your

teacher. Write the following informationforyour element on your index card: name, symbol, group number, atomic number, atomicmass, and metal, nonmetal, or metalloid.

2. On the other side of your indexcard, showthe number of protons and neutrons in thenucleus (e.g., 6p for six protons and 6n forsix neutrons for carbon).

3. Draw circles around the nucleus to represent the energy levels of your element.The number of circles you will need is thesame as the period the element is in on theperiodic table.

4. Draw dots on each circle to represent theelectrons in each energy level. Remember,elements in row 1 become stable with two

outer electrons, while levels two and three

become stable with eight electrons.

5. Look at the picture side only of four or fiveof your classmates' cards. Identify the element and the group to which it belongs.

© Conclude and Apply1. As you classify the elements according to

their group number, what pattern do yousee in the number of electrons in the outer

energy levels?

2. Atoms that give up electrons combine withatoms that gain electrons to form com

pounds. In your ScienceJournal, predictsome pairs of elements that combine inthis way.

ommuiiicating

Make a graph that relates the groups to thenumber of electrons in their outer energylevels. For more help, refer to the ScienceSkill Handbook.

LAB 693

u

Types of BondsReading Guide

*7V&a£ You'll Learn tWty> It's Important 1*Jew Vocabulary

• Describe ionic bonds and cova- Bond type determines how <i chemical bond

lent bonds. compounds mix and interact <> ionic bond

• Identify the particles produced with other compounds. <» covalent bond

by ionic bonding and by covalent „.. <» molecule

bonding. 9 Review Vocabulary <» polar molecule

• Distinguish between a nonpolar atom: the smallest particle of an (» nonpolar molecule

covalent bond and a polar cova element that still retains the properlent bond. ties of the element

Figure 7 Agoiter, an enlargement of the thyroid gland in the

neck, can be caused by iodine

deficiency.


Gain or Loss of ElectronsYou and a friend decide to go to the movies.When you arrive

at the theater, you discover that you do not have enough moneyto buy a ticket. Your friend has enough money for both ticketsand loans you the money. Now you both have enough moneyto go to the movies.

Recall that atoms also can loan electrons to other atoms so

that both can reach a stable energy level. When atoms gain, lose,or share electrons, an attraction forms that pulls the atomstogether to form a compound. This attraction is called a chemical bond. A chemical bond is the force that holds atomstogether in a compound. A compound has different physicaland chemical properties from those of the atoms that make upthe compound.

Some of the most common compounds are made by the lossand gain of just one electron. These compounds contain an element from Group 1 on the periodic table and an element fromGroup 17. Some examples are sodium chloride, commonly knownas table salt, andpotassium iodide, an ingredient in iodized salt.

INTEGRATE

Health

Why do people need iodized salt? A lackof iodine causes a wide range of problemsin the human body. The most obvious is

an enlarged thyroid gland, as shown in Figure 7, but theproblems can include mental retardation, neurological disorders, and physical problems.

A Bond Forms What happens when potassium and iodineatoms collide? A neutral atom of potassium has one electron inits outer level. This is not a stable outer energy level. Whenpotassium forms a compound with iodine, potassium loses oneelectron from its fourth level, and the third level becomes the

complete outer level. However, the atom is no longer neutral.The potassium atom has become an ion. When a potassiumatom loses an electron, the atom becomes positively chargedbecause there is one electron less in the atom than there are protons in the nucleus. The 1+ charge is shown as a superscriptwritten after the element's symbol, K+, to indicate its charge.Superscript means "written above."

The iodine atom in this reaction undergoes change as well.An iodine atom has seven electrons in its outer energy level.Recall that a stable outer energy level contains eight electrons.During the reaction with potassium, the iodide atom gains anelectron, leaving its outer energy level with eight electrons. Thisatom is no longer neutral because it gained an extra negativeparticle. It now has a charge of 1—and is called an iodide ion,written as I". The compound formed between potassium andiodine is called potassium iodide. The dot diagrams for theprocess are shown in Figure 8.

Reading Check What part ofan ion'ssymbolindicatesitscharge?

Another way to look at the electron in the outer shell of apotassium atom is as an advertisement to other atoms saying,"Available: One electron to lend." The iodine atom would havethe message, "Wanted: One electron to borrow." When the twoatoms get together, each becomes a stable ion. Notice that theresulting compound has a neutral charge because the positiveand negative charges of the ions cancel each other.

• •

Ik*

K'

• • •

• ft c

• » «

O * :• ' ft « « •

-»K

->

Ions and Nerve Cells

Ions are important inmany processes in yourbody. The movement ofmuscles is just one ofthese processes. Musclemovement would be

impossible without themovement of ions in and

out of nerve cells.

Research the type of ionsused by the nerve cells.

Figure 8 Potassium and iodinemust perform a transfer of one

electron. Potassium and iodine end

up with stable outer energy levels.Infer whythesizeof theionchanges with thechange inelectrondistribution.

+ • • •

o

K+

:|:

SECTION 2 Types of Bonds 695

Figure 9 This sparkler containsiron, which burns in air to produce

an ionic compound that contains

iron and oxygen.

Figure 10 Amagnesium atomgives an electronto each of twochlorine atoms to form MgCI2.

The Ionic BondWhen ions attract in this way, a bond is formed. An ionic

bond is the force of attraction between the opposite charges ofthe ions in an ionic compound. In an ionic bond, a transfer ofone or more electrons takes place. With this transfer of electronsto form an ionic compound, a large amount of energy isreleased, as shown in Figure 9.

Now that you have seen how an ionic bond forms when oneelectron is involved, see how it works when more than one elec

tron is involved. The formation of magnesium chloride, MgCl2,is another example of ionic bonding. When magnesium reactswith chlorine, a magnesium atom loses two electrons andbecomes a positively charged ion, Mg2+. At the same time, twochlorine atoms gain one electron each and become negativelycharged chloride ions, Cl~. In this case, a magnesium atom hastwo electrons to lend, but a single chlorine atom needs to borrowonly one electron. Therefore, it takes two chlorine atoms, asshown in Figure 10, to take the two electrons from the magnesium ion to form the compound magnesium chloride.

Zero Net Charge The result of this bond is a neutral compound. The compound as a whole is neutral because the sum ofthe charges on the ions is zero. The positive charge of the magnesium ion is equal to the negative charge of the two chloride ions.In other words, when different atoms form an ionic compound,electrons move from one atom to a different atom, but the overallnumber of protons and electronsof the combined atoms remainsequal and unchanged. Therefore, the compound is neutral.

Metals and nonmetals usually combine by forming ionicbonds.Looking at the periodic table, youwill see that the elementsthat bond ionically areoftenacross the tablefrom each other.Ioniccompounds are often crystalline solids with high melting points.

CI

CI Mg

CI

Magnesium 2 chlorine atoms

[:Q:]- Mg2+ [.'&]'

Magnesium chloride

696 CHAPTER22 Chemical Bonds

Sharing ElectronsSome atoms of nonmetals are unlikely to lose or gain elec

trons. For example, the elements in Group 14 of the periodictable have four electrons in their outer levels. They would haveto either gain or lose four electrons to have a stable outer level.Losing four electrons takes a great deal of energy. Each time anelectron is removed, the nucleus holds the remaining electronseven more tightly. These atoms become more chemically stableby sharing electrons, rather than by losing or gaining electrons.

The attraction that forms between atoms when they shareelectrons is known as a covalent bond. A neutral particle thatforms as a result of electron sharing is called a molecule, asshown in Figure 11.

Single Covalent Bonds Asingle covalent bond is made upof two shared electrons. Usually, one of the shared electronscomes from one atom in the bond and the other comes from the

other atom in the bond. A water molecule contains two singlebonds. In each bond, a hydrogen atom contributes one electronto the bond, and the oxygen atom contributes the other.The twoelectrons are shared, forming a single bond. The result of thistypeof bonding is a stable outer energylevel for each atom in themolecule.

Multiple Bonds A covalent bond also can contain more thanone pair of shared electrons. An example of this is the bond innitrogen (N2), shown in Figure 12.A nitrogen atomhas five electrons in its outer energy level and needs three more electrons tobecome stable. It does this by sharing three of its electrons withanothernitrogenatom. The other nitrogen atom also shares threeof its electrons. When each atom contributes three electrons, theyshare six electrons, or three pairs of electrons. Each pair of electrons represents a bond. Therefore, three pairs of electrons represent three bonds, or a triple bond. Each nitrogen atom is stablewith eight electrons in its outer energy level. In a similar way, abond that contains twoshared pairsof electrons isa double bond.Carbon dioxide is an example of a molecule with double bonds.

Covalent bonds form between nonmetallic elements. Theseelements are close together in the upper right-hand corner ofthe periodic table. Many covalent compounds are liquids orgases at room temperature.

• • • •

H* f\ *

• •

HFigure 11 Each ofthe pairs ofelectrons between the two hydro

gens and the oxygen is shared as

each atom contributes one elec

tron to the pair to make the bond.

Figure 12 The dot diagramshows that the two nitrogen atomsin nitrogen gas share sixelectrons.Explain which ofthese gaseswould require themost energy toreact with another element to form

a compound, H2 or N2.

*N. + •N. •> :N:!n:


Observing a Bond Type

Procedure SEHfc1. Complete the safety form.2. Turn on the faucet to

produce a thin streamof water.

3. Rub an inflated balloon

with wool or fur.

4. Bring the balloon near thestream of water, anddescribe what you see.

Analysis1. Explain your observations.2. Relate the attraction

between the balloon and

the water to the attraction

between the north and

south poles of twomagnets. Why mightwater act like A^1^^,a magnet? m^W a* %

viofl\e

Figure 13 The chlorine atomexerts the greater pull on the

electrons in hydrogen chloride,which forms hydrochloric acid in

water.

Explain why the chlorine atom hasagreater pull than the hydrogen atom.


Unequal Sharing Electrons are not always shared equallybetween atoms in a covalent bond. The strength of the attractionof each atom to its electrons is related to the size of the atom, thecharge of the nucleus, and the total number of electrons the atomcontains. Part of the strength of attraction has to do with how farthe electron being shared is from the nucleus. For example, amagnet has a stronger pull when it is right next to a piece of metalrather than several centimeters away. The other part of thestrength of attraction has to do with the size of the positive chargein the nucleus. Using a magnet as an example again, a strong magnet will hold the metal more firmly than a weak magnet will.

One example of this unequal sharing is found in a moleculeof hydrogen chloride, HCl, which is shown in Figure 13. Inwater, HCl is hydrochloric acid, which is used in laboratories andin industry to clean metal, and is found in your stomach, whereit helps to digest food. Chlorine atoms have a stronger attractionfor the shared electrons than hydrogen atoms do. As a result, theshared electrons in hydrogen chloride will spend more time nearthe chlorine atom than near the hydrogen atom. The chlorineatom has a partial negative charge, represented by a lowercaseGreek symbol delta followed by a negative superscript, 8". Thehydrogen atom has a partial positive charge, represented by a8+.

Reading Check What determines the strength ofattraction ofan atom?

Tug-Of-War You might think of a covalent bond as the rope ina tug-of-war, and the shared electrons as the knot in the centerof the rope. Figure 14 illustrates this concept. Each atom in themolecule attracts the electrons that they share. However, sometimes the atoms aren't the same size.

The same thing happens in tug-of-war. Sometimes one teamhas more people or stronger participants than the other. Whenthis is true, the knot in the middle of the rope ends up closer tothe stronger team. Similarly, the electrons beingshared in a molecule are held more closely to the atoms with the stronger pullor larger nucleus.

(partial negative

(partial positive • • charge)charge) 0-

8+

x

VdrochlorSiI ACID CON. i

1 "Ci__J_

• •

CI

• •

NATIONALGEOGRAPHIC VISUALIZING POLAR MOLECULES

Figure 14

fhen playing tug-of-war, if there are more—or stronger—teammembers on one end of the rope than the other, there is an unequalbalance of power.The stronger team can pull harder on the rope

and has the advantage. A similar situation exists in polar molecules, in whichelectrons are attracted more strongly by one type of atom in the moleculethan another. Because of this unequal sharing of electrons, polar moleculeshave a slightly negative end and a slightly positive end, as shown below.

CHLOROFORM In a molecule of chloroform

(CHCI,),or trichloromethane (tri klor oh MEthayn),the three chlorine atoms attract electronsmore strongly than the hydrogen atom does,creatinga partial negative chargeon the chlorineendof the molecule anda partial positive chargeon the hydrogen end.This polarmolecule isaclear, sweet-smelling liquid once widely used asananesthetic inhuman and veterinary surgery.

HYDROGEN FLUORIDE Hydrogen and fluorinereact to form hydrogen fluoride (HF). In an HFmolecule,the two atoms are bound togetherbya pair of electrons, one contributed by eachatom. However, the electrons are not shared

equally because the fluorine atom attracts themmore strongly than the hydrogen atom does.Theresultisa polar moleculewith a slightly positivecharge near the hydrogen end anda slightlynegative charge near the fluorine end.


Lin

8+

8-

H20Water

8+

Figure 15 The polarity ofwateris responsible for many of its

unique properties.Explain thecause ofpolarity.

Figure 16 Table sugar is anexample of a covalentcompoundthat is a crystalline solid soluble inwater. Ionic compounds, such as

table salt, are also soluble in water.


Polar or Nonpolar? For the molecule involved in this electron tug-of-war, there isanother consequence. Again, look at themolecule of hydrogen chloride. The unequal sharing of electrons gives each chlorine atom a slight negative charge and eachhydrogen atom a slight positive charge. The atom holding theelectron more closely always will have a slightlynegative charge.The charge is balanced but not equally distributed. This type ofmolecule is called polar.The term polar means "having oppositeends." A polar molecule is one that has a slightly positive endand a slightly negative end, although the overall molecule isneutral. Water is an example of a polar molecule, as shown inFigure 15.

ITfeidTngTrTecF What is a polar molecule?

Two atoms that are exactly alike can share their electronsequally, forming a nonpolar molecule. A nonpolar molecule isone in which electrons are shared equally in bonds. Such a molecule does not have oppositely charged ends. This is true of molecules made from two identical atoms or molecules that are

symmetric, such as CC14.

Properties of CompoundsRecall that atoms can form two types of bonds: covalent and

ionic. A compound whose atoms are held together by covalentbonds is a covalent compound.

Sugar is a covalentcompound. A compound that is composedof ions is an ionic compound. Table salt, NaCl, is an ionic com

pound. Looking at both substances,you would think theywould have similar physicaland chemical properties.

Both sugar and salt, asshown in Figure 16, are usedto change the taste of foods.You add sugar to lemonadeor iced tea to make the drink

sweet. Table salt is used to

enhance the flavors of foods,

such as meats and vegetables.Sugarand salt have different physical and chemicalproperties.

Covalent and Ionic Properties The chemical and physicalproperties of covalent compounds and ionic compounds are different. This difference in properties, as shown in Table 2, is dueto differences in attractive forces of the bonds.

Covalent Compounds The covalent bonds between atomsin molecules are strong. However, the attraction between individual molecules is weak. It is the weak forces between individual molecules that are responsible for the properties of covalentcompounds.

Melting and boiling points of covalent compounds are relatively lower when compared to ionic compounds. Sugar will meltat approximately 185°C, whereas table salt will melt at 801°C.Covalent compounds, which will form soft solids, have poorelectrical and thermal conductivity. Candles and propane gasshown in Figure 17 are covalent compounds.

Table 2 Comparison of Covalent and Ionic Compounds

Bond Type

Melting and BoilingPoints

Electrical

Conductivity

State at Room

Temperature

Forces Between

Particles

Covalent Compounds Ionic Compounds

Electron Sharing Electron Transfer

Lower Higher

Poor

Solid, liquid, orgas

Strong bonds betweenatoms; weak attraction

between molecules

Good

Solid

Strong attractionbetween positive and

negative ions

Figure 17 Candle wax andpropane are covalent compounds

that are insoluble in water.


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