8.2 The Nature of Covalent Bonding 8 - Mr. Leiker's Classroom

8.2 The Nature of Covalent Bonding 8.2

Connecting to Your World You know that without oxygen Guide for Reading 1 FOCUS

to breathe, you could not live. But did you know that oxygen plays Key Concepts Objectives
another important role in your life? High in the atmosphere, a different
• What is the result of electron 8.2.1 Describe how electrons are
form of oxygen, called ozone, forms a layer that filters sharing in covalent bonds? shared to form covalent bonds
out harmful radiation from the sun. The colors in and identify exceptions to the
this map indicate the concentrations of ozone in • How do electron dot structures octet rule.
various parts of Earth’s atmosphere. In this represent shared electrons?
section, you will learn how oxygen atoms can 8.2.2 Demonstrate how electron dot
join in pairs to form the oxygen you breathe and • How do atoms form double or structures represent shared
can also join in groups of three oxygen atoms to triple covalent bonds? electrons.
form ozone.
• How are coordinate covalent 8.2.3 Describe how atoms form dou-
The Octet Rule in Covalent Bonding bonds different from other ble or triple covalent bonds.
covalent bonds?
Recall that when ionic compounds form, electrons tend to be transferred 8.2.4 Distinguish between a cova-
so that each ion acquires a noble gas configuration. A similar rule applies • How is the strength of a lent bond and a coordinate
for covalent bonds. In covalent bonds, electron sharing usually occurs covalent bond related to its covalent bond and describe
so that atoms attain the electron configurations of noble gases. For example, bond dissociation energy? how the strength of a covalent
each hydrogen atom has one electron. But a pair of hydrogen atoms share bond is related to its bond dis-
these two electrons when they form a covalent bond in a hydrogen • How are oxygen atoms bonded sociation energy.
molecule. Each hydrogen atom thus attains the electron configuration in ozone?
of helium, a noble gas with two electrons. Combinations of atoms of the 8.2.5 Describe how oxygen atoms
nonmetals and metalloids in Groups 4A, 5A, 6A, and 7A of the periodic • What are some exceptions to are bonded in ozone.
table are likely to form covalent bonds. In this case the atoms usually the octet rule?
acquire a total of eight electrons, or an octet, by sharing electrons, so that Guide for Reading
the octet rule applies. Vocabulary
Build Vocabulary L2
Single Covalent Bonds single covalent bond
Word Parts The word structure comes
The hydrogen atoms in a hydrogen molecule are held together mainly by structural formula from the Latin verb struere, which
the attraction of the shared electrons to the positive nuclei. Two atoms held means “to build.” A structural formula
together by sharing a pair of electrons are joined by a single covalent bond. unshared pair is one that shows how a molecule is
Hydrogen gas consists of diatomic molecules whose atoms share only one built, that is, how the atoms are joined
pair of electrons, forming a single covalent bond. double covalent bond together by chemical bonds. Ask stu-
dents to define structural steel. (Steel is
triple covalent bond used to form the skeleton of buildings.)

coordinate covalent bond

polyatomic ion

bond dissociation energy

resonance structure

Reading Strategy

Identifying Main Idea/Details
List the main idea in the para-
graph following the heading
The Octet Rule in Covalent Bonding.
As you read, list examples of
how this rule is followed when a
single covalent bond, a double
covalent bond, and a triple cova-
lent bond form.

H ϩ H ⎯→ H H shared pair
of electrons
Hydrogen Hydrogen Hydrogen
molecule
atom atom Reading Strategy L2

1s Directed Reading/Thinking Activity
H Have students read the red headings
throughout the section first and ask
H Hydrogen molecule themselves questions about what they
1s might learn when they read the text.

Section 8.2 The Nature of Covalent Bonding 217

2 INSTRUCT

Section Resources Technology Explain that scientists attribute the
thinning of the ozone layer, in part, to
Print • Interactive Textbook with ChemASAP, the action of compounds called chlo-
• Guided Reading and Study Workbook, Problem-Solving 8.8, 8.10; Simulation 6, rofluorcarbons (CFCs), which have
Section 8.2 Assessment 8.2 been released into the atmosphere.
• Core Teaching Resources, • Go Online, Section 8.2 Ask, Why is ozone important in the
Section 8.2 Review atmosphere? (It filters out radiation
• Probeware Laboratory Manual, that could harm living beings on Earth.)
Quick Lab, p. 226
• Transparencies, T87–T89

Covalent Bonding 217

Section 8.2 (continued) An electron dot structure such as H:H represents the shared pair of

electrons of the covalent bond by two dots. The pair of shared electrons

The Octet Rule in forming the covalent bond is also often represented as a dash, as in H⎯H

Covalent Bonding For: Links on Valence for hydrogen. A structural formula represents the covalent bonds by dashes
Electrons
Discuss and shows the arrangement of covalently bonded atoms. In contrast, the
Visit: www.SciLinks.org
L2 Web Code: cdn-1082 molecular formula of hydrogen, H2, indicates only the number of hydrogen
atoms in each molecule.

Write electron configurations for car- The halogens also form single covalent bonds in their diatomic mole-
bon, nitrogen, oxygen, fluorine, and
neon on the chalkboard. Ask, How cules. Fluorine is one example. Because a fluorine atom has seven valence
many electrons would carbon, nitro-
gen, oxygen, and fluorine need to electrons, it needs one more to attain the electron configuration of a noble
share in order to achieve the same
electron configuration as neon? (4, 3, gas. By sharing electrons and forming a single covalent bond, two fluorine
2, and 1 respectively)
atoms each achieve the electron configuration of neon.

..F..... ϩ .F...... .. F.... .. F.. .. .. or .. .. .. ..
F.. F..

Fluorine Fluorine Fluorine
atom atom molecule

CLASS Activity 1s 2s 2p
F 2s 2p
Representing Molecules L2
F
1s

Purpose Students practice different

ways to represent molecules.

Materials paper and pencil Fluorine
molecule

Procedure Divide students into In the F2 molecule, each fluorine atom contributes one electron to com-
plete the octet. Notice that the two fluorine atoms share only one pair of
groups of three or four. Have them

practice drawing molecular diagrams, valence electrons. A pair of valence electrons that is not shared between

structural formulas, electron-dot struc- atoms is called an unshared pair, also known as a lone pair or a nonbond-

tures, and orbital diagrams for mole- ing pair.

cules such as OF2, SCl2, N2H4, CCl4, You can draw electron dot structures for molecules of compounds in
CHCl3, and C2H6.
much the same way that you draw them for molecules of diatomic ele-

Single Covalent Bonds ments. Water (H2O) is a molecule containing three atoms with two single
covalent bonds. Two hydrogen atoms share electrons with one oxygen

atom. The hydrogen and oxygen atoms attain noble-gas configurations by

sharing electrons. As you can see in the electron dot structures below, the

oxygen atom in water has two unshared pairs of valence electrons.

2H. ϩ ..O... . ..O.... ..H or ..O.. H
H
Hydrogen Oxygen
atoms atom H

Water molecule

Download a worksheet on Valence 1s 2s 2p
Electrons for students to complete, O
and find additional teacher support
from NSTA SciLinks. 1s 1s Water molecule
HH

Checkpoint What does a structural formula represent?

218 Chapter 8

Differentiated Instruction

Special Needs L1

Pair each student with a study partner. Have any atom, 1A to 7A, indicates the number of
valence electrons that atom has, and that it is
them use the periodic table and quiz each valence electrons that appear in the electron
dot structures.
other on writing electron dot structures for

single atoms and bonded atoms. Make sure

they understand that the Group number for

218 Chapter 8

You can draw the electron dot structure for ammonia (NH3), a suffocat- Discuss L2
ing gas, in a similar way. The ammonia molecule has one unshared pair of
First review the molecular and struc-
electrons. tural formulas, electron dot structures,
and orbital diagrams for fluorine,
3H. ϩ ..N.. . H H water, and ammonia molecules. If pos-
..HN.... ..H or ..N sible, display physical models. Call
Hydrogen Nitrogen H attention to the fact that fluorine has
atoms atom one half-filled orbital and forms one
H bond, oxygen has two half-filled orbit-
als and forms two bonds, and nitrogen
Ammonia molecule has three and forms three bonds.Tell
students carbon has two. Ask, How
1s 2s 2p many covalent bonds do you think
N carbon forms?(Students may logically
say two.) Tell students that CH2 does
1s 1s 1s Ammonia molecule not represent a stable molecule, but
HH H CH4 (methane) is a stable molecule.
Explain the concept of electron promo-
Methane (CH4) contains four single covalent bonds. The carbon atom tion, which allows carbon to form four
has four valence electrons and needs four more valence electrons to attain single covalent bonds. Point out that
a noble-gas configuration. Each of the four hydrogen atoms contributes elements in groups 3A and 4A pro-
one electron to share with the carbon atom, forming four identical carbon- mote s electrons to p orbitals, increas-
hydrogen bonds. As you can see in the electron dot structure below, meth- ing their bonding capacity. For
ane has no unshared pairs of electrons. example, boron’s electron configura-
tion is 1s22s22p1. Based on this config-
4H. ϩ .C.. . H uration, students might infer that
H..HC.... ..H or H C H boron can form only one covalent
Hydrogen Carbon bond. However, the chloride of boron
atoms atom H is BCl3 rather than BCl.The promotion
H of one 2s electron to the 2p orbital
1s allows for the formation of three bonds.
C Methane molecule Boron does not achieve a noble-gas
configuration, but it does achieve
2s and 2p added stability by forming three bonds
rather than one.
1s 1s 1s 1s
HH H H

Methane molecule

When carbon forms bonds with other atoms, it usually forms four
bonds. You would not predict this based on carbon’s electron configura-
tion, shown below.

1s 2 2s 2 2p 2

Checkpoint What is the electron dot structure of a methane molecule?

Section 8.2 The Nature of Covalent Bonding 219

Facts and Figures phosphorus to form five bonds.The amount Answers to...
of energy needed to promote an electron is
Expanding the Octet less than the energy released with the forma- Checkpoint It shows the
Nonmetals in the third row and beyond such tion of an extra bond. Sulfur can promote covalent bonds as dashes and
as phosphorus, sulfur, and iodine can form one 3s electron and one 3p electron and shows the arrangement of
more than four bonds because they have form two extra bonds. covalently bonded atoms.
empty d orbitals. Phosphorus, for example,
can unpair a 3s electron and promote it to an H
empty 3d orbital.The promotion allows Checkpoint H C H

H

Covalent Bonding 219

Section 8.2 (continued) If you tried to form covalent C ⎯ H bonds for methane by combining
the two 2p electrons of the carbon with two 1s electrons of hydrogen atoms,
CLASS Activity you would incorrectly predict a molecule with the formula CH2 (instead of
CH4). The formation of four bonds by carbon can be simply explained. One
Bonding for Second Row L2 of carbon’s 2s electrons is promoted to the vacant 2p orbital to form the
Elements following electron configuration.

Purpose Students gain understand- 1s2 2s and 2p

ing of covalent bonding and distin- This electron promotion requires only a small amount of energy. The pro-
motion provides four electrons of carbon that are capable of forming cova-
guish between covalent and ionic lent bonds with four hydrogen atoms. Methane, the carbon compound
formed by electron sharing of carbon with four hydrogen atoms, is much
bonding. more stable than CH2. The stability of the resulting methane more than
compensates for the small energy cost of the electron promotion. There-
Procedure Have students draw elec- fore, formation of methane (CH4) is more energetically favored than the
formation of CH2.
tron dot structures for each element in

the second row of the periodic table:

Li, Be, B, C, N, O, and F.Then have them

answer the following:

• Predict how many bonds each atom

must form to attain a noble-gas CONCEPTUAL PROBLEM 8.1

configuration. (1, 2, 3, 4, 3, 2, 1) Drawing an Electron Dot Structure
• Can lithium form a covalent bond
Hydrochloric acid (HCl (aq)) is prepared by dissolving gaseous
and reach stability? (no) hydrogen chloride (HCl (g)) in water. Hydrogen chloride is a
• Which elements can reach stability by diatomic molecule with a single covalent bond. Draw the electron
dot structure for HCl.
forming covalent bonds? (C,N,O,F)
• Can fluorine form an ionic bond?

(yes) 1 Analyze Identify the relevant concepts. 2 Solve Apply concepts to the situation.
• Are the bonds in nitrogen mole-
In a single covalent bond, a hydrogen and a
cules (N2) ionic or covalent? chlorine atom must share a pair of electrons. H. ϩ .C....l.. H ..C.. ..l ..
(covalent) Each must contribute one electron to the
bond. First, draw the electron dot structures Hydrogen Chlorine Hydrogen
for the two atoms. Then show the electron chloride molecule
sharing in the compound they produce. atom atom

CONCEPTUAL PROBLEM 8.1 In the electron dot structures, the hydrogen
atom and the chlorine atom are each correctly
Answers shown to have an unpaired electron. Through
electron sharing, the hydrogen and chlorine
7. a. Cl Cl b. Br Br c. I I atoms are shown to attain the electron config-
urations of the noble gases helium and argon,
8. a. H O O H b. Cl P Cl respectively.
Cl

Practice Problems Plus L2 Practice Problems

The following covalent molecules have 7. Draw electron dot structures for each molecule.
only single covalent bonds. Draw an a. chlorine b. bromine c. iodine
electron dot structure for each.
8. The following molecules have single covalent Problem Solving 8.8 Solve
Problem 8 with the help of an
a. NF3 F N F bonds. Draw an electron dot structure for each. interactive guided tutorial.
F
a. H2O2 b. PCl3 with ChemASAP

b. SBr2 Br S Br

220 Chapter 8

Facts and Figures the theory of acids and bases by defining an
acid as an electron pair acceptor and a base
Inventing Electron Dot Structures as an electron pair donor.The definitions
Gilbert Newton Lewis (1875–1946) was an encompass all Brønsted-Lowry acid-base
American chemist who invented electron reactions and include many others not previ-
dot structures, which are often called Lewis ously categorized as acid-base reactions.
structures or diagrams in his honor.These
structures supported Lewis’s theory of the
electron pair in chemical bonding. As a pro-
fessor of physical chemistry, he expanded

220 Chapter 8

Double and Triple Covalent Bonds Double and Triple

8.1 Molecular CompoundsSometimes atoms bond by sharing more than one pair of electrons. Atoms Covalent Bonds

form double or triple covalent bonds if they can attain a noble gas structure Simulation 6 Simulate the Discuss L2
covalent bonding between
by sharing two pairs or three pairs of electrons. A bond that involves two molecules. To introduce the discussion of multiple
covalent bonds, use the electron dot
shared pairs of electrons is a double covalent bond. A bond formed by sharing GuidewfitohCr hReemadAiSnAgP structure for the nitrogen molecule.
Ask students to show how the struc-
tChoreneYopnuaeimrcsiotgifhnetltgehcittnrookntYshoiastuaantrrioWpxleyogceornvldaalteonmt b,owTnihdteh.sseixtovyalmenocdeeellseacrtreomnsa,dweooufld Key Concepts ture of diatomic nitrogen can satisfy
the octet rule. (The nitrogen atoms can
ftohremsaamdeobuabsliec bpoiencdesb, yjosinheadrintoggtewtoheorfbiytssetilcekcstrionnusnwitisthinadnifoftehreerntowxyagyesn. • How are the melting points and share six electrons.) Have them com-
boiling points of molecular pare the bonding in ammonia with the
aEtvoemn .when the toy models are made of the same kind of pieces, the units compounds different from bonding in nitrogen gas.Then intro-
ionic compounds? duce the oxygen molecule. Ask stu-
..O.. .. ϩ th..O..e..yefonrtmnucmanb..be..Ore..s..dOai..n..ffdeorarernra..t..OnbgeecOam..u.. esnetosfotfhtehdeiffer- dents to draw a structure that obeys
• What information does a the octet rule. Ask, Why doesn’t oxy-
Oxygen Oxygen wooden barsOjxoyigneinng them. Atoms can molecular formula provide? gen form a triple bond? (Each oxygen
atom atom join in waymsotlhecautleare like these models, needs to share only two electrons to
Vocabulary achieve a stable electron configuration.)
1s 2s 2p with different possible ways of join- Explain that although a double bond in
O ing the atoms. In this section, you covalent bond the oxygen molecule fulfills the octet
molecule rule, it does not fit with experimental
will learn how atoms can share elec- diatomic molecule evidence that shows that the oxygen
molecular compound molecule contains two unpaired elec-
O trons to form a bond, called a cova- molecular formula trons.Thus the structure of O2 is an
exception to the octet rule. Help stu-
1s 2s 2p lent bonOdx, aygnednhmoowlectuhlee atoms join to Reading Strategy dents draw the electron dot structure
and orbital diagram for carbon dioxide.
form units called molecules. Building Vocabulary As you Ask, What type of bonds does carbon
In such an arrangement, all the electrons within the molecule would be read the section, write a definition form with the two oxygen atoms in
of each key term in your own CO2? (double covalent bonds) Note that
paired. Experimental evidence, however, indicates that two of the electrons words. carbon can form single, double, and tri-
ple bonds, but a quadruple bond is
in O2 are still unpaired. Thus, the bonding in the oxygen molecule (O2) does Figure 8.7 Oxygen and nitrogen impossible because of geometric
not obey the octet rule. You cannot draw an electron dot structure that ade- are the main components of restrictions. Have students draw dia-
Earth’s atmosphere. The oxygen grams for hydrogen cyanide (HCN) and
quately describes the bonding in the oxygen molecule. molecule is an exception to the formaldehyde (H2CO). Ask, What kind
octet rule. It has two unpaired of bonds does carbon form in each
MoAlneecleumleentswahonsedmMoleoculleesccuonlatairnCtrioplme bponodsuins nditsrogen (N2), a electrons. Three pairs of electrons of these molecules? (HCN: one single
major component of Earth’s atmosphere illustrated in Figure 8.7. In the are shared in a nitrogen carbon-to-hydrogen bond and one tri-
Innitrnoagteunrem, omleactuteler , teaakcehs nmitraongyenfoartmoms. hTahseonnoebulnesghaasreedlepmaiernotfse, lesuctcrhonass. molecule. ple carbon-to-nitrogen bond; H2CO: two
hAesliinumgleannidtrnoegoenn sahtoowmnhiansFfiigvuereva8l.e1n, ceexiestleacstraotonms. sE.aTchheyniatrreogmeonnaattoommiicn; single carbon-to-hydrogen bonds and
tthhaetniist,rothgeeyn cmoonlseicsut loefmsiunsgtlegaaitnomthsr.eYeoeulelcetarornnesdtoinhathvee tphreeveiloeuctsrochnacpotne-r one double carbon-to-oxygen bond) As
tfihgautraattoiomnsoof fnseoomn.e elements combine to form salts. Salts, such as sodium the discussion proceeds, provide mod-
chloOritdhee(rNcaoCml)p,oaur..enN..dc.rsy, sϩhtoawlli.enNv..ee.. sr,ocliadns with high melting points. els for the molecules discussed.
hav..eNv......eNry.. doifrfer..Nent pNro.. perties. Hydro-

gen chloride (HCNli)tr,ofgoernexaNmitrpolgee,nis a gas at room temperature. Water (H2O)

Nitrogen
is a liquid at rooamtomtemperaattoumre. These two mcoomlecpuoleunds are so different

from salts that you might correctly suspect that electrostatic attraction
between ions fails1sto exp2slain their2bponding. Such compounds are not ionic.

Their combinNing atoms do not give up electrons or accept electrons as

readily as sodNium does in combining with chlorine.
Instead, aat“otum1gssotfowge2atsrh”efro.rTthhee2aeptleocmtrsohnesldtatkoNegsiterpothlgaeecnrembbyoelstewhcuaelreeinngtheeleacttoromnss,
bonding the

are joined by a covalent bond.

Figure 8.1 The noble gases, including
helium and neon, are monatomic. That
means they exist as single atoms. Helium,
being less dense than air, is often used to
inflate balloons. The colors produced in
what we commonly call neon lights are a
result of passing an electric current
through one or more noble gases.

Section 8.2 The Nature of Covalent Bonding 221

Differentiated Instruction

Less Proficient Reader L1

Divide students into groups of three or four. OF2. Have them do the same for SCl2 and
CCl4. When students have mastered these,
Have them draw electron dot formulas for continue the exercise with CHCl3, N2H4, and
C2H6.
oxygen and fluorine, then combine the

drawings into the electron dot formula for

Covalent Bonding 221

Section 8.2 (continued) Table 8.1

Relate L1 The Diatomic Elements

Earth’s atmosphere is approximately 80 Name Chemical Electron dot Properties and uses
percent nitrogen gas, but surprisingly Fluorine formula structure
few nitrogen compounds exist com- Chlorine Greenish-yellow reactive toxic gas. Compounds of fluorine,
pared with the numerous compounds Bromine F2 ....F.. ..F.... a halogen, are added to drinking water and toothpaste to
of oxygen, which constitutes only 20 promote healthy teeth.
percent of the atmosphere. Students Cl2 ..C....l C....l..
may correctly surmise that the triple Greenish-yellow reactive toxic gas. Chlorine is a halogen
bond in N2 is harder to break than a Br2 ..B....r .. .. used in household bleaching agents.
double bond and considerably harder B..r
to break than a single bond.Thus, N2 is Dense red-brown liquid with pungent odor. Compounds
a stable molecule. Plant and animal life Iodine I2 ....I.. ..I.... of bromine, a halogen, are used in the preparation of
depends on nitrogen, but in order to photographic emulsions.
be usable to living systems, the ele- Hydrogen H2 H ⎯H
ment must be converted to a com- Nitrogen N2 ..N N.. Dense gray-black solid that produces purple vapors; a
pound, a process called nitrogen fixing. Oxygen O2 halogen. A solution of iodine in alcohol (tincture of iodine)
Nitrogen fixing occurs naturally when Inadequate is used as an antiseptic.
lightning provides the energy for
atmospheric nitrogen to react with Colorless, odorless, tasteless gas. Hydrogen is the
oxygen to form nitrogen oxides.The lightest known element.
oxides dissolve in rain and fall to the
ground where they can be utilized Colorless, odorless, tasteless gas. Air is almost 80%
by plants. Nitrogen fixing bacteria in nitrogen by volume.
the soil are also able to convert
atmospheric nitrogen to usable Colorless, odorless, tasteless gas that is vital for life. Air is
compounds. about 20% oxygen by volume.

Up to this point in your textbook, the examples of single and triple
covalent bonds have involved diatomic molecules. Table 8.1 lists the prop-
erties and uses of the elements that exist as diatomic molecules. Single,
double, and triple covalent bonds can also exist between unlike atoms. For
example, consider carbon dioxide (CO2), which is present in the atmo-
sphere and is used to carbonate many soft drinks as shown in Figure 8.8.
The carbon dioxide molecule contains two oxygens, each of which shares two
electrons with carbon to form a total of two carbon–oxygen double bonds.

....O.. ϩ ..C.. ϩ ..O.... ....O....C....O.... or ....O C O....

Oxygen Carbon Oxygen Carbon dioxide
atom molecule
atom atom

1s 2s and 2p
C

O O
1s
Figure 8.8 Carbon dioxide gas is 2s 2p 2p 2s 1s Carbon dioxide molecule
soluble in water and is used to
carbonate many beverages. A The two double bonds in the carbon dioxide molecule are identical to
carbon dioxide molecule has two each other. Carbon dioxide is an example of a triatomic molecule, which is
carbon–oxygen double bonds. a molecule consisting of three atoms.

222 Chapter 8

222 Chapter 8

Coordinate Covalent Bonds Coordinate Covalent

Carbon monoxide (CO) is an example of a type of covalent bonding differ- Bonds
ent from that seen in water, ammonia, methane, and carbon dioxide. A car-
bon atom needs to gain four electrons to attain the electron configuration Discuss L2

of neon. An oxygen atom needs two electrons. Yet it is possible for both A coordinate covalent bond is an
exception to the rule that covalent
atoms to achieve noble-gas electron configurations by a type of bonding bonding occurs between two half-
called coordinate covalent bonding. To see how, begin by looking at the empty orbitals of two atoms. In a coor-
double covalent bond between carbon and oxygen. dinate covalent bond, one atom has an
empty orbital and the other an orbital
..C.. ϩ ..O.... .. C.... O.... filled with an electron pair that is not
yet involved in a chemical bond. Note
Carbon Oxygen Carbon that the bonding still involves only one
atom atom monoxide pair of electrons and one pair of orbit-
als but one atom provides both of the
1s 2s 2p shared electrons.
C

O

1s 2s 2p Carbon monoxide molecule

With the double bond in place, the oxygen has a stable configuration but Discuss L2
the carbon does not. As shown below, the dilemma is solved if the oxygen
also donates one of its unshared pairs of electrons for bonding. Ask students to compare the electron

..C....O.... ..C......O.. dfoortasthryudcrtougreenfoiroanm(Hm+o).nAiafte(Nr Hfo3r)manindg

Carbon monoxide the three nitrogen-to-hydrogen bonds,
molecule

A coordinate covalent bond is a covalent bond in which one atom contrib- nitrogen has an unshared pair of elec-
utes both bonding electrons. In a structural formula, you can show coordi-
nate covalent bonds as arrows that point from the atom donating the pair trons.The hydrogen ion has no elec-
of electrons to the atom receiving them. The structural formula of carbon
monoxide, with two covalent bonds and one coordinate covalent bond, is trons available for bonding. Ask, How
C O . In a coordinate covalent bond, the shared electron pair comes
from one of the bonding atoms. Once formed, a coordinate covalent bond is Figure 8.9 The polyatomic do you explain the existence of the
like any other covalent bond. ammonium ion (NH4ϩ), present
" in ammonium sulfate, is an ammonium Hio+nm, NuHst4o+v?e(rTlahpetehmepfitllyed
The ammonium ion (NH4ϩ) consists of atoms joined by covalent important component of orbital of the
bonds, including a coordinate covalent bond. A polyatomic ion, such as fertilizer for field crops, home
NH4ϩ, is a tightly bound group of atoms that has a positive or negative gardens, and potted plants. orbital in ammonia.The electrons from
charge and behaves as a unit. The ammonium ion forms when a pos-
itively charged hydrogen ion (Hϩ) attaches to the unshared 223 nitrogen are attracted to both the nitro-
electron pair of an ammonia molecule (NH3). Most plants
need nitrogen that is already combined in a compound rather gen nucleus and the hydrogen nucleus. A
than molecular nitrogen (N2) to grow. The fertilizer shown in
Figure 8.9 contains the nitrogen compound ammonium sulfate. bond forms when electrons are simulta-

neously attracted to two nuclei.This type

of bond is called a coordinate covalent

bond.) Ask students to explain the

unshared electron pair bonding in CO2 and then explain the
bonding in CO using coordinate cova-
H ϩ ..HN.... ..H
H H..HN.... .. H H lent bonding.
H or H N
H
H

Hydrogen Ammonia Ammonium
ion molecule ion

(proton) (NH 3 ) (NH 4 )

Checkpoint What is a polyatomic ion?

Answers to...

Checkpoint a group of
atoms that has a positive or nega-
tive charge and behaves as a unit

Covalent Bonding 223

Section 8.2 (continued) Most polyatomic cations and anions contain both covalent and coordi-
nate covalent bonds. Therefore, compounds containing polyatomic ions
Discuss L2 include both ionic and covalent bonding.

Have students write the electron dot As another example, draw the electron dot structure of the polyatomic
structure for SO2. Emphasize that the ion SO32Ϫ. Start by drawing the electron dot structures for the oxygen and
structure should satisfy the bonding sulfur atoms, and the two extra electrons indicated by the charge. Then join
requirements of all three atoms. Stu- two of the oxygens to sulfur by single covalent bonds.
dents should find that, to satisfy the
octet rule for all the atoms, they must O S Oϩ OS O
write a structure in which one oxygen O O
atom is double bonded to sulfur.The
other oxygen is single bonded by a Then join the remaining oxygen by a coordinate covalent bond, with
coordinate covalent bond in which the
electrons are donated by sulfur. Point sulfur donating one of its unshared pairs to oxygen, and add the two extra
out that experimental evidence indi-
cates that both sulfur-oxygen bonds electrons. Put brackets about the structure and indicate the 2Ϫ charge,
are identical. Explain that this evidence
indicates that the bonding in SO2 must giving the result shown. c..O........OS..........O.... ..d 2Ϫ
be some intermediate between a sin- ..O......OS..........O.... .. ϩ ..
gle and double bond. Ask, How does
the formation of a coordinate cova- Each of the atoms of the completed structure has eight valence electrons,
lent bond differ from that of a cova- satisfying the octet rule. Without the two extra electrons, two of the oxygens
lent bond? (In a covalent bond, each would be electron-deficient.
atom provides one electron, in a coordi-
nate covalent bond, both electrons are Table 8.2 lists electron dot structures of some common compounds
provided by the same atom.) with covalent bonds.

Table 8.2 Some Common Molecular Compounds

Name Chemical Structure Properties and uses
Hydrogen formula
peroxide H Colorless, unstable liquid when pure. It is used as rocket
H2O2 ..O.. ¬O.... fuel. A 3% solution is used as a bleach and antiseptic.
Sulfur ¬
dioxide SO2 ¬ Oxides of sulfur are produced in combustion of petroleum
products and coal. They are major air pollutants in industrial
Relate L2 Sulfur SO3 H areas. Oxides of sulfur can lead to respiratory problems.
trioxide ....O¬¬ S.. O......
Have students think of everyday exam- NO uu Oxides of nitrogen are major air pollutants produced by the
ples in which the shape of an object is Nitric oxide NO2 ....O¬¬ S u O.. .. .. combustion of fossil fuels in automobile engines. They irri-
as important as its composition. For Nitrogen N2O O.. .. .. tate the eyes, throat, and lungs.
example, several keys might be made dioxide HCN
of the same metal, but only one will fit HF ....O ¬¬ N... Colorless, sweet-smelling gas. It is used as an anesthetic
into a particular lock. Nitrous HCI commonly called laughing gas.
oxide ....O . Colorless, toxic gas with the smell of almonds.
Hydrogen ¬¬ N ..O....
cyanide Two hydrogen halides, all extremely soluble in water. Hydro-
Hydrogen u gen chloride, a colorless gas with pungent odor, readily dis-
fluoride solves in water to give a solution called hydrochloric acid.
Hydrogen ..O.... ¬ N¬¬ N..u
chloride
H ¬ C ¬¬ N..
H¬ .F.....
H¬ C....l..

224 Chapter 8

224 Chapter 8

Remember, the electron dot structure for a neutral molecule contains CONCEPTUAL PROBLEM 8.2
the same number of electrons as the total number of valence electrons in
the combining atoms. The negative charge of a polyatomic ion shows the Answers _
number of electrons in addition to the valence electrons of the atoms 9. H O
present. Because a negatively charged polyatomic ion is part of an ionic
compound, the positive charge of the cation of the compound balances F_
these additional electrons.
10. F B F
F
O 2_
CONCEPTUAL PROBLEM 8.2 O 2_
11. a. O S O b. O C
Drawing the Electron Dot Structure of a Polyatomic Ion OO
O_
The polyatomic hydronium ion (H3Oϩ), which is found in acidic
mixtures such as lemon juice, contains a coordinate covalent bond. 12. H O C
The H3Oϩ ion forms when a hydrogen ion is attracted to an unshared O
electron pair in a water molecule. Draw the electron dot structure of
the hydronium ion. Practice Problems Plus L2

1 Analyze Identify the relevant concepts. 2 Solve Apply the concepts to this situation. Draw the electron dot structure of
the polya_tomic chlorate anion,ClO3؊.
H3Oϩ forms by the addition of a hydrogen ion H ϩ ..HO.... ..H H..HO.... ..H or H H H
to a water molecule. Draw the electron dot O. . O Cl O
structure of the water molecule. Then, add the
hydrogen ion. Oxygen must share a pair of Hydrogen Water Hydronium O
electrons with the added hydrogen ion to form ion molecule ion
a coordinate covalent bond.
(proton) (H2O) (H3O )

The oxygen in the hydronium ion has eight TEACHER Demo

valence electrons, and each hydrogen shares

two valence electrons. This satisfies the needs Bond Energies L2

of both hydrogen and oxygen for valence elec-

trons. The water molecule is electrically neu- Purpose Students observe two reac-
tions in which bonds are formed and
tral, and the hydrogen ion has a positive compare ionic and covalent bond
energies.
charge. The combination of these two species

must have a charge of 1ϩ, as is found in the

hydronium ion.

Practice Problems Materials 10- to 15- cm piece of clean
magnesium ribbon, a small piece of
9. Draw the electron dot structure of the hydroxide 12. Draw the electron dot structure for the hydro- charcoal, tongs, Bunsen burner, filters
ion (OHϪ). gen carbonate ion (HCO3Ϫ). Carbon is the cen- for viewing
tral atom, and hydrogen is attached to oxygen in
10. Draw the electron dot structure of the poly- this polyatomic anion. Safety Wear safety goggles and lab
atomic boron tetrafluoride anion (BF4Ϫ). apron.
Problem-Solving 8.10 Solve
11. Draw the electron dot structures for sulfate Problem 10 with the help of an Procedure Using tongs, hold the
(SO42Ϫ) and carbonate (CO32Ϫ). Sulfur and interactive guided tutorial. piece of magnesium in a Bunsen
carbon are the central atoms, respectively. burner. CAUTION! Tie back long hair
with ChemASAP and loose clothing and wear safety gog-
gles. Do not look directly at the flame.
Section 8.2 The Nature of Covalent Bonding 225 Observe through filters. Discuss with
students the large amount of heat and
light given off in the formation of MgO.
Write the balanced equation on the
board.

2Mg + O2 → 2MgO + energy
Using tongs, place a small piece of
charcoal in the Bunsen burner flame
and try to ignite it. Write the balanced
chemical equation.

C + O2 → CO2 + energy

Expected Outcomes Students note

that much less energy is given off in

forming CO2 than in forming MgO. Ask,
What kind of bonds are in MgO?

(ionic) What kind of bonds are in

CO2? (covalent) Ionic bond energies
are, in general, greater than covalent

bond energies.

Covalent Bonding 225

Section 8.2 (continued) Quick LAB

Bond Dissociation Strengths of Covalent Bonds
Energies
Purpose Procedure you estimate best fits the points for
Quick LAB To compare and contrast each set of data. (Your graph should
the stretching of rubber Sensor version available in the Probeware have three separate lines.) The x-axis
Strengths of Covalent Bonds L2 bands and the dissociation Lab Manual. and y-axis intercepts of the lines should
Objective After completing this activ- energy of covalent bonds. pass through zero, and the lines should
ity, students will understand that the 1. Bend the coat hanger to fit over the top extend past 1 kg on the x-axis. Deter-
dissociation energy of a covalent bond Materials of a door. The hook should hang down mine the slope of each line in cm/kg.
increases in order from single bond to on one side of the door. Measure the
double bond to triple bond. • 1 170-g (6-oz) can of length of the rubber bands (in cm). Analyze and Conclude
Skills Focus Solving, interpreting food Hang a rubber band on the hook 1. Assuming the rubber bands are mod-
created by the coat hanger.
Prep Time 10 minutes • 2 454-g (16-oz) cans of els for covalent bonds, what can you
Advance Prep Obtain the necessary food 2. Place the 170-g can in the plastic bag. conclude about the relative strengths
materials in advance.You may wish to Use the paper clip to fasten the bag to of single, double, and triple bonds?
ask students to help with this task by • 3 No. 25 rubber bands the end of the rubber band. Lower the
bringing cans of food, coat hangers, • metric ruler bag gently until it is suspended from 2. How does the behavior of the rubber
and plastic grocery bags to class. • coat hanger the end of the rubber band. Measure bands differ from that of covalent
Class Time 30 minutes • plastic grocery bag and record the length of the stretched bonds?
• paper clip rubber band. Using different combina-
Teaching Tips • graph paper tions of food cans, repeat this process
• Use tape to secure the hanger in • pencil three times with the following masses:
place if necessary. • motion detector (optional) 454 g, 624 g, and 908 g.
• Have groups of students record the
data and then create a graph as a 3. Repeat Step 2, first using two rubber
class using an overhead projector. bands to connect the hanger and the
Expected Outcome As the mass of paper clip, and then using three.
the load increases, the stretch of the
rubber band or bands increases. 4. Graph the length difference: (stretched
For a given mass, a single rubber band rubber band) Ϫ (unstretched rubber
stretches farther than a double and a band) on the y-axis versus mass (kg) on
double rubber band stretches farther the x-axis for one, two, and three rub-
than a triple. ber bands. Draw the straight line that
Analyze and Conclude
Bond Dissociation Energies
1. Triple covalent bonds are stronger
than double covalent bonds, which A large quantity of heat is released when hydrogen atoms combine to form
are stronger than single covalent hydrogen molecules. This suggests that the product is more stable than the
bonds. reactants. The covalent bond in the hydrogen molecule (H2) is so strong
that it would take 435 kJ of energy to break apart all of the bonds in 1 mole
2. The change in bond dissociation (about 2 grams) of H2. (You will study the mole, abbreviated mol, in Chap-
energies in going from a carbon-car- ter 12.) The energy required to break the bond between two covalently
bon single bond to a carbon-carbon bonded atoms is known as the bond dissociation energy. This is usually
double bond to a carbon-carbon tri- expressed as the energy needed to break one mole of bonds, or 6.02 ϫ 1023
ple bond is nearly constant.The bonds. The bond dissociation energy for the H2 molecule is 435 kJ/mol.
change in length of one, two, and
three rubber bands, as given by the A large bond dissociation energy corresponds to a strong covalent
slopes of the lines, is not constant. It bond. A typical carbon–carbon single bond has a bond dissociation energy
is large going from one to two rub- of 347 kJ/mol. Typical carbon–carbon double and triple bonds have bond
ber bands and small going from two dissociation energies of 657 kJ/mol and 908 kJ/mol, respectively. Strong
to three rubber bands. carbon–carbon bonds help explain the stability of carbon compounds.
Compounds with only C ⎯ C and C ⎯ H single covalent bonds, such as
methane, tend to be quite unreactive. They are unreactive partly because
the dissociation energy for each of these bonds is high.

226 Chapter 8

For Enrichment L3

Have students draw a graph of actual bond
dissociation energies versus number of
bonds for carbon-carbon single, double, and
triple bonds. Have students write a state-
ment that relates this graph to their conclu-
sions in the activity.

226 Chapter 8

Resonance Resonance

Ozone in the upper atmosphere blocks harmful ultraviolet radiation from Discuss L2
the sun. At the lower elevations shown in Figure 8.10, it contributes to smog.
The ozone molecule has two possible electron dot structures. Although the electron dot structures of
O2 on page 221 show that all electrons
..O.... ..O.. ....O.... d ....O....O.. ..O.... .. are paired, experimental evidence
reveals that two electrons are
Notice that the structure on the left can be converted to the one on the unpaired. Ask, Would you consider
right by shifting electron pairs without changing the positions of the oxy- oxygen a reactive gas? (yes) What
gen atoms. could account for oxygen’s reactiv-
ity? (Unpaired electrons are reactive.)
As drawn, these electron dot structures suggest that the bonding in After a brief discussion of the other
ozone consists of one single coordinate covalent bond and one double exceptions to the octet rule that are
covalent bond. Because earlier chemists imagined that the electron pairs presented in this section, remind stu-
rapidly flip back and forth, or resonate, between the different electron dot dents that many bonds do follow the
structures, they used double-headed arrows to indicate that two or more octet rule.
structures are in resonance.
Relate L2
Double covalent bonds are usually shorter than single bonds, so it was
believed that the bond lengths in ozone were unequal. Experimental mea- Students may be interested in knowing
surements show, however, that this is not the case. The two bonds in ozone that substances containing unpaired
are the same length. This result can be explained if you assume that the electrons can be identified through a
actual bonding in the ozone molecule is the average of the two electron dot phenomenon called paramagnetism.
structures. The electron pairs do not actually resonate back and forth. When molecules with unpaired elec-
trons are placed in a magnetic field,
The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of they tend to be drawn into the field.
the extremes represented by the resonance forms. These substances are paramagnetic. In
contrast, molecules in which all elec-
The two electron dot structures for ozone are examples of what are still trons are paired tend to be pushed
referred to as resonance structures. A resonance structure is a structure that from a magnetic field.These sub-
occurs when it is possible to draw two or more valid electron dot structures stances are diamagnetic.
that have the same number of electron pairs for a molecule or ion. Reso-
nance structures are simply a way to envision the bonding in certain mol-
ecules. Although no back-and-forth changes occur, double-headed arrows
are used to connect resonance structures.

Checkpoint What notation is used to show that the two covalent bonds in
O3 are the same?

Figure 8.10 Although ozone high above the Paramagnetism should not be con-
ground forms a protective layer that absorbs fused with ferromagnetism, which is
ultraviolet radiation from the sun, at lower the familiar attraction of metals such as
elevations ozone is a pollutant that contributes iron, cobalt and nickel to a magnetic
to smog. field.The property of paramagnetism is
the evidence that shows that the oxy-
Section 8.2 The Nature of Covalent Bonding 227 gen molecule has unpaired and thus
cannot be described exactly by appli-
cation of the octet rule. However, the
oxygen bond does have double bond
character; its bond length and bond
energy are similiar to those of double
bonds in other molecules that conform
to the octet rule.

Answers to...

Checkpoint double-headed
arrows

Covalent Bonding 227

Section 8.2 (continued) Exceptions to the Octet Rule

Exceptions to the Octet The octet rule provides guidance for drawing electron dot structures. For
Rule some molecules or ions, however, it is impossible to draw structures that
satisfy the octet rule. The octet rule cannot be satisfied in molecules
TEACHER Demo whose total number of valence electrons is an odd number. There are also
molecules in which an atom has fewer, or more, than a complete octet of
A Resonance Hybrid L2 valence electrons. The nitrogen dioxide (NO2) molecule, for example, con-
tains a total of seventeen, an odd number, of valence electrons. Each oxy-
gen contributes six electrons and the nitrogen contributes five. Two
plausible electron dot structures can be drawn for the NO2 molecule.

Purpose Students observe the forma- ..O.. . ¬O.... ..
tion of nitrogen dioxide and write its “N
resonance structures.
..O.... . “ O.. ..
¬N

Materials small piece of copper 1s 2s 2p
metal, concentrated nitric acid, evapo- O 2s 2p
rating dish, fume hood N
O
Safety Wear safety goggles, gloves, Nitrogen dioxide molecule
and lab apron. Perform the experiment 1s
under an efficient hood.

Procedure Place a small piece of cop- An unpaired electron is present in each of these structures, both of which
per in an evaporating dish and cover it fail to follow the octet rule. It is impossible to draw an electron dot struc-
with concentrated HNO3. While stu- ture for NO2 that satisfies the octet rule for all atoms. Yet, NO2 does exist as
dents observe the reaction, write the a stable molecule. In fact, it is produced naturally by lightning strikes of the
balanced equation for the reaction on sort shown in Figure 8.11.
the board.
A number of other molecules also have an odd number of electrons. In
Cu + 4HNO3 → Figure 8.11 Lightning is one these molecules, as in NO2, complete pairing of electrons is not possible. It
Cu(NO3)2 + 2H2O + 2NO2 + energy means by which nitrogen and is not possible to draw an electron dot structure that satisfies the octet rule.
oxygen in the atmosphere Examples of such molecules include chlorine dioxide (ClO2) and nitric
Tell students that NO2 is one of the pol- produce nitrogen dioxide. oxide (NO).
lutants in automobile exhaust. It gives
smog its reddish-brown color and is Several molecules with an even number of valence electrons, such as
very reactive and poisonous. Have stu- some compounds of boron, also fail to follow the octet rule. This may hap-
dents try to write the electron dot pen because an atom acquires less than an octet of eight electrons. The
structure for NO2.They will be unable boron atom in boron trifluoride (BF3), for example, is deficient by two elec-
to find a way to arrange the 17 elec- trons, and therefore is an exception to the octet rule. Boron trifluoride
trons around the central nitrogen and readily reacts with ammonia to make the compound BF3·NH3. In doing so,
the two oxygen atoms so that the octet the boron atom accepts the unshared electron pair from ammonia and
rule is obeyed. Refer students to the completes the octet.
two resonance structures on this page,
each with an unpaired electron on the ..F.... H ..F.... H
nitrogen atom. Ask them to draw
another plausible resonance structure. .. .. ¬ B ϩ ..N ¬ H .. .. ¬ B d N ¬ H
(They could draw a structure with the F.. F..
unpaired electron on an oxygen atom.)
..F.... H ..F.... H

Checkpoint Give two examples of exceptions to the octet rule.

Expected Outcome A reddish-brown 228 Chapter 8
gas is produced.

Facts and Figures electron on oxygen atoms. When free radi-
cals interact, they share their unpaired elec-
Free Radicals trons and create a dimer.Thus, dinitrogen
Nitrogen dioxide has an odd number of tetroxide, N2O4 consists of two nitrogen
valence electrons (17), so one electron must dioxide molecules joined by a N-N covalent
be unpaired. Molecules with unpaired elec- bond.The bond is temperature dependent;
trons are called free radicals and tend to be at higher temperatures the bond breaks as
reactive.Two resonance structures for NO2 shown in an equilibrium diagram on p. 557.
appear on this page.These have the
unpaired electron on the nitrogen atom.Two
other possible structures place the unpaired

228 Chapter 8

Figure 8.12 Phosphorus pentachloride, used as a 3 ASSESS
chlorinating and dehydrating agent, and sulfur
hexafluoride, used as an insulator for electrical equipment, Evaluate Understanding L2
are exceptions to the octet rule. Interpreting Diagrams
How many valence electrons does the sulfur in sulfur Ask student to write electron dot struc-
hexafluoride (SF6 ) have for the structure shown in the tures for SBr2 and C3H8. Have pairs of
figure? students decide whether their struc-
tures are correct and work together to
C l Cl F F F correct any errors. Have students
F ƒ F define a coordinate covalent bond in
ƒ √ ≈ √ their own words. (A coordinate covalent
√ S ≈ bond is formed when one atom contrib-
Cl P ≈ utes both bonding electrons.) Ask stu-
ƒ dents to explain resonance structures.
ƒ l Cl (Resonance structures occur when two or
F more valid electron dot structures can be
C written for a molecule.)

Phosphorus pentachloride Sulfur hexafluoride Elements Handbook

A few atoms, especially phosphorus and sulfur, sometimes expand the CFCs react with ozone, thinning the
octet to include ten or twelve electrons. Consider phosphorus trichloride ozone layer. Banning the use of CFCs
(PCl3) and phosphorus pentachloride (PCl5). Both are stable compounds should help decrease the amount of
in which all of the chlorines are bonded to a single phosphorus atom. CFCs reaching Earth’s ozone layer.
Covalent bonding in PCl3 follows the octet rule because all the atoms have
eight valence electrons. However, as shown in Figure 8.12, the electron If your class subscribes to the
dot structure for PCl5 can be written so that phosphorus has ten valence Interactive Textbook, use it to
electrons. review key concepts in Section 8.2.

8.2 Section Assessment with ChemASAP

13. Key Concept What electron configurations 22. Use the bond dissociation energies of H2 Answers to...
do atoms usually achieve by sharing electrons to (435 kJ/mol) and of a typical carbon–carbon
form covalent bonds? bond (347 kJ/mol) to decide which bond is Checkpoint NO2, BF3
stronger. Explain your reasoning. Figure 8.12 In sulfur hexafluoride,
14. Key Concept How is an electron dot structure the sulfur atom must have twelve
used to represent a covalent bond? Handbook valence electrons.

15. Key Concept When are two atoms likely to Ozone Read the feature on ozone on page R31 of
form a double bond between them? A triple bond? the Elements Handbook. Describe the effect of CFCs
on the ozone layer. Explain why the United States
16. Key Concept How is a coordinate covalent has banned the use of CFCs in aerosols.
bond different from other covalent bonds?
Assessment 8.2 Test yourself
17. Key Concept How is the strength of a covalent on the important concepts
bond related to its bond dissociation energy? of Section 8.2.

18. Key Concept Draw the electron dot resonance with ChemASAP
structures for ozone and explain how they describe
its bonding. Section 8.2 The Nature of Covalent Bonding 229

19. Key Concept List three ways in which the
octet rule can sometimes fail to be obeyed.

20. What kinds of information does a structural for-
mula reveal about the compound it represents?

21. Draw electron dot structures for the following

molecules, which have only single covalent bonds.

a. H2S b. PH3 c. ClF

13. the configurations of noble gases Section 8.2 Assessment There are also molecules in which an
14. Two dots represent each covalent bond. atom has fewer, or more, than a com-
15. when they can attain a noble-gas struc- sponds to a strong covalent bond. plete octet of valence electrons.
18. O O O ←→ O O O 20. the arrangement of atoms in a molecule
ture by sharing two pairs or three pairs
of electrons The actual bonding of oxygen atoms in 21. a. S H b. H P H c. Cl F
16. The shared electron pair comes from one of ozone is a hybrid, or mixture, of the ex- HH
the bonding atoms.In other covalent bonds tremes represented by the resonance
each bonding atom provides an electron. forms. 22. The H-H bond is stronger because it has a
17. A large bond dissociation energy corre- 19. The octet rule cannot be satisfied in greater dissociation energy.
molecules whose total number of va-
lence electrons is an odd number.

Covalent Bonding 229