CHAPTER 12: CHEMICAL BONDING

INTRODUCTION

There are one hundred and nine elements now known, which seems like a lot of elements, yet theelements can combine to produce a far greater number of different molecules. One of the mostimportant substances in our environment, water, is made from two hydrogen atoms and oneoxygen atom which have bonded together. Understanding how and why the elements combine isa fundamental chemical concept that helps chemists predict the structures and properties of newmolecules.

GOALS FOR THIS CHAPTER

1. Know the different types of bonds that can form between atoms. (Section 12.1)2. Know why different kinds of bonds form. (Section 12.1)3. Know which atom in a covalent bond (if any) is more electronegative. (Section 12.2)4. Know how electronegativity varies from left to right across the periodic table, and from

bottom to top. (Section 12.2)5. Be able to tell which molecules will be polar, based on bond polarity. (Section 12.3)6. Show how representative metals and nonmetals react to form ions with noble gas electron

configurations. (Section 12.4)7. Predict formulas of ionic compounds based on noble gas configurations of ions. (Section

12.4)8. Understand the structure of ionic compounds. (Section 12.5)9. Predict the size of an anion or cation, relative to the size of the neutral atom, and based on

location within the group of the periodic table. (Section 12.5)10. Be able to write Lewis structures for ionic and covalent compounds and molecules with

double and triple bonds. (Sections 12.6 and 12.7)11. Be able to determine the molecular structure, electron arrangement, and bond angles

between pairs of atoms for a given molecule. (Section 12.8)12. Be able to state the premises of the VSEPR model. (Section 12.9)13. Be able to apply the VSEPR model to molecules with double bonds. (Section 12.10)

QUICK DEFINITIONS

Bond

Bond energy

Ionic bond

A force between atoms which holds them together as a unit.(Section 12.1)

The energy required to break a bond. (Section 12.1)

A bond formed when electrons are transferred from one atom toanother. (Section 12.1)

217

Ionic compound

Covalent bond

Polar covalent bond

Electronegativity

Dipole moment

Lewis structure

Duet rule

Octet rule

Bonding pair

Lone pairs

Single bond

Double bond

Triple bond

Resonance

The kind of bond which forms when a metal reacts with a nonmetaL(Section 12.1)

A bond formed when electrons are shared between two atoms.(Section 12.1)

A bond formed when electrons are shared unequally between twoatoms. One atom has a partial positive charge, and the other atomhas a partial negative charge. (Section 12.1)

A measure of the attraction an atom has for the electrons the atomshares in a covalent bond. (Section 12.2)

Molecules which have a polar covalent bond and have distinctpositive and negative ends. These molecules are said to have adipole moment, centers of positive and negative charge. (Section12.3)

A diagrammatic representation of a molecule showing how valenceelectrons are arranged among the atoms in the molecule. (Section12.6)

When hydrogen shares electrons, it tries to fill its valence electronorbital with two (a duet of) electrons. (Section 12.6),

Atoms other than hydrogen try to fill their valence electron orbitalswith eight (an octet of) electrons. (Section 12.6)

A pair of valence electrons shared in a covalent bond. (Section12.6)

Pairs of electrons not shared in a covalent bond. (Section 12.6)

One pair of electrons shared between two atoms. (Section 12.7)

Two pairs of electrons between two atoms. (Section 12.7)

Three pairs of electrons shared between two atoms. (Section 12.7)

A situation which occurs when more than one Lewis structure canbe drawn for a molecule. (Section 12.7)

218 12 Chemical Bonding

Resonance structures

Molecular structure

Bond angle

Linear

Trigonal planar

Tetrahedral structure

VSEPRmodel

Trigonal pyramid

Effective pair

PRETEST

The possible Lewis structures which can be drawn for one molecule.(Section 12.7)

The arrangement in space of atoms in a molecule. Also called thegeometric structure. (Section 12.8)

An angle described by the location of two or three atoms in space.Bond angles help describe the shape of a molecule. (Section 12.8)

A molecular structure that occurs when pairs of electrons whichspread out on either side of a central atom to form bond angles of180 degrees. (Sections 12.8 and 12.9)

A molecular structure which occurs when pairs of electrons whichspread out flat in space to form a triangle shaped molecule withbond angles of 120 degrees. (Section 12.8)

A molecular structure which occurs when there are four pairs ofelectrons which spread out to the four corners of a tetrahedron toform bond angles of 109.5 degrees. (Sections 12.8 and 12.9)

The Valence Shell Electron Pair Repulsion model says that electronsshared in a bond tend to spread out away from other bondingelectrons as far as possible. (Section 12.9)

A molecular structure which occurs when there are three sharedpairs of electrons and one lone pair of electrons, as in ammonia.(Section 12.9)

Two pairs of electrons (a double bond) shared between two atomsthat act as a single unit. (Section 12.10)

1. What force holds atoms together in a covalent bond?

2. What kind of bond forms between lithium atoms and oxygen atoms?

3. Which atom is the least electronegative, Mg, Ba, or CI?

4. Is the bond formed between S and CI ionic, covalent or polarcovalent?

Pretest 219

5. How many electrons do °and Si need to transfer or share to complete an octet?

6. Is N3- smaller or larger than the neutral atom, N?

7. What is the Lewis structure of Si02?

8. What is the Lewis structure of CIO-?

9. Use VSEPR to determine the molecular structure of S02'

10. Use VSEPR to determine the molecular structure of HCN.

PRETEST ANSWERS

1. The attraction of both nuclei for a pair of electrons holds the atoms together in a covalentbond. (12.1)

2. Two metallic lithium atoms lose electrons to a non-metal oxygen atom to form lithiumoxide, Li20 . The bond between the lithium ions and the oxide ion is an ionicbond. (12.1)

3. Barium is farthest to the left of the periodic table, and nearest the bottom. It is the leastelectronegative of the elements Mg, Ba and Cl. (12.2)

4. The electronegativity of S is 2.5 while that of Cl is 3.0. The difference is 0.5. This is apolar covalent bond. (12.2)

5. Oxygen is in Group 6 of the periodic table. It has 6 valence electrons and needs to shareor transfer 2 electrons to form an octet. Silicon has 4 valence electrons. It needs to sharefour electrons to form an octet. (12.4)

6. The nitride anion, N3-, is larger than a neutral nitrogen atom. (12.5)

7. The silicon atom must share two pairs of electrons with each oxygen atom to form an octet.The Lewis structure is (12.6 and 12.7)

. .:0:: Si ::0:. .

220 12 Chemical Bonding

8. Ions have the same number of electrons as a neutral atom, plus or minus the charge on theion. The CIO' ion has 14 valence electrons. Its Lewis structure is (12.6 and 12.7)

[ :ci:o:]-.. ..

9. One Lewis structure of S02 is

.S. .·0:. . . .

• O·...Its molecular structure is trigonal planar.

10. The Lewis structure of HCN is

Its molecular geometry is linear.

CHAPTER REVIEW

12.1 TYPES OF CHEMICAL BONDS

How Does a Bond Form Between Two Atoms?

(12.9)

(12.9)

A chemical bond can form between any two atoms when the new combination of atoms is morestable than are the individual atoms.

When a metal forms a bond with a nonmetal, electrons are transferred from the metal to thenonmetal, creating oppositely charged ions. The oppositely charged ions are attracted to eachother which produces an ionic bond. Two nonmetals can also form a bond. When twononmetals bond, they share electrons to form a covalent bond.

Potassium is an alkali metal from Group 1 of the periodic table, and bromine is a halogen(nonmetal) from Group 7. They can form a chemical bond when potassium loses its one valenceelectron to bromine. The potassium ion with a positive charge is attracted to the bromide ionwith a negative charge and the ionic compound KBr is formed.

Carbon and chlorine are both nonmetals on the right end of the periodic table. Reactionsbetween metals and nonmetals result in the formation of ionic bonds. But when two nonmetalscombine they form a chemical bond by sharing electrons. Each nucleus attracts the electrons andas a result, the electrons are held between each nucleus.

Chapter Review 221

12.2 ELECTRONEGATIVITY

What Happens When Electrons Are Not Shared Equally?

Sometimes electrons are not shared equally between two bonding atoms. The electrons areattracted to one of the atoms more than the other. Electronegativity is a measure of the amountof attraction an atom has for the electrons it shares. Unequal sharing results in the formation ofpolar covalent bonds.

In bonds which form between two atoms of the same element, the electrons are shared equallybetween the two atoms. The attraction of each identical nucleus for the electrons is the same.For example, in a molecule of Nz the bonding electrons are shared equally by both nitrogenatoms.

In bonds which form between two dissimilar atoms such as hydrogen and chlorine, the electronsare shared unequally. The tendency of an atom in a bond to attract the electrons it shares towarditself is often greater for one atom than for the other. The measure of this attraction is calledelectronegativity. A difference in electronegativy between two atoms results in unequal sharing.Chlorine has an electronegativity value of 3.0 and hydrogen has a value of 2.1. Because chlorinehas a value greater than hydrogen, the electrons in the bond are more likely to be found closer tochlorine. A listing of electronegativity values is given in Figure 12.3 of your textbook.

There are some periodic trends in electronegativity. With the exception of the noble gases,elements on the upper right of the periodic table have the highest electronegativity values.Electronegativity values generally increase from left to right, and from bottom to top. If youforget the exact electronegativity value for an atom, you should be able to tell which atom in abond is more electronegative just from the position of the two atoms in the periodic table relativeto each other.

In the bond formed between nitrogen and hydrogen, hydrogen has an electronegativity value of2.1, while nitrogen has a value of 3.5. The electronegativity difference is 1.4 units. Becausenitrogen has a larger electronegativity value, the nitrogen atom attracts the electrons more than Hdoes. We say that nitrogen has a partial negative charge because the probability of finding theshared electrons near the nitrogen atom is greater than near the hydrogen atom. In this bondhydrogen has a partial positive charge because the probability of finding the shared electron nearit is smaller than near the nitrogen atom.

Both atoms in the carbon-sulfur bond have the same electronegativity value. The sharedelectrons are attracted equally by both atoms. This is an example of a covalent bond betweentwo dissimilar atoms which is not polar. Neither atom is partially positive nor partially negative.

222 12 Chemical Bonding

12.3 BOND POLARITY AND DIPOLE MOMENTS

How Does the Polarity ofa Bond Affect the Polarity ofa Molecule?

The bond between hydrogen and fluorine in HF is a polar bond. The fluorine atom bears apartial negative charge, and the hydrogen a partial positive charge. Diatomic molecules such asHF which have a center of positive charge and a center of negative charge are said to have adipole moment.

partially positive end8+ <r---II...... H-F ~IIIIIII--- partially negative end

Some molecules with more than two atoms also have a dipole moment. Water has two atoms ofhydrogen and one atom of oxygen. Each of the bonds between hydrogen and oxygen is a polarbond. The oxygen bears a partial negative charge, while each of the hydrogen atoms bears apartial positive charge. The end of the water molecule with the oxygen atom has a center ofnegative charge, and the end with the two hydrogen atoms has a center of positive charge.

partially positive end partially negative end

The dipole moment of a water molecule plays an important role in the properties of water. Forexample, polar water molecules attact each other. The partially positive end of one watermolecule is attracted to the partially negative end of another. So water molecules stick together.A great amount of heat is required to change liquid water to a gas. As a result, water on theearth's surface remains in the liquid state. Polar water molecules also interact with ioniccompounds. The partially positive end of water is attracted to negative ions of an ioniccompound and dissolves them in solution.

12.4 STABLE ELECTRON CONFIGURATIONS AND CHARGES ON IONS

How Can You Determine How Many Electrons Will Be Lost or Gained When Ions Form?

With a few exceptions atoms tend to form bonds by achieving a noble gas configuration ofvalence electrons. This means most atoms share or transfer enough electrons to have eight (anoctet) in their valence shells. Magnesium, a metal from Group 2 of the periodic table, loses twoelectrons to a nonmetal.

By losing two electrons, magnesium achieves the same stable valence electron configuration asneon, which is 1s22i2p6. Elements tend to gain, lose or share electrons so that their valence

electron configuration is the same as the nearest noble gas.

Chapter Review 223

Each chlorine atom in a chlorine molecule gains an electron from a metal to form two chlorideions.

A chlorine atom which has an electron configuration of 122s22p63i3p5 can achieve the valence

electron configuration of the noble gas argon, li2i2p63s23p6, by gaining 1 electron.

Most representative elements follow the octet rule, but there are exceptions among thenonrepresentative elements.

12.5 IONIC BONDING AND STRUCTURES OF IONIC COMPOUNDS

What Is the Structure ofIonic Compounds?

Potassium chloride forms a regular crystalline structure made of potassium and chloride ions heldtogether by the attractions between the oppositely charged ions. The smaller cations alternate inthe structure with larger anions. In the diagram below, the black spheres represent the anions,while the white spheres represent the cations. The diagram shows only a small part of apotassium chloride crystal.

Ionic compounds can be made from a metal and a polyatomic ion, as well as from a metal and anonmetal. The ionic compound sodium sulfate is composed of two sodium ions, Na\ and asulfate ion, SO/-. The sulfate ion as a unit has a 2- charge. Two sodium ions and the sulfate ion

are bonded together by attractions.

What is the Size ofan Ion, Compared With an Atom?

Anions are larger than the parent atoms because they have one or more extra electrons, whichincrease the distance from the nucleus to the outermost electron. Cations are usually smaller thanparent atoms.

224 12 Chemical Bonding

12.6 LEWIS STRUCTURES

How Can You Write Lewis Structures For Simple Molecules?

Lewis structures show the arrangement of the valence electrons in compounds. When writingLewis structures of ionic compounds, show the valence electron configuration after electronshave been transferred from the cation to the anion. You can also draw Lewis structures forcovalent molecules. When you first begin to draw Lewis structures, follow the steps from yourtextbook until you have gained some experience. These steps are repeated below.

Steps For Writing Lewis Structures

Step 1

Step 2

Step 3

Obtain the sum of the valence electrons from all of the atoms. Do not worryabout keeping track of which electrons come from which atoms. It is the totalnumber of electrons that is important.

Use one pair of electrons to form a bond between each pair of bound atoms.For convenience, a line (instead of a pair of dots) is usually used to indicateeach pair of bonding electrons.

Arrange the remaining electrons to satisfy the duet rule for hydrogen and theoctet rule for the second-row elements.

Example:What is the Lewis structure of lithium iodide?

Lithium iodide is an ionic compound. Lewis structures of ionic compounds are drawn byremoving one or more electrons from the metal to produce an ion with a noble gas config­uration of valence electrons. A bromine atom has seven valence electrons, while a lithiumatom has one. When an ionic bond is formed between lithium and bromine, an electron isremoved from lithium and transferred to bromine. Bromine achieves an octet with thesame valence electron configuration as the noble gas krypton, while lithium achieves aduet with the same valence electron configuration as helium. When writing Lewisstructures of ionic compounds, remember to show the charges present on the ions.

L·+ ·B···­1 • r.

We do not put a dash between the lithium and the bromide ions because there are no sharedelectrons.

Chapter Review 225

Example:What is the Lewis structure of Br2?

Let's follow the three steps for writing Lewis structures given in section 12.6 of yourtextbook. Step 1. Determine the total number of valence electrons in all atoms of themolecule. In this molecule each bromine atom has seven valence electrons, so the totalnumber of valence electrons is fourteen. Step 2. Place a pair of electrons between eachpair of atoms which are bonded together. In this molecule, arrange one pair of electronsbetween the two bromine atoms. Step 3. If there are electrons remaining, distribute themaround the atoms in the molecule so that hydrogen satisfies the duet rule and other atomssatisfy the octet rule. In this molecule the number of valence electrons left is twelve.Arrange these electrons around the bromine atoms as unshared pairs until each bromineatom has an octet.

.. ..:Br-Br:..

Example:What is the Lewis structure of HCI?

Step 1. There are eight valence electrons in HCI, one contributed by hydrogen and sevenby chlorine. Step 2. One electron pair will be shared between hydrogen and chlorine.

H-CI

Step 3. The six remaining electrons should be distributed around the atoms until completeduets or octets are achieved. By sharing an electron pair with chlorine, hydrogen hasachieved a duet, so the remaining six electrons must be arranged as unshared pairs aroundthe chlorine atom.

H-Cl:

Now, both hydrogen and chlorine are satisfied.

12.7 LEWIS STRUCTURES OF MORE COMPLEX MOLECULES

How Can You Write Lewis Structures For Molecules with Double and Triple Bonds?

Example:What is the Lewis structure of~CO?

For more complicated structures, use the same three rules we have used previously.Step 1. The total number of valence electrons is twelve, two from the hydrogens, fourfrom carbon, and six from oxygen. Step 2. Arrange a pair of electrons between eachatom.

226 12 Chemical Bonding

..

H

IH-C-O

Step 3. We have used six electrons between the atoms. There are now six electrons leftunused. Each hydrogen satisfies the duet rule, so distribute the remaining six electronsaround the oxygen atom.

H

IH-C-O:

In this Lewis structure the total number of valence electrons is OK, but carbon does nothave an octet. There must be another solution. If carbon and oxygen share two pairs ofelectrons, then the remaining four electrons can be arranged as unshared electrons aroundthe oxygen. All the atoms are now satisfied according to the octet rule. As you can see,the first solution will not always work. You must find the arrangement which accounts forall the valence electrons, and satisfies the octet or duet rule for all atoms.

H

I .H-C=O:.

12.8 MOLECULAR STRUCTURE

What Is Meant by Molecular Structure?

The molecular structure of a molecule is the three-dimensional shape the molecule assumes inspace. We can often define molecular structure by looking at the angle formed by atoms in amolecule. This angle is called the bond angle. Typical bond angles are 180° for linearmolecules and 120° for trigonal planar molecules. Some molecules have bond angles of 106°and are bent, while others have bond angles of 109.5° and are tetrahedral in shape.

12.9 MOLECULAR STRUCTURE: THE VSEPR MODEL

What Is the VSEPR Model?

The VSEPR model says that the structure of a molecule depends on the spreading out in spaceof electron pairs as far away from each other as possible. When there are two electron pairsaround an atom, each pair spreads out so that there is one pair on each side of an atom. Thisproduces a linear structure. We can illustrate this with some hypothetical atoms, x and y. x is

Chapter Review 227

the central atom and is bonded to two y atoms. When the two electron pairs shared betweenatom x and atom y spread out, they move to opposite sides of the x atom, producing an angle of180°.

180°~

y-x-y

When there are three pairs of electrons around the x atom, the y atoms spread out to form a flatplane, with equal bond angles of 120°. This type of structure is called trigonal planar.

When there are four electron pairs surrounding the x atom, the y atoms appear to produce bondangles of 90°. But, the electron pairs can spread out even further in three dimensions to form atetrahedron. The bond angles in this shape are 109.5°, which is greater than 90°. This is called atetrahedral arrangement.

yI:\109.5'

y/i'Zyy

How Can the Shape ofa Molecule Be Predicted Using the VSEPR Model?

It is possible to predict the shape of a molecule whose shape you do not know. For example,carbon tetrachloride has the formula CCI4• To determine the molecular structure for CCI4, wecan follow the steps in section 12.9 of your textbook. Step 1. Draw the Lewis structure of themolecule. Let's draw the Lewis structure for carbon tetrachloride. There are a total of thirty-twovalence electrons in CCI4, four from carbon and seven from each of the four chlorine atoms.Arrange four pairs between the carbon atom and each of the chlorine atoms.

228 12 Chemical Bonding

CI

ICI-C-CI

ICI

This gives carbon an octet. Distribute the remaining electrons around the chlorine atoms toproduce

..:Cl:

.. I ..:CI-C-CI:.. I ..

.o:..Step 2. Count the electron pairs and arrange them as far apart as possible. From the Lewisstructure, we can see that there are four pairs of electrons around the central atom, carbon.Step 3. All four pairs of electrons are shared between carbon and chlorine atoms. All fourchlorine atoms will occupy a comer of the tetrahedron. Step 4. When four atoms occupy thecomers of a tetrahedron, the molecular shape is a tetrahedron and the bond angle is 109.5°.

CI

t\l0905'CI/ I"CI

CI

12.10 MOLECULAR STRUCTURE: MOLECULES WITH DOUBLE BONDS

How Can the Shape ofMolecules With Double Bonds be Predicted Using the VSEPR Model?

The VSEPR model can predict the molecular geometry of molecules with double bonds as wellas those with only single bonds. We can use the sulfate ion as an example. In each of the tworesonance structures sulfur shares six electron pairs with four oxygen atoms. Two of the bondsto oxygen are double bonds. Sulfur shares two pairs of electrons with two of the oxygen atomsto form the double bonds.

Chapter Review 229

. .• O"

.. II ..:9'-Ii-Q:

.0.

and

..:0:

. I .:o=s=o:. I .

:0:..

The VSEPR model as applied to molecules with double bonds says that two pairs of electrons actas one region of electron density. The two pairs of electrons become an "effective pair". Sowhen counting the pairs of electrons surrounding an atom a double bond counts as one pair. Thesulfate ion has four effective pairs of electrons. The molecular geometry of molecules with foureffective pairs of electrons is tetrahedral.

LEARNING REVIEW

1. What are the two kinds of bonds which can form between atoms?

2. What kind of bond forms between

a. two identical atoms?b. a metal and a nonmetal?

3. What is meant by a polar covalent bond?

4. Which of the choices has an ionic bond?

a. COb. CaBrzc. HBr

d. Clz

5. Arrange the following atoms based on electronegativity. Put the most electronegativeatom on the right and the least electronegative atom on the left.

P Al CI Mg

6. Which bond is the most polar? Which bond is the least polar?

a. P-CIb. H-Hc. N-Hd. C-F

230 12 Chemical Bonding

7. Write electron configurations for both reactants and products for the reactions below.

a. Mg + Cl2 ----- Mg2+ + 2cr

b. 2Li + S ----- 2Li+ + S2-

8. When two nitrogen atoms combine to form a nitrogen molecule, how many electrons areshared to give each atom a complete octet?

9. How many electrons do each of the atoms below need to gain, lose or share to achieve anoble gas valence electron configuration?

a. Sb. Mgc. C

10. In which of the atom/ion pairs is the ion smaller than the atom?

a. S/S2-

b. Ca/Ca2+

c. Li/Li+

d. I/f

11. Draw Lewis structures for these ionic compounds.

a. MgS

b. N~O

12. Write valence electron configurations for the following atoms.

a. Bb. Src. Krd. CI

13. Draw Lewis structures for the molecules or ions below.

a. H2S

b. HCCHc. PO 3-

4

d. HIe. PCl3

Learning Review 231

14. Some molecules have Lewis structures which violate the octet rule. Draw a probableLewis structure for BeIz.

15. Use the VSEPR model to determine the molecular structure of each of the moleculesbelow.

a.b.c.

d.

,0H-C-H

ANSWERS TO LEARNING REVIEW

1. When atoms combine they form either ionic bonds or covalent bonds. In an ionic bond,electrons are completely transferred. In a covalent bond, electrons are shared betweenatoms. Ionic bonds usually form when a metal and a nonmetal react. Covalent bondsusually form when two nonmetals react.

2. a.b.

When two identical atoms bond, a nonpolar covalent bond forms.When a metal and a nonmetal bond, an ionic bond forms.

3. Electrons shared between two atoms are not always shared equally. Sometimes, theelectrons are attracted to one of the atoms more than the other. The atom in a bond whichattracts the electron pair will have an extra electron part of the time and so bear a partialnegative charge. The atom which does not strongly attract the electron pair it shares willbe electron deficient part of the time and so bears a positive charge. The kind of covalentbond where the electrons are not shared equally is called a polar covalent bond.

4. Ionic bonds are formed when a metal loses an electron to a nonmetal. Among thesechoices, the only bond between a metal and a nonmetal is the bond formed betweencalcium and bromine to form calcium bromide. So the correct answer is b.

5. Elements on the right side of the periodic table have higher electronegativity values thando elements on the left side of the periodic table. The atom with the highestelectronegativity would be CI, because it is in the upper righthand corner of the periodictable; then P, then AI, and Mg, on the left side of the periodic table is the lowest.

232 12 Chemical Bonding

Mg Al P CI