Chemistry - A Molecular Science

Chapter 5 The Covalent Bond

Table 5.3

Average bond energies and bond lengths of bonds

involving C, N, and O

Bond

Bond Energy (kJ/mol)

Bond Length (Å)

Bond

Bond Energy (kJ/mol)

Bond Length (Å)

C-O 358

1.43

N-O

201

1.44

C=O 799

1.23

N=O

607

1.20

C≡

O

1072 1.13

O-O

204

1.48

C-C 347

1.54

O=O

498

1.21

C=C 612

1.33

N-N

163

1.47

C≡

C

820 1.20

N=N

418

1.24

C-N 305

1.47

N≡

N

941 1.10

C=N 615

1.27

C≡

N

891 1.15

CO

Example 5.3

shown in each Lewis structure must equal the sum of the valence electrons of the atoms in the molecule.

An important feature of a bond is its

bond order

, which is the number of shared pairs

it contains. The chlorine-chlorine bond is said to be a

single bond

or to have a bond order

of one because it contains a single pair of shared electrons. The bond in O

contains two 2

shared pairs of electrons, which gives it a bond order of two; it is a

double bond

. The

three shared pairs of electrons in N

form a 2

triple bond

, or a bond with a bond order of

three. As shown in Table 5.3,

the bond strength of a specific bond increases and its bond

length decreases as its bond order increases

. However, the bond length also depends on

the bound atoms (Section 5.1), so this gene

ralization cannot be applied to bonds of

different types. Thus, we can predict that a C-C bond is longer than a C

C bond, but we ≡

cannot compare an H-H bond with a C

C bond. Indeed, the H-H bond (0.74 Å) is shorter ≡

than the C

C bond (1.20 Å) even though the bond or≡

der of the H-H bond is lower because

the bound atoms are much smaller. Example 5.3

Draw the Lewis structure for carbon monoxide, determine the carbon-oxygen bond order, and describe the bond dipole. Carbon monoxide is CO, so

1. ER = (2 atoms)(8 electrons/atom) = 16 electrons.
   2. VE = 4 from carbon + 6 from oxygen = 10 valence electrons, or five pairs.

Our Lewis structure must show five pairs of electrons.

3. SP = ½

(16 - 10) = 3 pairs must be shared.

4. The three shared pairs give each

atom six electrons, so each needs one

lone pair to obtain

an octet. See Lewis structure in the margin.

The C-O bond contains three shared pairs, so it is a triple bond;

i.e

., the bond order is 3.

Oxygen is more electronegative than carbon, so the bond dipole would point from the carbon to the oxygen.

5.6

DETERMINING LEWIS STRUCTURES

The same prescription used to determine the number of shared pairs in diatomic molecules can be used for polyatomic molecules. In or

der to determine the Lewis structure of any

molecule obeying the octet rule, simply use the following prescription:

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