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
- 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.
- SP = ½
(16 - 10) = 3 pairs must be shared.
- 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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