Modern inorganic chemistry

48 STRUCTURE AND BONDING

Table 2.10

BOND ENERGIES

Bond In compound Average thermochemical(kJmor (^1) ) bond energy
C^H
N— H
0— H
F—H
Cl— H
C— Cl
N— Cl
Si— Cl
C— C
C=C
C==C
N^N
N=N
0— O
0=0
CH 4
NH 3
H 2 O
HF
HC1
CC1 4
NC1 3
SiCl 4
C 2 H 6
C 2 H 4
C 2 H 2
N 2 H 4
N 2
H 202
(^02)
416
391
467
566
431
327
193
391
346
598
813
160
946
146
498
in a few cases does the covalent bonding extend throughout the
whole structure and in these cases a 'giant molecule' is produced.
In diamond, each carbon atom has four covalent links tetrahedrally
arranged. Since the bonds are strong the molecule is very stable and
extremely hard. Carborundum (Si—C) and boron nitride have
similar structures and properties. The high melting points of these
solids correctly indicates that the covalent bonds are usually
stronger than ionic bonds.

COVALENT BOND LENGTHS

As in the case of ions we can assign values to covalent bond lengths
and covalent bond radii. Interatomic distances can be measured by,
for example, X-ray and electron diffraction methods. By halving the
interatomic distances obtained for diatomic elements, covalent
bond radii can be obtained. Other covalent bond radii can be deter-
mined by measurements of bond lengths in other covalently bonded
compounds. By this method, tables of multiple as well as single
covalent bond radii can be determined. A number of single covalent
bond radii* in nm are at the top of the next page.

• While bond energies increase in, for example, the sequence C—C, C=C. CEE^
  {Table 2.10). bond radii decrease: C=C gives C = 0.067. feC gives C = 0,060 nm