Physical Chemistry Third Edition

20.4 Heteronuclear Diatomic Molecules 863

Table 20.4 Selected Values of Average

Bond Energies

Bond Bond Energy (kJ mol−^1 )

C–C 343

CC 615

C–Cl 326

C–F 490

C–H 416

C–N 290

C–O 351

Cl–Cl 244

F–F 158

H–Cl 432

H–H 436

H–F 568

H–N 391

N–N 160

N–O 176

O–O 144

O–H 464

From F. T. Wall,Chemical Thermodynamics, 3rd ed.,

W. H. Freeman, San Francisco, 1974, p. 63.

A rule of thumb is that if the difference between the electronegativities of two

elements is greater than 1.7, a bond between those elements will be primarily ionic.

A difference of less than 1.7 corresponds to a polar covalent bond, and a nonpolar

covalent bond requires a difference of zero.

Exercise 20.21

Classify the bonds between the following pairs of elements as purely covalent, polar covalent,

and primarily ionic: (a) Li and H, (b) C and O, (c) N and Cl, (d) H and F, (e) Li and F, and

(f) F and F.

The Valence-Bond Method for Heteronuclear Diatomic

Molecules

A valence-bond wave function like that of Eq. (20.3-7) or Eq. (20.3-20) corresponds to

equally shared electrons. Inclusion of both of the two ionic terms in Eq. (20.3-10) does

not correspond to ionic bonding, because one ionic term corresponds to bond polarity

in one direction and the other to bond polarity in the opposite direction. For a polar

molecule such as LiH a modified valence-bond wave function can be constructed by

including only one ionic term:

ΨMVBcVBΨVB+cIΨI (20.4-17)

whereΨVBcontains the covalent terms:

ΨVBψ 1 sLi(1)α(1)ψ 1 sLi(2)β(2)[ψ 2 sp(1)Li(3)ψ 1 sH(4)+ψ 1 sH(3)ψ 2 sp(1)Li(4)]

×[α(3)β(4)−β(3)α(4)] (20.4-18)