Physical Chemistry Third Edition

20.4 Heteronuclear Diatomic Molecules 855

z

y

2 s (a)

(b)



z

y

2 pz



z

y

2 sp(1)



z

y

2 sp(2)



Figure 20.15 Orbital Regions for Nonhybridized and Hybridized Atomic Orbitals.

(a) The nonhybridized orbital regions. (b) The hybridized orbital regions.

has cylindrical symmetry around the bond axis, as is always the case with a sigma

orbital.

The 3σmolecular orbital has roughly equal coefficients for the 2sand 2patomic

orbitals with a nearly negligible contribution from the hydrogen 1sorbital. We approx-

imate it by the 2sp(2)Li hybrid orbital. The 4σorbital can be approximated as a linear

combination of the 2sp(1)Li hybrid orbital and the hydrogen 1sorbital. Our new orbitals

only roughly approximate the Hartree–Fock–Roothaan orbitals but can be used for a

qualitative description of the electronic states of the LiH molecule. With two electrons

in the nonbonding 1sLi orbital and two in the bonding 2σorbital, we have a single

bond.

Exercise 20.16

a.Is the 3σmolecular orbital a bonding, an antibonding, or a nonbonding orbital?

b.Estimate the coefficients to represent the 4σmolecular orbital as a linear combination of the

2 sp(1)Li hybrid orbital and the 1sH orbital. Is this a bonding, an antibonding, or a nonbonding

orbital?

Dipole Moments

In the approximate 2σbonding orbital the coefficient of the 1sH orbital is roughly twice

as large as the coefficient of the 2sp(1)Li orbital. There is thus a larger probability of

finding an electron in the vicinity of the hydrogen nucleus than in the vicinity of the