Physical Chemistry Third Edition

860 20 The Electronic States of Diatomic Molecules

The energies of the LCAOMOs and hybrid orbitals are shown schematically in the

correlation diagram of Figure 20.17. The line segment for the bonding orbital is drawn closer

to the fluorine side to indicate that this orbital has a greater contribution from the fluorine

atomic orbital. The ground-state electron configuration is

(1sF)^2 (2sp(2)F)^2 (2pxF)^2 (2pyF)^2 (1σ)^2

Exercise 20.18

Describe the bonding in LiF. How much ionic character do you think there will be in the bond?

A qualitative description of the chemical bonding in carbon monoxide (CO) is

obtained by constructing LCAOMOs similar to those of N 2 , except that the coeffi-

cients for the C orbital and the O orbital will not have equal magnitudes. We use all of

the atomic orbitals in the first two shells of both atoms as our basis set, but construct

LCAOMOs with no more than two atomic orbitals in each linear combination. The

2 pxand 2pyorbitals will combine to form pi molecular orbitals, while the 2pzorbitals

will combine to make a sigma molecular orbital. Oxygen’s effective nuclear charge is

greater than that of carbon so its orbital energies will lie somewhat lower than those

of carbon, and the bonding LCAOMOs will have somewhat larger coefficients for the

oxygen orbitals than for the carbon orbitals. The electron configuration of the molecule

is the same as that of the N 2 molecule, although the orbitals are different:

(σ 1 s)^2 (σ∗ 1 s)^2 (σ 2 s)^2 (σ∗ 2 s)^2 (π 2 px)^2 (π 2 py)^2 (σ 2 pz)^2

1 s

1 sF 1 sF

H atom

atomic

orbitals

Molecular

orbitals

Atomic

orbitals

(with

hybridization)

Atomic

orbitals

(nonhybridized)

2 sF

2 pzF2pxF^2 pyF

1   

2    *

2 sp(2)F

2 sF (replaced)

2 pzF (replaced)

Orbital energy (schematic)

Figure 20.17 Correlation Diagram for Hydrogen Fluoride.