Physical Chemistry Third Edition

20.3 Homonuclear Diatomic Molecules 845

Orbital energy

Atomic

orbitals

1 sA 1 sA

2 sA 2 sB

2 pxA2pyA2pzA2pzB2pzB2pyB

Atomic

orbitals

Molecular

orbitals

*g 2 px

(^) g 2 pz
u 2 s
u 1 s
(^) g 2 s
(^) g 1 s
u 2 pz
(^) u 2 px (^) u 2 py
g 2 py
Figure 20.12 Correlation Diagram for Homonuclear Diatomic Molecules.
the appropriate value ofZare used. From Figure 19.4 it is apparent that the atomic
orbital energies depend on the nuclear charge, and the LCAOMO energies also depend
on nuclear charge as well as on internuclear distance. The order of the LCAOMO
energies in the figure is correct for the elements lithium through nitrogen for distances
near the equilibrium internuclear distance. For oxygen and fluorine theσg 2 pzbonding
orbital is lower in energy than theπu 2 pxandπu 2 pybonding orbitals. In a sigma orbital
the electron moves in a region more directly between the nuclei and would be more
effective in screening the nuclear charges from each other than in a pi orbital. With the
larger nuclear charges of the O and F atoms this screening is apparently more important
than with the other atoms.
We now have enough orbitals for the ground states of all of the homonuclear diatomic
molecules of elements of the first and second rows of the periodic table. The electron
configurations are shown in Table 20.2 along with the bond orders. In B 2 and O 2 the
final two electrons are assigned to different degenerate space orbitals with parallel spins
in accordance with Hund’s first rule, which applies to molecules as well as to atoms.
Hund’s second and third rules do not apply, becauseLis not a good quantum number
for molecules. The electron configurations in Table 20.2 convey all of the information
conveyed by writing an orbital wave function.
Two degenerate orbitals are analogous to a subshell and could be lumped together.
The ground-state electron configuration of O 2 would then be written
(σg 1 s)^2 (σ∗u 1 s)^2 (σg 2 s)^2 (σ∗u 2 s)^2 (σg 2 p)^2 (πu 2 p)^4 (π∗g 2 p)^2
This electron configuration does not explicitly show that the final two antibonding
electrons occupy different space orbitals.