Physical Chemistry Third Edition

850 20 The Electronic States of Diatomic Molecules

Exercise 20.14

a.Write a simple valence-bond wave function for the ground state of diatomic fluorine.

Construct the bonding factor from 2pzorbitals and place the “lone pairs” in the nonbonding

2 s,2px, and 2pyorbitals.

b.Write a modified valence-bond wave function for the ground state of diatomic fluorine,

including ionic terms.

Excited States of Homonuclear Diatomic Molecules

Excited states are represented in the LCAOMO approximation by electron configura-

tions other than that of the ground state. Term symbols can be written for an excited

electron configuration.

EXAMPLE20.8

a.Write the term symbols that can occur for the excited Be 2 electron configuration

(σg 1 s)^2 (σ∗u 1 s)^2 (σg 2 s)^2 (σ∗u 2 s)(σu 2 p1)

b.Which term will have the lowest energy?

c.What is the bond order for this electron configuration?

d.Do you think the Be 2 molecule could exist in the excited state corresponding to this

electron configuration?

Solution

a.ML1 and there are two u orbitals, so that the term symbols are^3 Πgand^1 Πg.

b.The lowest energy term symbol will have the maximum value ofS, according to Hund’s

rule:^3 Πg

c.Bond order(1/2)(5−3) 1

d.Since the bond order is nonzero, the molecule should exist in this configuration.

PROBLEMS

Section 20.3: Homonuclear Diatomic Molecules

20.14Give the term symbol or symbols for each of the
excited electron configurations of the hydrogen
molecule:
a. (σg 1 s)(πu 2 p1)

b. (πu 2 p1)(σ∗g 2 p,−1)

20.15Give the term symbol or symbols for each of the excited
electron configurations of the hydrogen molecule:

a.(σg 2 p0)^2

b.(σg 1 s)(σg 2 p0)

20.16Give the term symbol for the ground-state electron

configuration of each of the second-row homonuclear

diatomic molecules (Li 2 through F 2 ).

20.17Excited states of diatomic neon exist, although the ground

state has bond order zero. Give the electron configuration

and term symbol for two different excited states that

might exist.