1 (311)

Prévia do material em texto

SOLUTIONSMANUAL TO ACCOMPANY ATKINS' PHYSICAL CHEMISTRY 303
D9D.4 �e Coulomb integral is essentially the energy of an electron when it occupies
an atomic orbital in the molecule.
�e resonance integral is a contribution to the energy of a molecule that can be
associated with an electron interacting with more than one nucleus at once.
Solutions to exercises
E9D.1(b) �e molecular orbital energy level diagram for a heteronuclear diatomic AB
is similar to that for a homonuclear diatomic A2 (Fig. 9C.11 on page 361 or
Fig. 9C.12 on page 361) except that the atomic orbitals on A and B are no longer
at the same energies. As a result the molecular orbitals no longer have equal
contributions from the orbitals on A and B; furthermore, it is more likely that
the s and p orbitals will mix. From simple considerations it it not possible to
predict the exact ordering of the resulting molecular orbitals, so the diagram
shown Fig. 9.7 is simply one possibility. Note that because the heteronuclear
diatomic no longer has a centre of symmetry the g/u labels are not applicable.
�e electronic con�gurations are:
(i) XeF (15 valence electrons) 1σ2 2σ2 3σ2 1π4 2π4 4σ1; (ii) PN (10 valence elec-
trons) 1σ2 2σ2 3σ2 1π4; (iii) SO– (13 valence electrons) 1σ2 2σ2 3σ2 1π4 2π3.
A BMolecule
3s or 5s
3p of 5p
3σ
2σ
4σ
1σ
1π
2π
2s
2p
Figure 9.7
E9D.2(b) �e molecular orbital diagram of IF is similar to the one shown in the solu-
tion to Exercise E9D.1(b) where the orbitals on atom A are 5s and 5p. It is
not possible to predict the precise energy ordering of the orbitals from simple
considerations, so this diagram is simply a plausible suggestion.
IF has 7+7 = 14 valence electrons, therefore the ground state electron con�gu-
ration is 1σ2 2σ2 3σ2 1π4 2π4. IF+ has one fewer electron than IF, therefore it has
one fewer electron on the antibonding 2π orbital. IF– has one more electron
than IF, therefore it has one more electron accommodated on the antibonding
4σ orbital. Hence the order of decreasing bond order is IF+, IF, IF– , which is
expected to be the same as the order of increasing bond length.