The Foundations of Chemistry

MOLECULAR ORBITAL ENERGY LEVEL DIAGRAMS

Figure 9-5 shows molecular orbital energy level diagrams for homonuclear diatomic mole-
cules of elements in the first and second periods. Each diagram is an extension of the
right-hand diagram in Figure 9-2, to which we have added the molecular orbitals formed
from 2sand 2patomic orbitals.
For the diatomic species shown in Figure 9-5a, the two  2 porbitals are lower in energy
than the  2 porbital. Molecular orbital calculations indicate, however, that for O 2 , F 2 , and
hypothetical Ne 2 molecules, the  2 porbital is lower in energy than the  2 porbitals (see
Figure 9-5b).

9-2

Spectroscopic data support these

orders.

9-2 Molecular Orbital Energy Level Diagrams 357

Figure 9-5 Energy level diagrams for first- and second-period homonuclear diatomic
molecules and ions (not drawn to scale). The solid lines represent the relative energies of
the indicated atomic and molecular orbitals. (a) The diagram for H 2 , He 2 , Li 2 , Be 2 , B 2 , C 2 ,
and N 2 molecules and their ions. (b) The diagram for O 2 , F 2 and Ne 2 molecules and their
ions.

2 p 2 p

π 2 py

σ 2 p

(^)

σ 2 p
H 2 through N 2
1 s 1 s
2 s 2 s
Atomic orbitals Molecular orbitals Atomic orbitals
2 p 2 p
π 2 py
O 2 through Ne 2
1 s 1 s
2 s 2 s
Atomic orbitals Molecular orbitals Atomic orbitals
(a) (b)
(^)

π 2 py
(^)

π 2 pz
π 2 pz
(^)

σ 2 s
σ 2 s
(^)

σ 1 s
Energy (not drawn to scale)
(^)

σ 2 p
(^)

π 2 py
(^)

π 2 pz
π 2 pz
σ 2 p
(^)

σ 2 s
σ 2 s
σ 1 s σ 1 s
(^)

σ 1 s
“Homonuclear” means consisting only
of atoms of the same element.
“Diatomic” means consisting of two
atoms.