Page 291 - Instant notes

H4
MOLECULAR ORBITAL THEORY OF
DIATOMIC MOLECULES II

Key Notes
Molecular orbital energy levels may be represented by a
molecular orbital energy level diagram, which illustrates the
relative energy of the molecular and atomic orbitals, and their
relationship. The bonding molecular orbital is of lower energy
than that of the separate atomic orbitals, and the converse is true
for the antibonding orbital. The available electrons are placed in
pairs into the molecular orbitals, with the lowest energy
molecular orbitals being filled first. In the hydrogen molecule,
both electrons occupy the bonding molecular orbital, giving an
overall bonding interaction. In He 2 , two electrons occupy each of
the σ and σ* orbitals, giving no net bonding.
In second row diatomic molecules, linear combinations of the p
orbitals give both a σ orbital and two degenerate π orbitals. As
the molecular orbitals are qualitatively unchanged for the second
row diatomic molecules, the same molecular orbital diagram may
be used by entering the correct number of electrons. Detailed
analysis reveals that the highest occupied π and σ orbitals
exchange positions between the elements nitrogen and oxygen.
Core orbitals do not make a significant contribution to the
bonding, as each pair of bonding and antibonding orbitals is fully
occupied, leaving no net bonding contribution.
Molecular orbitals in homonuclear molecules may be described
in terms of their symmetry with respect to a point of inversion at
the center of the bond. A molecular orbital whose sign is
unchanged by inversion is termed gerade, g, and one whose sign
is inverted is ungerade, u.
The total spin angular momentum quantum number, Σ, describes
the spin of the electrons in the molecule, and is quoted as its
multiplicity, (2Σ+1). The orbital angular momentum is described
by a quantum number A. For an electron in a a orbital, Λ=0, for
an electron in a π orbital, Λ=±1, etc. The term symbol
representing a specific electron configuration is written in the
1
form 2Σ+1 Λ parity . The ground state of nitrogen is written as Σ g , for
example.