The relative energies of the original atomic orbitals and the bonding and antibonding
molecular orbitals are shown in a molecular orbital energy-level diagram
like that in FIG. 4.27. The increased energy of an antibonding orbital is about equal
to or a little greater than the lowering of the energy of the corresponding bonding
orbital.
Molecular orbitals are built from linear combinations of atomic orbitals: when
atomic orbitals interfere constructively, they give rise to bonding orbitals; when
they interfere destructively, they give rise to antibonding orbitals. N atomic
orbitals combine to give N molecular orbitals.
4.10 Electron Configurations of Diatomic Molecules
In the molecular orbital description of homonuclear diatomic molecules, we first
build all possible molecular orbitals from the available valence-shell atomic orbitals.
Then we accommodate the valence electrons in molecular orbitals by using the
same procedure used in the building-up principle for atoms (Section 2.6). That is,
1. Electrons are accommodated in the lowest-energy molecular orbital, then in
orbitals of increasingly higher energy.
2. According to the Pauli exclusion principle, each molecular orbital can
accommodate up to two electrons. If two electrons are present in one orbital,
they must be paired.
3. If more than one molecular orbital of the same energy is available, the electrons
enter them singly and adopt parallel spins (Hund’s rule).