Homonuclear diatomic molecules: molecular orbital (MO) theory

An overview of the MO model

In molecular orbital (MO) theory, we begin by placing the nuclei of a given molecule in their equilibrium positions and then calculate the molecular orbitals (i.e. regions of space spread over the entire molecule) that a single electron might occupy. Each MO arises from interactions between orbitals of atomic centres in the molecule, and such interactions are:

•  allowed if the symmetries of the atomic orbitals are compatible with one another;
•  efficient if the region of overlap between the two atomic orbitals is significant;
•  efficient if the atomic orbitals are relatively close in energy.

An important ground-rule of MO theory is that the number of MOs that can be formed must equal the number of atomic orbitals of the constituent atoms.

Each MO has an associated energy and, to derive the electronic ground state of a molecule, the available electrons are placed, according to the aufbau principle, in MOs beginning with that of lowest energy. The sum of the individual energies of the electrons in the orbitals (after correction for electron–electron interactions) gives the total energy of the molecule.