Molecular orbital theory was proposed by F. Hund and R.S. Mulliken in 1932.Bonds is localized to two atoms as well as molecule. This theory involves involve distribution of electrons and sharing of electrons by the nuclei There is no place of resonance in this theory. Satisfactory explanation of paramagnetic character of oxygen. The energy of overlapping orbitals must be comparable and there must be similarities in their symmetry.
In this theory , a molecule is considered quite different from the corresponding atoms. This theory includes the orbitals having different energy levels and the orbitals involves are known as molecular orbitals.
It forms a wave function for an electron in a molecule. Each molecule can accommodate maximum two electrons having opposite spin in its orbital.