Molecular Orbital (MO) Theory Definition:

The Molecular Orbital Theory aims to explain the consequence of atomic orbital overlap. The theory introduces molecular orbitals, which are orbitals associated with an entire molecule rather than an individual atom.

Molecular Orbital (MO) Theory Explained:

In our definition of the valence bond theory, we learned about the ‘nature’ of atomic orbital overlaps. The MO theory, however, takes this one step further and talks about the ‘consequence’ of the atomic orbital overlap/interaction. According to this theory, after the interaction, the atomic orbitals cease to exist and give rise to two molecular orbitals. The two orbitals formed are called the bonding MO (lower energy) and anti-bonding MO (higher energy). These orbitals can be thought of as a cloud of electrons associated with the entire molecule rather than the individual atoms.

Note: Since orbitals (atomic as well as molecular) are wavefunctions, they can combine either constructively to form bonding molecular orbitals or destructively to form antibonding molecular orbitals.

For Example –

Simple diatomic homonuclear molecule – consider the formation of the Hydrogen molecule. In this case, the 1s atomic orbitals from each Hydrogen atom overlap and give rise to one-two molecular orbitals. (the degenerate levels of the two hydrogen atoms will be separated as the two nuclei come closer to each other) See the diagram below.

Heteronuclear molecule –