Molecular Orbitals

Valence bond theory is able to explain many aspects of bonding, but not all. To complement this theory, we use another called the molecular orbital (MO) theory. Molecular orbital theory is a more sophisticated model for understanding the nature of chemical bonding.

MO theory takes the idea of atomic orbitals overlapping to a new level, where new molecular orbitals are generated using a mathematical process called linear combination of atomic orbitals (LCAO).

Molecular orbitals share many similarities with atomic orbitals:

–          They are filled from lowest energy to highest energy (Aufbau principle).

–          They can hold a maximum of two electrons of opposite spin per orbital (Pauli exclusion principle).

The major difference between atomic and molecular orbitals is that atomic orbitals represent electron density in space associated with a particular atom. Molecular orbitals are associated with the entire molecule, meaning the electron density is delocalized (spread out) over more than one atom.