Molecular Orbital theory - an introduction

Molecular orbital theory considers the molecule as a whole, rather than as a localised structure that is described in valence bond theory. Electrons are allocated to molecular orbitals in much the same way as in atomic orbitals. As interactions may involve two or more nuclei, the system is said to be delocalised or multi-centred.

Molecular Orbital theory considers the molecule as a whole system rather than as a localised structure. Molecular orbitals belong to molecules in much the same was as atomic orbitals belong to atoms.

Electrons are allocated to molecular orbitals using the same principles as atomic orbitals.

The following rules apply in the formation of molecular orbitals:

• The number of molecular orbitals formed is equivalent to the number of atomic orbitals involved in their formation
• For every bonding molecular orbital formed, an antibonding molecular orbital is also formed
• The symmetry of the atomic orbitals must be compatible (same sign of wave)
• The atomic orbitals must be close to each other in energy
• The region of the orbital overlap must be significant