Molecular Orbital Theory (MOT)

Molecular Orbital Theory (MOT) is based on the wave mechanical model of atom. In contrast to the localized bonding in VBT, the molecular orbital theory visualizes the bonding to be delocalized in nature - spread over the whole molecule.

The atomic orbitals of the constituent atoms combine to generate new types of orbitals (called molecular orbitals). These are spread over the whole molecule - they are delocalized. These new orbitals, do not belong to any one atom but extend over the entire region of the bonded atoms.

These molecular orbitals are created by Linear Combination of Atomic Orbitals (LCAO) approach in which, the atomic orbitals of comparable energies and of suitable symmetry combine to give rise to an equal number of molecular orbitals.

The available electrons then fill these orbitals in the order of increasing energy as in the Aufbau principle used in the electron configurations of atoms.

Example: Hydrogen Molecule

The two hydrogen atoms have an electron each in their respective 1s orbitals. In the process of bond formation the atomic orbitals of two hydrogen atoms can combine in two possible ways. In one, the MO wave function is obtained by addition of the two atomic wave functions whereas in the other the MO is obtained by subtraction of the atomic orbitals.

The molecular orbital obtained by the addition of atomic orbitals is of lower energy than that of the atomic orbitals and is called a bonding orbital. On the other hand, the orbital obtained by subtraction of atomic orbitals is of higher energy and is called an anti-bonding orbital.

The bonding orbital obtained above is denoted as σ1s while the anti- bonding orbital is denoted as σ*1s. There are a total of 2 electrons in a hydrogen molecule, according to Aufbau principle these are filled into σ1s orbital.

Bond Order

Bond order (b.o.) = ½(n_b - n_a)

Where, n_b and n_a refer to the number of electrons present in bonding and anti-bonding molecular orbitals respectively. For hydrogen molecule the bond order will be ½(2–0) = 1. It means that there is a single bond between two hydrogen atoms.