211.

Define moleuclar orbital. What are the salient features of Molecular orbital theory?

A molecular orbital may be defined as a region in space associated with the nuclei of bonded atoms in a molecule where the probability of finding a particular electron is maximum.

Salient features:

(i) Molecular orbitals are formed by the linear combination of atomic orbitals having nearly the same energies.

(ii) Molecular orbitals are associated with the nuclei of the bonded atom in a molecule.

(iii) The number of molecular orbitals formed is equal to the number of combining atomic orbitals.

(iv) When two atomic orbitals combine, they form two new orbitals called bonding molecular orbital and anti-bonding molecular orbital. The bonding molecular orbital has lower energy whereas anti-bonding molecular orbital has higher energy than the atomic orbitals.

(v) The shapes of the molecular orbitals formed depend on upon the type of the combining atomic orbitals.

(vi) The bonding molecular orbitals are represented as σ, δ etc. whereas the corresponding antibonding molecular orbitals are represented as  etc. 

(vii) The filling of the molecular orbitals takes place according to the same rule as those of atomic orbitals. These are: 

 (a) Molecular orbitals are filled in order of their increasing energies (Aufbau principle).
(b) Molecular orbital can have maximum two electrons and these must have opposite spin (Pauli’s exclusion principle).
(c) A pairing of electrons in the degenerate molecular orbitals does not take place until each of them has one electron each (Hund’s rule of maximum multiplicity).