Write the salient features of Molecular Orbital Theory.

Explanation

M.O.T. was proposed by Hund and Mulliken.
1) Formation of MOs : The atomic orbitals of the bonded atoms with similar energies and symmetrical with respect to the inter-nuclear axis lose their identities, merge together and give rise to a set of new orbitals, named, 'molecular orbitals'.
2) Now in the molecule, the electrons will be influenced by the nuclei of all atoms, since the m.o. covers all the nuclei.
3) The m.o.'s are formed by LCAO method.
4) The number of m.o.'s produced is equal to the number of a.o's that combine.
5) Half of these m.o.'s possess lower energy than the a.o's from which they form. They are called, bonding m.o's. The remaining half wil have higher energy. They are called, anti bonding m.o's
6) The shape of m.o's is determined by the shapes of a.o's from which they form.
7) Axial overlap of two a.o's gives `sigma` m.o's while parallel overlap gives `pi` m.o's.
8) Based on spectroscopic studies, the relative order of energy of the m.o's of ten m.o's of `Li_(2), Be_(2), B_(2), C_(2) " and " N_(2)` are :
`sigma_(1s) lt sigma_(1s)^(ast) lt sigma_(2s) lt sigma_(2s)^(ast) lt pi_(2p_(y))=pi_(2p_(z)) lt sigma_(2p_(x)) lt pi_(2p_(y))^(star)=pi_(2p_(z))^(star) lt sigma_(2p_(x))^(star)`
But in case of `O_(2), F_(2), Ne_(2)` molecules, the relative order 1s
`sigma_(1s) lt sigma_(1s)^(star) lt sigma_(2s) lt sigma_(2s)^(star) lt sigma_(2p_(x)) lt pi_(2p_(y))=pi_(2p_(z)) lt pi_(2p_(y))^(star) =sigma_(2p_(z))^(star) lt sigma_(2p_(x))^(star)`
(position of `sigma2p_(x)` m.o's changes)
9) Electrons occupy the m.o's which reflects Aufbau principle, Hund's rule of maximum multiplicity and Pauli's exclusion principle.
10) Electrons present in bonding m.o's help to stablise the bond, while those present in anti bonding m.o's more than destabilize the bond.
11) The inner orbitals which do not take part in bonding are called, 'non-bonding orbitals'.