Give features of Molecular orbital $( \mathrm{MO} )$ theory:

Molecular orbital ($MO$) theory was developed by$ F$. Hund and $R.S.$ Mulliken in $1932$ . The features of this theory are as under:

$(i)$ The electrons in a molecule are present in the various molecular orbitals as the electrons of atoms are present in the various atomic orbitals.

$(ii)$ The atomic orbitals of comparable energies and proper symmetry combine to form molecular orbitals.

$(iii)$ While an electron in an atomic orbital is influenced by one nucleus, in a molecular orbital it is influenced by two or more nuclei depending upon the number of atoms in the molecule. Thus, an atomic orbital is monocentric while a molecular orbital is polycentric

$(iv)$ The number of molecular orbital formed is equal to the number of combining atomic orbital. When two atomic orbitals combine, two molecular orbitals are formed. One is known as bonding molecular orbital while the other is called antibonding molecular orbital.

$(v)$ The bonding molecular orbital has lower energy and hence grater stability then the corresponding antibonding molecular orbital.

$(vi)$ Just as the electron probability distribution around a nucleus in an atom is given by an atomic orbital, the electron probability distribution around a group of nuclei in molecule is given by a molecular orbital.

$(vii)$ The molecular orbital like atomic orbitals are filled in accordance with the Aufbau principle obeying the Paulie's exclusion principal and the Hund's rule.