The main points are
(i) When two atomic orbitals combine they loose their original identity and form new orbitals which are termed as molecular orbitals.
(ii) Only those atomic orbitals combine to form molecular orbitals which have comparable energies and proper orientation. For example 1 s would only combine with \(1 \mathrm{~s}\) and not with \(2 s .2 p_x\) would only combine with \(2 p_x\) of other atom and so on.
(iii) The number of molecular orbitals formed is equal to the number of atomic orbitals which combine with one another.
(iv) The two atomic orbitals combine to give bonding molecular orbital and anti-bonding molecular orbital. The bonding molecular orbital has lower energy than anti-bonding molecular orbital and hence greater stability than the other.
(v) The filling up of molecular orbitals takes place according to the same rules as those of atomic orbitals.