Molecular orbital theory

Limitation of VSEPR theory

1. It cannot explain why oxygen molecule is paramagnetic
2. It cannot explain formation of H₂⁺ molecule.   And may more

MOLECULAR ORBITAL THEORY (MOT)

· Basic idea of MOT is that atomic orbitals of individual atoms combine to form molecular orbitals. Electrons in molecule are present in the molecular orbitals which are associated with several nuclei.

· The molecular orbital formed by the addition of atomic orbitals is called the bonding molecular orbital ( s ).

· The molecular orbital formed by the subtraction of atomic orbital is called antibonding molecular orbital (s *).

· The sigma (s ) molecular orbitals are symmetrical around the bond-axis while pi (p ) molecular orbitals are not symmetrical.

• Sequence of energy levels of molecular orbitals changes for diatomic molecules like Li₂, Be₂, B₂, C₂, N₂ is  (For n £ 14) ;  s1s < s^*1s < s2s < s^*2s < p2p_y  = p2p_Z < s2p_x < p^*2p_y  = p^*2p_Z  < s^*2p_x
•  Sequence of energy levels of molecular orbitals changes for diatomic molecules like O₂, F₂, Ne₂ ( for n >14) is s1s < s^*1s < s2s < s^*2s < s2p_x < p2p_y  = p2p_Z < p^*2p_y  = p^*2p_Z  < s^*2p_x
•  Bond order (b.o.) is defined as one half the differences between the number of electrons present in the bonding and the antibonding orbitals. 
•  Bond Order =

      N_B = Bonding electrons                                        N_A = anti-bonding electrons