A molecular orbital energy level diagram is a qualitative descriptive tool explaining chemical bonding in molecules in terms of molecular orbital theory in general and the Linear combination of atomic orbitals molecular orbital method in particular.
Molecular orbital energy depends upon the two factors:
(i) The energies of the atomic orbitals gets combine to form molecular orbitals.
(ii) The overlapping between the atomic orbitals.
Greater the overlapping, more the bonding orbital is lowered and the antibonding orbital is raised in energy relative to atomic orbitals. For e.g; the extent of overlapping in case of s - orbital is greater than that in p - orbital. Consequently, the energy of a s2pz is lesser than the energy of bonding p2px or p2py MOs.
Now, 1s atomic orbitals of two atoms form two molecular orbitals named as s1s and s*1s. The 2s and 2p orbitals form four bonding molecular orbitals and four antibonding molecular orbitals as:
s2s, s2pz , p2px , p2py
s*2s, s *2pz, p*2px, p*2py
The energy levels of these molecular orbitals have been examined experimentally by spectroscopic methods. The order of increasing molecular orbital’s energy is obtained by the combination of 1s, 2s and 2p orbitals of two atoms is:
s1s, s*1s, s2s, s*2s, s2pz, p2Px = p2py, p*2px = p*2py, s*2pz
However, experimental proofs for some diatomic molecules have shown that above sequence of energy levels of MOs is not correct for all molecules. For e.g., for homonuclear diatomic molecules of second row elements in periodic table such as B2, C2, N2, the s*2pz molecular orbital is higher in energy than p2px andp 2py MOs. The order of MOs for such type of molecules is:
s1s, s *1s, s 2s, s 2s, p2Px = p 2py, s *2pz, p *2px = p *2py,
But for molecules O2 onwards (O2, F2), the first order of energies of molecular orbital is correct. Thus, for diatomic molecules of second period, there are two types of energy level diagram of Mos.
The main difference between the two types of sequences is that for molecules O2, F2 and Ne2 the s2pz, molecular orbital has lower energy than p2px and p2py MOs while in the case of molecules Li2, Be2, B2, C2 and N2, s2pz, molecular orbital is higher in energy than p2px and p2py MOs.