The relative energies of molecular orbitals depend upon the following two factors:

(i) Atomic orbitals energies for the combination to form molecular orbitals.

(ii) The extent of overlapping between the atomic orbitals.

The greater the overlap, the more the bonding orbital is lowered and the anti-bonding orbital is raised in energy relative to atomic orbitals. For e.g., the extent of overlapping in case of sigma – orbital is more than that in pi – orbital. Consequently, the energy of a σ2pzis lower than the energy of bonding π2pxor π2pyMOs.

Now, 1s atomic orbitals of two atoms form two molecular orbitals designated as σ1s and σ*1s. The 2s and 2p orbitals (eight atomic orbitals on two atoms) form four bonding molecular orbitals and four anti-bonding molecular orbitals as:Bonding MOs:σ2s, σ2pz, π2px, π2py

Anti-bonding MOs:σ*2s, σ*2pz, π*2px,π*2py

The energy levels of these molecular orbitals have been determined experimentally by various methods. The order of increasing energy of molecular orbitals 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

Energy increases ———————————->

But for molecules O2onwards (O2, F2), the first order of energies of MOs is correct.Thus, for diatomic molecules of second period (Li2to Ne2), there are two types of energy levels of Mos. For molecules Li2, Be2, B2, C2and N2the molecular orbital energy level diagram