Energy Level Diagram for Molecular Orbitals

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

(i) the energies of the atomic orbitals combining 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 antibonding orbital is raised in energy relative to atomic orbitals. For e.g., the extent of overlapping in case of s - orbital is more than that in p - orbital. Consequently, the energy of a s2p_z is lower than the energy of bonding p2p_x or p2p_y MOs.

Now, 1s atomic orbitals of two atoms form two molecular orbitals designated as s1s and s*1s. The 2s and 2p orbitals (eight atomic orbitals on two atoms) form four bonding molecular orbitals and four antibonding molecular orbitals as:

Bonding MOs:

s2s, s2p_z , p2p_x , p2p_y

Antibonding MOs:

s*2s, s *2p_z, p*2p_x, p*2p_y

The energy levels of these molecular orbitals have been determined experimentally by spectroscopic 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, s2p_z, p2P_x = p2p_y, p*2p_x = p*2p_y, s*2p_z

However, experimental evidence 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 such as B₂, C₂, N₂, the s*2p_z molecular orbital is higher in energy than p2p_x and p 2p_y MOs. The order of MOs for these molecules is:

s1s, s *1s, s 2s, s 2s, p2P_x = p 2p_y, s *2p_z, p *2p_x = p *2p_y, s *2p_z.

But for molecules O₂ onwards (O₂, F₂), the first order of energies of MOs is correct.

Thus, for diatomic molecules of second period (Li₂ to Ne₂), there are two types of energy levels of Mos. For molecules Li₂, Be₂, B₂, C₂ and N₂ the molecular orbital energy level diagram is shown in figure 1.15 (a).

fig 1.15 (a) - Molecular orbital energy level diagram for Li₂, Be₂, C₂ and N₂ molecules

On the other hand, for the molecules O₂, F₂ and Ne₂ the molecular orbital energy level diagram is shown in fig.1.15 (b).

fig 1.15 (b) - Molecular orbital energy level diagram for O₂, F₂ and Ne₂ molecules

The main difference between the two types of sequences is that for molecules O₂, F₂ and Ne₂ the s2p_z, molecular orbital is lower in energy than p2p_x and p2p_y MOs while in the case of molecules Li₂, Be₂, B₂, C₂ and N₂, s2p_z, molecular orbital has higher energy than p2p_x and p2p_y MOs.