Molecular Orbital Energy

Introduction

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

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 s2p_z is lesser than the energy of bonding p2p_x or p2p_y 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:

Bonding molecular orbitals:

s2s, s2p_z , p2p_x , p2p_y

Antibonding molecular orbitals:

s*2s, s *2p_z, p*2p_x, p*2p_y

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, s2p_z, p2P_x = p2p_y, p*2p_x = p*2p_y, s*2p_z

Here the energy of molecular orbitals increases from s1s to s*2p_z.

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*2p_z molecular orbital is higher in energy than p2p_x andp 2p_y MOs. The order of MOs for such type of 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.

Here the energy of molecular orbitals increases from s1s to s*2p_z.

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.

Conclusion for Molecular Orbital Energy

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