The Role of Antibonding Electrons

Consider, for example, the n bond in ethylene, which is formed by just such a two-center interaction of AOs (Section 1.13). The bond energy of 

The reason for this can be seen if we consider the distribution of the electrons in the molecule. Each electron occupies a MO given by 

It will be seen from this argument that the antibonding MO is far more antibonding than the bonding MO is bonding. A corollary of this is that the 

The decrease in bond energy for homopolar bonds is due to an increase in the number of repulsive lone-pair interactions, carbon having no lone pairs, nitrogen one, oxygen two, and fluorine three. 

---