 The reduction in energy of these electrons is the driving force for chemical bond formation.Whenever mixing for an atomic orbital is not possible for reasons of symmetry or energy, a non-bonding MO is created, which is often quite similar to and has energy level equal or close to its constituent AO, thus not contributing to bonding energetics.The resulting electron configuration can be described in terms of bond type, parity and occupancy for example dihydrogen 1σ"g"2.