The He+22 ion has only two valence electrons (two from each He atom minus two for the +2 charge). We can now fill the molecular orbital diagram :
The two electrons occupy the lowest-energy molecular orbital, which is the bonding (σ 1s) orbital, giving a (σ 1s)2 electron configuration. So the bond order is
= 1/2[Nb - Na] = (2 - 0)/2 = 2/2 = 1
He+22 is therefore predicted to contain a single He-He bond. Thus it should be a stable species.
Let us examine the He2 molecule, formed from two He atoms with 1s2 electronic configurations.
With a total of four valence electrons, both the σ 1s bonding and (σ * 1s) antibonding orbitals must contain two electrons. This gives a (σ1s)2 (σ1s)2 electronic configuration.
Bond order = 1/2[Nb - Na] = (2 - 2)/2 = 0
which indicates that the He2 molecule has no net covalent bond and is not a stable species.
