[cruckshank]

Using molecular orbital theory how can I explain why F2, O2 and N2 are common, whilst C2, Li2 and B2 are rare, existing only under certain conditions?

 I've drawn the diagrams out but can't really see any particular contrast between the two sets other than C2, Li2 and B2 have only their first bonding molecular orbitals filled, whilst the F2, O2 and N2 have quite a few molecular orbitals beyond that filled.

Thanks in advance.

[mjc123]

What alternative bonding arrangements are possible for C, B and Li? Why are they not used by N, O and F?