Here is the molecular orbital (MO) diagram to fill the valence electrons into:
s2s*
s2s
The asterisk (*) indicates the "antibonding" molecular orbital. Both MOs are described as sigma (s) bonds, made from combining the valence 2s orbitals of each atom in an additive ("s2s + s2s") and subtractive ("s2s - s2s") manner (they call this "linear combinations of atomic orbitals", LCAO).
And remember,
bond order = number of bonding electrons - number of antibonding electrons
2
Li2 has two valence electrons that go into the s2s bonding MO:
The bond order is
2 - 0 = 1 (a single bond)
2
The Li22+ ion would have zero valence electrons and therefore no bond (the bond order would be zero):
Alternatively, we could try to make a bond with the remaining inner two 1s electrons from each lithium ion (4 electrons total). But here again we would wind up with a bond order of zero:
The beryllium dimer Be2 also has a total of 4 valence electrons which go into the MO diagram as follows:
The bond order will be
2 - 2 = 0
2
so there is no bond here either.
But the Be22+ ion has only 2 valence electrons which go into the s2s bonding MO just like Li2:
and the bond order is
2 - 0 = 1 (a single bond)
2
Hope this is helped!
Steve
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