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Inorganic : Molecular Orbital Diagrams
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 Message 2 of 2 in Discussion 
From: MSN Nickname·Steve·  in response to Message 1Sent: 10/27/2004 8:16 AM
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:
 
              
       s2s*
 
        ! ¡   
       s2s
 

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):
 
0 - 0  =  0
  2
 
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:
 
       ! ¡   
       s1s*
 
        ! ¡   
       s1s
 
 
2 - 2  =  0
  2
 
 
 
 
The beryllium dimer Be2 also has a total of 4 valence electrons which go into the MO diagram as follows:
 
        ! ¡   
       s2s*
 
        ! ¡   
       s2s


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:
 
 
              
       s2s*
 
        ! ¡   
       s2s
 
and the bond order is
 
2 - 0  =  1   (a single bond)
  2
 
 
 
Hope this is helped!
 
 
Steve