Resonance: When Lewis Structures Fail
There are some molecules and ions for which we cannot write Lewis
structures that agree with experimental measurements of bond length and
bond energy. One example is the formate ion, CHO2-,
which is produced by neutralizing formic acid, HCHO2.
The skeletal structure for this ion is:
O
H C
O
and, following the usual steps, we would write its Lewis
structure as
This structure suggests that one carbon-oxygen bond should be
longer than the other, but experimental evidence shows that they are in
fact identical. In fact, the C-O bond lengths are about halfway between
that expected for a single bond and that expected for a double bond. The
Lewis structure doesn't match the experimental evidence, and there's no
way to write one that does unless we use 1.5 electrons per bond.
We get around this problem with resonance. The Lewis structure
can be shown as two structures
The term resonance is often misleading. The word itself suggests
that the actual structure flip-flops back and forth between the two
structures shown. This is not the case.
There is a simple way to determine when resonance should be
applied to Lewis structures. If you find that you must move electrons to
create a double bond while following the procedure, the number of
resonance structures is equal to the number of equivalent choices for
the location of the double bond.
eg 2. An SO3 molecule showing resonance structures. 
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