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:
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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