The Predicted Stabilities of Resonance Contributors

All resonance contributors do not necessarily contribute equally to the resonance hybrid. The degree to which each one contributes depends on its predicted stability. Because resonance conttibutors are not real, their stabilities cannot be measured. Therefore, the stabilities of resonance conttibutors have to be predicted based on molecular features found in real molecules. 

The greater the predicted stability of a resonance contributor, the more it contributes to the structure of the resonance hybrid. 

The examples that follow illustrate these points. 

B is less stable than A because B has separated charges and one of its oxygens has a positive charge. 

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