Too many structures resonance hybrids

There is a rather large class of molecules for which one has no difficulty writing Lewis structures in fact, we can write more than one valid structure for a given molecule. This raises a new problem how do we decide which one to use  

The answer is simple if the structures are all equivalent, we use them all. As an example, consider the nitrate ion N03 , for which three Lewis structures can be written  

These structures differ only in which oxygen atom is attached by the double bond. Since there is no reason to prefer one over another, the N03 ion is regarded as a superposition, or hybrid of these three structures. 

The term resonance has been used to describe this phenomenon, which is indicated in the above structures by the double-ended arrows. The choice of this word was unfortunate, because it connotes the existence of some kind of dynamic effect that has led to the mistaken idea that the structure is continually alternating between the three possibilities. The correct interpretation is simply that none of the hybrid structures completely represents the molecule the real structure is a superposition of the individual contributing structures. 

Thus there is no true double bond in N03 what we have instead are three 4/3 bonds . A more realistic way of representing the structure might be to show the fractional bonds as dotted lines  

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