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I was studying about the resonance and here's my doubt:
How many resonating structures are there in nitrobenzene? Is it 5 (as drawn) or is it 4 because first and the last structures are same or is it 3 because second and fourth structures are also same?
In short, do we count fifth structure when we take the number of resonating structures?
We never really need to know the number of resonance structures. The whole concept is qualitative, so we can't draw any quantitative conclusions from it. But if you want to do it anyway, then of course 1 and 5 are the same, while 2 and 4 are not (they have positive charges on different atoms).
Also, both oxygens are in fact equivalent, so you might want to add the structures with their roles reversed.
If you consider strict symmetry rules, you can only draw 3 really different resonance structures (the first three, from left to right), because all other structures you can draw are symmetrically equivalent to one of those 3 if you rotate it properly, those structures are only useful to show you that there is delocalization of electronic density within those atoms.
