I attach the a file containing two sets of resonance structures. Can you please identify which is the correct resonance form of Nitrate ion?
Both I and II can be identified as resonance, since the configurations involved correspond to energetically degenerate states.
It seems to me that Anga's answer and your option II are the same sets of resonance structures and are probably the preferred structures.
From the second structure, it is clear that the central atom Nitrogen carries a positive charge, while two of the three oxygen atoms carry a negative charge each. But from the structure of Nitric acid from which Nitrate ion is formed, Hydrogen atom is bonded to one of the oxygen atom, while Nitrogen donates a pair of electrons to one of the other two oxygen atoms and engages in double bond with the other. Will this mean that the structure of nitrate ion due to removal of a Hydrogen atom from nitric acid be structure 1?
Bonds in nitrate anion are thoroughly delocalised and as such all three of equal length (if there is no force field present or is homogenous).
Your II option and Anga's answer are my preferred choice. Also, Erik Rakovsky makes an important point about the N-O bond distances as well. Coordination of nitrate to a metal can modify both the sigma- and pie-bonding of the nitrate group on coordination, thus, changing the N-O bond lengths. Addison et. al. published an excellent review on structural aspects of coordinated-nitrate complexes: http://pubs.rsc.org/en/Content/ArticleLanding/1971/QR/qr9712500289#!divCitation
We published a recent chapter on NOx (NO, NO2, NO3) isomerization and looked at how coordination affects mode of binding of NOx : http://www.sciencedirect.com/science/article/pii/S0898883814000117
So this is like the bonding in benzene: the 6 electrons are spread across all SIX carbon atoms !!
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