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Why is the below structure not considered a valid structure for the nitrate ion? Is it because adding a double bond reduces the formal charges present on the nitrogen? Does the above form exist anywhere?

possible structure of the nitrate ion

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Nitrate actually exists as a superposition of these three resonance forms:

resonance forms of the nitrate ion
(Source: Wikipedia)

Essentially, a lone pair from an oxygen with a negative formal change forms a double bond to nitrogen. However, as nitrogen is a period II element, and hence must obey the octet rule, it can not have 5 bonds to other atoms. The already-present double bond then turns into a single bond and the pair of electrons moves onto the oxygen. This "movement" can happen with all three of the oxygen atoms and all three of the bonds. This is called "resonance".

Due to resonance, the energy of the system is minimised making it more stable.

Your Lewis structure is not far off. The "true" structure of nitrate is:

hybrid resonance structure of the nitrate ion (Source: Wikipedia)

Approximately, each oxygen atom contributes one third of a bond with the nitrogen, lowering the formal charge on each atom from what you have written, which does not exist in nature.

The true structure is sort of an average of the properties of each possibility for where the electrons could be located, including the three resonance forms above, your structure, and any other configuration. However, these other forms are of minuscule probability to be formed (perhaps read up on wavefunctions and electron distributions if you're interested), and so the main contributors to the "real" structure are the three resonance forms.

Note that none of these structures ever exist in isolation for more than the most infinitesimal of time periods.

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    $\begingroup$ Technically, the asker's suggested canonical structure (and indeed many others) also plays some role in describing the nature of the nitrate ion, but its importance is significantly smaller than the three main forms with one double bond. $\endgroup$ – Nicolau Saker Neto Mar 19 '14 at 1:30
  • $\begingroup$ Edited to address that. Hopefully this is more accurate and still clear now. $\endgroup$ – Michael Scholz Mar 19 '14 at 1:38
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    $\begingroup$ None of the resonance structures exist, not even in the most infinitesimal time frame. That's why it is a superposition. The OP suggested a valid Lewis structure, which I think has an important (non-negligible) contribution to the overall electronic structure. Probability should not be taken as an indicator that this structure exists, the probability expresses the contribution of this hypothetical state to overall state. This Q&A is related and explains the structure in the molecular orbital picture. $\endgroup$ – Martin - マーチン Feb 24 '16 at 4:11
  • $\begingroup$ I'm a bit confused about resonance structures. Does the electron "circulate" between the oxygen atoms so that the ion constantly switches between the resonance structures, or do all three resonance structures exist for a single ion at the same time? (Because I don't think 1/3 of an electron can exist) $\endgroup$ – user3932000 Jun 5 '16 at 20:26
  • $\begingroup$ Downvoting because this answer suggests that a small, finite number of resonance structures can be drawn and that were it. $\endgroup$ – Jan Oct 24 '17 at 8:39
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Your resonance structure is valid. There are many more valid resonance structures, the most common ones being the three which feature one $\ce{N=O}$ double bond and two $\ce{N-O}$ single bonds with a single formal positive charge on nitrogen and two single formal negative charges one on each of the single-bonded oxygens.

These four resonance structures are by no means the end! A multitude of further, more minor contributors can also be written. Each of these can be considered to contribute to the overall picture which cannot really be displayed accurately in the Lewis formalism. While the structure you propose contributes less than the three with a single double bond, it still contributes more than some others.

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