Why is O=N-O-O- not a valid predicted structure for the nitrate ion?

[nobahar]

Hello!

I was drawing the lewis structure for NO₃^- and was wondering why O=N-O-O^- isn't a 'valid' predicted structure (Double-bonded O to nitrogen, a single-bonded oxygen to nitrogen, a lone pair on nitrogen, and a single-bonded oxygen to oxygen with a negative charge on the terminal oxygen. For the nitrate ion its N with three oxygens attached, with a + charge on the nitrogen and two oxygens with negative charges. Explanations for the nitrate ion start with "Nitrogen is the central atom", and the structure then follows; why assume this, particularly in more general cases? If it isn't assumed, then why can't you arrive at the structure I gave first of all?

Any help appreciated,
Nobahar.

 

[Nino MMP]

The reason that O=N-O-O- isn't a valid predicted structure is because it breaks the octet rule. The octet rule states that elements in the second period and beyond will form bonds until they have eight electrons in their outer shell. In this structure, the nitrogen has only six electrons in its outer shell, which is not enough to satisfy the octet rule. A more correct structure would be O-N-O-O^2-, where each of the oxygen atoms has two pairs of electrons (four electrons total) and the nitrogen has three pairs of electrons (six electrons total). This satisfies the octet rule and is thus a valid predicted structure for NO3-.