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Why couldn't nitrate have two double bonds with oxygen molecules and one single bond with the last oxygen molecule that would have a -1 charge? I believe that all the electron totals still work and all of the atoms have an octet.
Two oxygen double bonds doesn't work for nitrate because we want to ensure that the most stable version of the structure is being formed. Having a 5 bond nitrogen would break the octet rule, because nitrogen doesn't have a d-orbital to accommodate more valence electrons. This is why instead, a formal charge of +1 is placed on the nitrogen atom, and 2 charges of -1 on the single bonded oxygen atoms.
Everyone here is correct: but the reason why it breaks the rule and something like sulfur doesn't is because nitrogen, being n=2 only has l=1, and ml=-1,0,1. Because of this, the maximum amount of electrons that can be filled into nitrogen are the p orbitals, the px, py, and pz orbitals (2 electrons in each orbital, 6 in total+ 2 from s orbital=8 electrons max). Because there are only 8 electrons max, we can only have a maximum of 4 covalent bonds in total. However, with something like sulfur, it can break this rule because it's in the n=3 period, with l=2, ml=-2,-1,0,1,2. Because l=2, this means sulfur can have d orbitals. While sulfur, in completing its octet doesn't actually normally use these d orbitals, when it needs to have 5 bonds (10 electrons) for example and expand its octet, it'll access these d orbitals and use d orbitals to bond with other elements.
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