When several Lewis structures are possible, the most stable structure will place a negative formal charge on the most electronegative/electropositive atom.

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The nitrate ion, NO3-, the nitrogen is bonded to two oxygen's which are both single bonded to the nitrogen, and also to a third oxygen atom, wich is double bonded to the nitrogen has an overall charge of (-1.)

I can't make the web insert extra spaces so I'll use ampersands.

&&&&&&..-&&&&&&&&&&&&&&&&..-&&&&&&&&&&&&&&&&&&&&&&&&&&
&&&&..O..&&&&&&&&&&&&&&..O..&&&&&&&&&&&&&&&..O..&&&&&&
&..&&&|&&&..-&&&&&&-..&&&|&&&..&&&&&&&&-..&&&||&&..-&&
&&O&=&N&-&O..&&&&&&..O&-&N&=&O&&&&&&&&&..O&-&N&-&O&..&
&..&&&+&&..&&&&&&&&&..&&&+&&&..&&&&&&&&&..&&&+&&&&..&&

What is so important to note that the positive charge goes on the the very electronegative N (usually nitrogens don't have to put up with accepting positive charges) because oxygen is even more electronegative.