What is the Lewis structure for a nitrate ion?
Electrons to distribute:
If you count up the valence electrons, the number is indeed 24 on the above representation. For the nitrate ion, the nitrogen is formally QUATERNIZED, i.e. it bears a formal positive charge.
The two singly bound oxygens bear a formal negative charge, and this representation is consistent with the negative charge of the ion (i.e. each of these 2 oxygen atoms is associated with 7 electrons rather than the 6 associated with the formally neutral, formally doubly bound O atom). Of course, all the oxygen atoms are formally equivalent.