Nitrate has 24 valence electrons and three equivalent resonance contributors. A single drawing shows one N=O bond and two N–O⁻ bonds, but the real ion has three equivalent N–O positions.
Nitrogen contributes 5 electrons, three oxygens contribute 18, and the negative charge adds 1: 5 + 18 + 1 = 24. Nitrogen is central and is bonded to all three oxygen atoms. In one contributor, nitrogen has one double bond and two single bonds.
How to draw nitrate
Count 24 electrons.
Add one electron for the −1 charge.
Draw three N–O single bonds.
Place nitrogen in the center and enclose the finished ion in brackets.
Complete oxygen octets.
After placing lone pairs, nitrogen still lacks an octet.
Form one N=O double bond.
Convert a lone pair on any oxygen into a bonding pair.
Add charges and resonance.
Nitrogen is +1, two single-bonded oxygens are −1, and the double-bonded oxygen is 0. Move the double bond to create three contributors.
Formal charges
The charges in one contributor add to −1: N(+1) + two O(−1) + one O(0). A drawing without the positive charge on nitrogen or the negative charges on single-bonded oxygens is incomplete.
Resonance and geometry
The atom arrangement does not change between contributors; only electron placement changes. Three electron domains around nitrogen give trigonal planar geometry with ideal 120° angles.
Common mistakes
Drawing only one contributor and treating one N–O bond as permanently different.
Forgetting the brackets and overall − charge.
Using 23 electrons instead of adding one for the anion.
Giving second-period nitrogen more than eight surrounding electrons.