The nitrite ion is ##NO_2^(-1)##

To determine the oxidation value of each element there are a few basic rules that we can use a a starting point.

• by themselves are 0
• Alkai Metals (Na, K, …) are +1 -Alkaline Earth Metals (Mg Ca…) are +2
• Oxygen is always -2

Since the oxygen in the nitrite ion is always -2 and there are 2 oxygen atoms in this polyatomic ion the charge for the oxygen is -4

Since the entire polyatomic nitrite ion has an overall ionic charge of -1 the nitrogen ion in the molecule must have a charge of +3.

Combining the -4 of the oxygen and the +3 of the nitrogen we achieve the -1 charge of the nitrite ion.

Therefore the oxidation state of nitrogen in a nitrite polyatomic molecule is ##-3##.

I hope this was helpful. SMARTERTEACHER