Final answer:
Nitrogen has a negative oxidation number in NH4Cl, where it exists in the form of an ammonium ion (NH4+) with nitrogen having an oxidation state of -3 to balance the +1 of each of the four hydrogen atoms.
Step-by-step explanation:
The question is asking about the oxidation number of nitrogen in various compounds. To determine which compound has nitrogen with a negative oxidation number, we need to apply the rules for oxidation states. In compounds like HNO3, NO2, N2O, and NaNO2, nitrogen generally has a positive oxidation state because it is combined with oxygen, which typically has an oxidation state of -2. However, in NH4Cl (ammonium chloride), nitrogen has a negative oxidation number because hydrogen has a +1 oxidation state, and since there are four hydrogens, the nitrogen must balance this with an oxidation state of -3 to make the overall charge of the ammonium ion neutral.