In which of the following is the oxidation number of nitrogen zero?

The oxidation number of an element is defined as the charge that it would carry if all the shared electrons are assigned to the more electronegative atom. The sum of the oxidation numbers of all atoms in a molecule must be equal to the charge of the molecule. In NH₃, the oxidation number of nitrogen is -3 because hydrogen has an oxidation number of +1 and the sum of oxidation numbers in the compound is zero. In NaNO₃, the oxidation number of nitrogen is +5 because sodium has an oxidation number of +1 and oxygen has an oxidation number of -2. In HNO₂, the oxidation number of nitrogen is +3 because hydrogen has an oxidation number of +1 and oxygen has an oxidation number of -2. In NOI₃, the oxidation number of nitrogen is +5 because oxygen has an oxidation number of -2 and iodine has an oxidation number of +7. The only option where nitrogen has an oxidation number of zero is N₂, because it is a diatomic molecule and the oxidation number of each nitrogen atom is zero.