The oxidation number for NO3, or nitrate, is -1. This is because oxygen always has an oxidation number of -2. The three oxygen atoms have a combined oxidation of -6, corresponding to their electromagnetic charge and the lone nitrogen has a charge, or oxidation number, of +5.
Combined together, the -6 charge of the oxygen atoms and the +5 charge of the nitrogen produce a molecule with a net charge of -1. Nitrate is considered an ion because it has a net charge that is not neutral. Nitrate readily forms ionic compounds by combining with different elements; these compounds are also called nitrates. For example, saltpeter, the gunpowder component, is the common name for potassium nitrate. Within these compounds, the NO3 molecular unit retains its -1 charge.