The oxidation number or state is assigned to show how many electrons are associated with an atom when it is in a compound. For example, the oxidation state of Cu2+ is +2 because it has two fewer electrons than in the neutral state. Elemental copper as in a piece of copper Cu(s) would have an oxidation state of zero (as all elements do). Oxygen gas O(g) is zero, and oxide is -2. Review your CHM151 notes on oxidation numbers, or watch this movie: Question: what is the oxidation number of sulfur in the sulfate ion SO4