In which of the following is the oxidation number of sulphur equal to -2?

S₈, H₂S, SO₂, and SO₃^2- are all compounds that contain sulfur, but the oxidation number of sulfur is only -2 in one of them. The oxidation number of an atom is a measure of the number of electrons that it has gained or lost in a chemical reaction. In S₈, the oxidation number of sulfur is zero because it is in its elemental state and has not gained or lost any electrons. In H₂S, the oxidation number of sulfur is -2 because hydrogen has an oxidation number of +1 and the overall charge of the compound is 0. In SO₂, the oxidation number of sulfur is +4 because oxygen has an oxidation number of -2 and the overall charge of the compound is 0. In SO₃^2-, the oxidation number of sulfur is +4 because oxygen has an oxidation number of -2 and the overall charge of the ion is -2. Therefore, the only compound in which the oxidation number of sulfur is -2 is H₂S.