Oxidation number rules: (1) Free element = 0. (2) Monoatomic ion = charge. (3) H = +1 (except metal hydrides: -1). (4) O = -2 (except peroxides: -1, superoxides: -1/2, OF2: +2). (5) Sum of oxidation numbers = charge of species. Balancing redox in acidic medium (half-reaction method): (a) Write half-reactions. (b) Balance atoms other than O, H. (c) Balance O with H2O. (d) Balance H with H+. (e) Balance charge with electrons. (f) Multiply to equalise electrons, add half-reactions. In basic medium: after balancing in acidic medium, add OH- to both sides to neutralise H+.