Oxidation-reduction reactions, or redox reactions, occur in many chemical and biochemical systems. The process involves the complete or partial transfer of electrons from one atom to another. Oxidation and reduction processes are complementary. For every oxidation, there is always a corresponding reduction process. This is because for a substance to gain electrons in a chemical reaction, another substance must be losing these electrons. Oxidation is defined as a process by which an atom or ion loses electrons. This can occur in several ways: • Addition of oxygen or other electronegative elements to a substance:. . . 2 Mg(s)+O2(g) → 2 MgO(s) . . .2 Mg(s)+O2(g) → MgCl2 (s). . . • Removal of hydrogen or other electropositive elements from a substance: . . . H2S(g)+Cl2(g) → 2 HCl(g)+S(s) . . .Here, H2S is oxidized. • The direct removal of electrons from a substance: . . . 2 FeCl2 (s)+Cl2(g) → 2 FeCl3 (s) . . . Fe2+ → Fe3+ +e− . . . Reduction is defined as the process by which an atom or ion gains electrons. This can occur in the following ways: • Removal of oxygen or other electronegative elements from a substance: . . . MgO(s)+H2(g) → Mg(s)+H2O(g). . . • Addition of hydrogen or other electropositive elements to a substance: . . . H2(g)+Br2(g) → 2 HBr(g). . . 2 Na(s)+Cl2(g) → 2 NaCl(s). . . Here, chlorine (Cl2) is reduced. • The addition of electrons to a substance: . . . Fe3+ +e− → Fe2+ . . . Oxidation number or oxidation state is a number assigned to the atoms in a substance to describe their relative state of oxidation or reduction. These numbers are used to keep track of electron transfer in chemical reactions. Some general rules are used to determine the oxidation number of an atom in free or combined state. 1. Any atom in an uncombined (or free) element (e.g., N2, Cl2, S8, O2, O3, and P4) has an oxidation number of zero. 2. Hydrogen has an oxidation number of +1 except in metal hydrides (e.g., NaH, MgH2) where it is −1. 3. Oxygen has an oxidation number of −2 in all compounds except in peroxides (e.g., H2O2, Na2O2) where it is –1.