The oxidation number is a value assigned to the atoms in a chemical reaction to determine which atoms in a reaction have been oxidized and reduced. When an atom increases its oxidation number, it is said to have been oxidized. Reduction is indicated by a decrease in the oxidation number of an atom. Reduction and oxidation are always paired so that a reduced atom is always accompanied by an oxidized atom. Oxidation-reduction reactions are frequently called redox reactions.
Write the formula for the reaction. Each substance in the reaction will have an oxidation number equal to the charge of the substance. Atoms in elemental form have an oxidation number of zero. For example, the oxidation number for a sulfur atom in the elemental state is zero. The sum of the oxidation numbers for sodium chloride (NaCl) is also zero.
Find the oxidation number for each atom in the chemical formula for both the reactants and products in the chemical reaction. Monoatomic ions are assigned an oxidation number equal to their charges. For example, since the sodium in sodium chloride is Na+(+1 charge), it's assigned a +1 oxidation number while the chlorine ion is Cl- (-1 charge) and is assigned a -1 oxidation number. Hydrogen atoms in the compounds are assigned a +1 oxidation number except for metal hydrides where the oxidation number is -1. Oxygen atoms are assigned a -2 oxidation number except when bonded with fluorine, in which case they are assigned a +2 or, in the case of peroxides, where the oxygen atoms are assigned a -1 value.
Verify the oxidation numbers by adding the oxidation numbers of each atom in each compound of the reaction. The sum of the oxidation numbers should equal the charge on the substance.
Identify the oxidized atoms by determining which atoms have had an increase in their oxidation number. Reduced atoms will show a decrease in the oxidation number.