oxidation number

The oxidation number is a number that indicates the degree of ionization. Thus, in the compound sodium oxide, Na2O, sodium (Na+) has an oxidation number of +1, and oxygen (O2-) an oxidation number of -2. In covalent and coordination (complex) compounds, the oxidation number is the electric charge that the atoms would have had if the compound was ionic; for example, in the ion (CuCl4)2-, regarded as formed from Cu2+ and 4Cl-, copper has an oxidation number of +2. Oxidation numbers are often used in the names of chemical compounds, as in iron(II) chloride (FeCl2) and iron(III) chloride (FeCl3), formerly called ferrous and ferric chloride.