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 (O^2–) 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.