Topic summary
Oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom assuming all of its bonds to other atoms are fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. Conceptually, the oxidation state may be positive, negative or zero. Oxidation state is useful for estimating the distribution of electric charge within molecules, especially for those containing primarily ionic bonds, and less so for those with primarily covalent bonds.
The oxidation state of an atom is capable of qualitative explanation of patterns in chemical reactions. It does not represent an atom's actual charge. The oxidation states of atoms in a given compound may vary depending on the electronegativity scale used. It is also relevant to the nomenclature conventions of inorganic compounds.
Oxidation states are typically represented by integers which may be positive, zero, or negative. In some cases, the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetiteFe3O4 (see below). The highest known oxidation state is +9, displayed by iridium in the tetroxoiridium(IX) cation (IrO+4). An even higher +10 is predicted to be possible for platinum in tetroxoplatinum(X), PtO2+4. The lowest oxidation state is −5, as for boron in Al3BC and gallium in pentamagnesium digallide (Mg5Ga2).
In Stock nomenclature, which is commonly used for inorganic compounds, the oxidation state is represented by a Roman numeral placed after the element name inside parentheses, e.g. Iron(III) oxide, or as a superscript after the element symbol, e.g. Fe2O3. The term oxidation was first used by Antoine Lavoisier to signify the reaction of a substance with oxygen. Much later, it was realized that the substance, upon being oxidized, loses electrons, and the meaning was extended to include other reactions in which electrons are lost, regardless of whether oxygen was involved. The increase in an atom's oxidation state through a chemical reaction is referred to as oxidation, and a decrease in oxidation state reduction; the two always occur as pairs in redox reactions. For pure elements, the oxidation state is zero.
Oxidation state, which idealizes bonds as being wholly ionic, can be understood as an opposite to formal charge, which assumes bonds are wholly covalent. Both can be useful for predicting the charge distributions of molecules.