Oxidation numbers
The oxidation state, sometimes referred to as oxidation number, is an indicator of the degree of oxidation (loss of electrons) of an atom in a chemical compound. Conceptually, the oxidation state, which may be positive, negative or zero, is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic, with no covalent component. This is never exactly true for real bonds.
The term oxidation was first used by Antoine Lavoisier to signify 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.
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 magnetite (Fe
3O
4). The highest known oxidation state is reported to be +9 in the iridium tetroxide cation (IrO4+). It is predicted that even a +10 oxidation state may be achievable by platinum in the platinum tetroxide dication (PtO42+). The lowest oxidation state is −4, as for carbon in methane or for chromium in [Cr(CO)4]4−.
The increase in oxidation state of an atom, through a chemical reaction, is known as an oxidation; a decrease in oxidation state is known as a reduction. Such reactions involve the formal transfer of electrons: a net gain in electrons being a reduction, and a net loss of electrons being an oxidation. For pure elements, the oxidation state is zero.
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