The oxidation state, often called the 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 Lavoisier to mean reaction of a substance with oxygen. Much later, it was realized that the substance on being oxidized loses electrons, and the use of the term "oxidation" was extended to include other reactions in which electrons are lost.

Oxidation states are typically represented by small integers. 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 cation IrO+
4,^[1] while the lowest known oxidation state is −5 for boron, gallium, indium, and thallium. The possibility of +9 and +10 oxidation states in platinum group elements, especially iridium(IX) and platinum(X), has been discussed by Kiselev and Tretiyakov.^[2]

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.

There are various methods for determining oxidation states/numbers.

In inorganic nomenclature the oxidation state is determined and expressed as an oxidation number represented by a Roman numeral placed after the element name.

In coordination chemistry, oxidation number is defined differently from oxidation state.

The oxidation state, often called the 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 Lavoisier to mean reaction of a substance with oxygen. Much later, it was realized that the substance on being oxidized loses electrons, and the use of the term "oxidation" was extended to include other reactions in which electrons are lost.

Oxidation states are typically represented by small integers. 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 cation IrO+
4,^[1] while the lowest known oxidation state is −5 for boron, gallium, indium, and thallium. The possibility of +9 and +10 oxidation states in platinum group elements, especially iridium(IX) and platinum(X), has been discussed by Kiselev and Tretiyakov.^[2]

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.

There are various methods for determining oxidation states/numbers.

In inorganic nomenclature the oxidation state is determined and expressed as an oxidation number represented by a Roman numeral placed after the element name.

In coordination chemistry, oxidation number is defined differently from oxidation state.