Oxidation State

In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by integers, which can be positive, negative, or zero. In some cases, the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetite (Fe₃O₄). The highest known oxidation state is +8 in the tetroxides of ruthenium, xenon, osmium, iridium, and hassium, and some complexes involving plutonium, while the lowest known oxidation state is −4 for some elements in the carbon group.

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.

The definition of the oxidation state listed by IUPAC is as follows:

Oxidation state: A measure of the degree of oxidation of an atom in a substance. It is defined as the charge an atom might be imagined to have when electrons are counted according to an agreed-upon set of rules:

• (1) the oxidation state of a free element (uncombined element) is zero
• (2) for a simple (monoatomic) ion, the oxidation state is equal to the net charge on the ion (For example, Cl- has an oxidation state of -1)
• (3) hydrogen has an oxidation state of 1 and oxygen has an oxidation state of −2 when they are present in most compounds. (Exceptions to this are that hydrogen has an oxidation state of −1 in hydrides of active metals, e.g. LiH, and oxygen has an oxidation state of −1 in peroxides, e.g. H₂O₂ or −1/2 in superoxides, e.g. KO₂)
• (4) the algebraic sum of oxidation states of all atoms in a neutral molecule must be zero, while in ions the algebraic sum of the oxidation states of the constituent atoms must be equal to the charge on the ion.

For example, the oxidation states of sulfur in H₂S, S₈(elementary sulfur), SO₂, SO₃, and H₂SO₄ are, respectively: −2, 0, +4, +6 and +6. The higher the oxidation state of a given atom the greater its degree of oxidation; the lower the oxidation state the greater its degree of reduction.