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Oxidation state

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An oxidation state of an element is a number which indicates the number of electrons taken from (oxidation) or given to (reduction) the element when it bonds with another element to make a chemical compound. This is normally given as a + or - number. Electrons are counted as negative charges; so when an electron is added, the element is in a -1 oxidation state. When an ion is written, its oxidation state is in superscript after the chemical symbol, such as Fe2+. This means iron in its +2 oxidation state, also known as ferrous iron. Normally compounds with a high + oxidation state are oxidizing agents because they have too few electrons. An example would be potassium permanganate, which contains manganese in its +7 oxidation state.

Oxidation state is not completely accurate, though. Many times, especially in more covalently bonded molecules, electrons are not completely taken away from one atom and given to the other atom. The electron may be away from the atom about 80% of the time. The more covalent the bonding gets in an atom, the more difficult it is to find out the oxidation state of each atom.

The official IUPAC definition of oxidation state is:[1]

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; (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); (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 (elemental sulfur), SO2, SO3, and H2SO4 are, respectively: -2, 0, +4, +6 and +6. The higher the oxidation state of a given atom, the greater is its degree of oxidation; the lower the oxidation state, the greater is its degree of reduction.


  1. IUPAC Gold Book