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.
Read more about Oxidation State: Some General Rules For Determining Oxidation States Without Use of Lewis Structures, Calculation of Formal Oxidation States With A Lewis Structure, Elements With Multiple Oxidation States, Oxidation Number, History, Unusual Formal Oxidation States
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... The oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate ... The peroxochromate(V) is another example of the +5 oxidation state ... Compounds of chromium(IV) (in the +4 oxidation state) are slightly more common than those of chromium(V) ...
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—Sextus Empiricus (2nd or 3rd cen., A.d.)