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The oxidation number of an element in a molecule or complex is the charge that it would have if all the ligands (basically, atoms that donate electrons) were removed along with the electron pairs that were shared with the central atom[1]. It's used in the inorganic nomenclature of inorganic compounds. It is represented by a Roman numeral; the plus sign is omitted for positive oxidation numbers. The oxidation number is placed either as a right superscript to the element symbol, e.g. Fe<sup>III</sup>, or in parentheses after the name of the element, e.g. iron(III): in the latter case, there is no space between the element name and the oxidation number. The oxidation number can also be written with a number and either a + or - sign after it. If the element creates a positively charged ion, the oxidation number will have a + sign after it, (example-hydrogen 1+). If the element creates a negatively charged ion, the oxidation number will have a - sign after it, (example-oxygen 2-). The change in the oxidation number represents the number of electrons gained or lost in a chemical reaction.