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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 is 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^III, 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 number represents the number of electrons gained or lost in a chemical reaction.

The oxidation number is usually numerically equal to the oxidation state of the central atom. However, for a variety of reasons, the oxidation state of transition metals can be difficult to determine^[2]. The most-accepted answer is that the electron pairs forming the coordination bonds are mostly associated with the ligands: this is a good approximation for most Werner-type complexes, but much less true for organometallic compounds as well as for certain hydrido complexes, dithiolene complexes and nitrosyl complexes.

1. All species in their elemental form are given the oxidation number of zero.
2. All monoatomic ions have the same oxidation number as the charge on the ion. e.g. Mg²⁺ has the oxidation number of +2.
3. All combined hydrogen has an oxidation number of +1 (except metal hydrides where its oxidation number is -1).
4. All combined oxygen has an oxidation number of -2 (except peroxides where the oxidation number is -1, and compounds with fluorine where it can be positive).
5. In polyatomic species, the sum of the oxidation numbers of the element in the ion equals the charge on that species (we can use this to find the oxidation number of elements in polyatomic species).
6. Group 1 elements such as K and Na and Group 2 elements such as Mg always have a +1 and +2 oxidation state in compounds, respectively.
7. Cl has a range of oxidation states when bonded to O. However, its oxidation number is always -1 when bonded to ionic compounds.

The oxidation number of sulfur in sulfuric acid (H₂SO₄) can be calculated from the rules above. Because this is a polyatomic species, the individual oxidation numbers must sum to equal the overall charge, which in this case is zero. Hydrogen has an oxidation number of +1, so the sum of the oxidation numbers of H₂ = 2. Oxygen has an oxidation number of -2, so the sum of oxidation numbers of O₄ = -8. Since the overall sum must equal zero, the oxidation state of sulfur can be calculated as +6 (8-2).