Oxidation Number Versus Oxidation State
The oxidation number is usually numerically equal to the oxidation state and so the terms oxidation state and oxidation number are often used interchangeably. However, oxidation number is used in coordination chemistry with a slightly different meaning since the rules used for counting electrons are different: every electron belongs to the ligand, regardless of electronegativity. Also, oxidation numbers are conventionally represented with Roman numerals while oxidation states are given in Arabic numerals. The oxidation number of a central atom may be part of the compound's name (for example iron(II,III) oxide); the oxidation state of atoms is not included in compound names.
The oxidation state can differ from the oxidation number in a few cases where the ligand atom is less electronegative than the central atom (e.g., in iridium phosphine complexes), resulting in a formal oxidation state that is different from the oxidation number.
Oxidation numbers are assigned to all atoms in a chemical equation to identify which atom is oxidized and which is reduced in a chemical process. Five rules can be applied to assigning oxidation numbers:
- The oxidation number of an element in its elementary form is zero. For example, hydrogen, oxygen, nitrogen, chlorine, phosphorus, sulphur, copper, iron etc. have oxidation number equal to zero.
- The sum of the oxidation numbers of all the atoms in an ion or molecule is its net charge.
- In compounds with non-metals, the oxidation number of hydrogen is +1. However, when hydrogen is compounded with a metal, its oxidation number reduces to −1 (the metal being a more electropositive element).
- Oxygen has been assigned oxidation number of −2 in its compounds. However, there are certain exceptions also. In peroxides such as hydrogen peroxide, the oxidation number of oxygen is −1. In oxygen difluoride the oxidation number of oxygen is +2 while in dioxygen difluoride, oxygen is assigned an oxidation number +1 (fluorine being a more electronegative element).
- The atom with higher electronegativity, typically a nonmetallic element, is assigned negative oxidation number while the other atom, often a metallic element, is given positive oxidation number.
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