oxidation number

oxidation state:

see valencevalence,
combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound.
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Oxidation Number

(also oxidation state), the numerical value of the electrostatic charge of an atom in a molecule. When assigning an oxidation number, it is assumed that the electron pairs that effect the bond in the molecule are completely shifted in the direction of the more electronegative atoms. The oxidation number of a hydrogen atom in compounds that do not contain a metal is arbitrarily set as +1. The rules for computing the oxidation number are given in OXIDATION-REDUCTION REACTION and VALENCE.

In chemistry, the concept of an oxidation number is used to formulate oxidation-reduction equations and to classify inorganic compounds, particularly coordination compounds, to which the standard classical definitions of valence do not conveniently apply.

In many cases, the oxidation number coincides with neither the valence nor the actual number of electrons that participate in bond formation; for example, carbon is always tetravalent in organic compounds, but the oxidation number for the C atom in the compounds CH₄, CH₃OH, and HCOOH is -4, -2, and +2, respectively. In compounds whose atoms are similar in electronegativity, it becomes difficult to determine precisely which atom is attracting the electron pair. For example, the carbon and sulfur atoms in a CS₂ molecule exhibit nearly identical electronegativity, yet the oxidation numbers in this compound can be +4 or –4 for the C atom and —2 or +2 for the S atom.