A chemical reaction in which an atom or ion loses electrons to another atom or ion.

An important concept of chemical reactions which is useful in systematizing the chemistry of many substances. Oxidation can be represented as involving a loss of electrons by one molecule and reduction as involving an absorption of electrons by another. Both oxidation and reduction occur simultaneously and in equivalent amounts during any reaction involving either process.

Oxidation number

The oxidation state is a concept which describes some important aspects of the state of combination of the elements. An element in a given substance is characterized by a number, the oxidation number, which specifies whether the element in question is combined with elements which are more electropositive or more electronegative than it is. It further specifies the combining capacity which the element exhibits in a particular combination. A scale of oxidation numbers is defined by assigning to an oxygen atom in an ion such as SO₄²⁻ the value of 2−. That for sulfur as 6+ then follows from the requirement that the sum of the oxidation numbers of all the atoms add up to the net charge on the species. The value of 2− for oxygen is not chosen arbitrarily. It recognizes that oxygen is more electronegative than sulfur, and that when it reacts with other elements it seeks to acquire two more electrons, by sharing or outright transfer from the electropositive partner, so as to complete a stable valence shell of eight electrons.

Although oxidation number is in some respects similar to valence, the two concepts have distinct meanings. In the substance H₂, the valence of hydrogen is 1 because each H makes a single bond to another H, but the oxidation number is 0, because the hydrogen is not combined with a different element. See also Valence.

When the oxidation number of an atom in a species is increased, the process is described as oxidation, no matter what reagent produces it; when a decrease in oxidation number takes place, the process is described as reduction, again without regard to the identity of the reducing agent. The term oxidation has been generalized to imply combination of an element with an element more electronegative than itself.

Reactions

In an oxidation-reduction reaction, some element decreases in oxidation state and some element increases in oxidation state. The substances containing these elements are defined as the oxidizing agents and reducing agents, and they are said to be reduced and oxidized, respectively. The processes in question can always be represented formally as involving electron absorption by the oxidizing agent and electron donation by the reducing agent. For example, reaction (1) can be regarded as the sum of the two partial processes, or half-reactions, (2) and (3). Similarly, reaction (4) consists of the two half-reactions (5) and (6), with half-reaction (5) being taken five times to balance the electron flow from reducing agent to oxidizing agent.
1

2

3

4

5

6

Each half-reaction consists of an oxidation-reduction couple; thus, in half-reaction (6) the reducing agent and oxidizing agent making up the couple are manganous ion, Mn²⁺, and permanganate ion, MnO₄⁻ , respectively; in half-reaction (5) the reducing agent is I⁻ and the oxidizing agent is I₂. The fact that MnO₄⁻ reacts with I⁻ to produce I₂ means that MnO₄⁻ in acid solution is a stronger oxidizing agent than is I₂. Because of the reciprocal relation between the oxidizing agent and reducing agent comprising a couple, this statement is equivalent to saying that I⁻ is a stronger reducing agent than Mn²⁺ in acid solution. Reducing agents may be ranked in order of tendency to react, and this ranking immediately implies an opposite order of tendency to react for the oxidizing agents which complete the couples. In the list below some common oxidation-reduction couples are ranked in this fashion:

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A chemical reaction in which an atom or ion loses electrons to another atom or ion. Also called redox.

The noun has one meaning:

Meaning #1: a reversible chemical reaction in which one reaction is an oxidation and the reverse is a reduction
  Synonyms: oxidoreduction, redox