Reduction is a process which involves the gain of electrons. Any substance form which electron can be readily removed, such as metals, may act as reducing agents. A reducing agent is a substance that can give electrons. The gain of electrons during reduction may result in :

(i)    Decrease of positive charge on atom or ion undergoing reduction.
 For example,

                                                                 Na⁺ + e   →     Na
                                                                 Fe³⁺ + e   →    Fe²⁺
                                                                 Sn⁴⁺ + 2e   →    Sn²⁺
                                                                O₂ + 2e     →    2O^2-

(ii)    Increase of negative charge on atom or ion undergoing reduction.
For example.

                                                                MnO₄^- + e    →    MnO₄^2-
                                                  [ Fe ( CN )₆ ]^3- + e    →     [ Fe (CN )₆ ] ^{4 -}

Oxidation-reduction reactions are an important general class of chemical reactions. In general, the reaction between a metal and a non-metal is an oxidation-reduction reaction in which the metal behaves as a reducing agent and is oxidized and the non-metal behaves as an oxidizing agent and is reduced.

Developments of definitions of oxidation ad reduction and of oxidizing agents can be summarized by an equilibrium:
                               Oxidation
Reducing agent      <==>        Oxidizing agent + Electrons
                              Reduction

Reduction-oxidation systems are also abbreviated as Redox systems.

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