Reduction

1. If, in a reaction, the oxidation number of an atom in a reactant decreases, the atom is said to undergo reduction.

eg. 1:



eg. 2:



eg. 3:



eg. 4:



2. If, in a species subjected to a reaction, the sum of oxidation numbers of atoms that participate in the overall reaction decreases, the species is said to undergo reduction.

eg. 1:



In 1, the sum of oxidation numbers of atoms that participate in the reaction decreases from 0 to -2; 1 is reduced.

eg. 2:



In 2, the sum of oxidation numbers of atoms that participate in the reaction decreases from -2 to -4; 2 is reduced.

eg. 3:



In 3, the sum of oxidation numbers of atoms that participate in the reaction decreases from -2 to -4; 3 is reduced.

eg. 4:



In 4, the sum of oxidation numbers of atoms that participate in the reaction decreases from -4 to -6; 4 is reduced.

In most reductions, the species reduced either gains hydrogen (eg. 1 and 3), loses oxygen (eg. 2), or both.  Traditionally, in casual usage, a reduction reaction involving neither is not referred to as a reduction.

see also oxidation

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