Why is gaining electrons called reduction?

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Answer 1 · 2015-05-24T23:59:29

In the early days of chemistry, oxidation was defined as a gain of oxygen atoms, and reduction was a loss of oxygen atoms.

For example, $HgO$ $"HgO"$ decomposes on heating into mercury and oxygen:

$2HgO ⟶ 2Hg + O_{2}$ $"2HgO" ⟶ "2Hg" + "O"_2$

The $Hg$ $"Hg"$ was said to be reduced because it lost an oxygen atom.

Eventually, chemists realized that the reaction involved a transfer of electrons from $O$ $"O"$ to $Hg$ $"Hg"$ .

$O^{2 -} \to O + {2e}^{-}$ $"O"^(2-) → "O" + "2e"^(-)$
${Hg}^{2 +} + 2 e^{-} \to Hg$ $"Hg"^(2+) + 2"e"^(-) → "Hg"$

${Hg}^{2 +} + O^{2 -} \to HgO$ $"Hg"^(2+) + "O"^(2-) → "HgO"$

Since $Hg$ $"Hg"$ gained electrons in the process, chemists added a second definition:

$G$ $color(red)("G")$ ain of $E$ $color(red)("E")$ lectrons is $R$ $color(red)("R")$ eduction.