Question

Why is gaining electrons called reduction?

Answer 1

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"` decomposes on heating into mercury and oxygen:

`"2HgO" ⟶ "2Hg" + "O"_2`

The `"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"` to `"Hg"`.

`"O"^(2-) → "O" + "2e"^(-)`
`"Hg"^(2+) + 2"e"^(-) → "Hg"`

`"Hg"^(2+) + "O"^(2-) → "HgO"`

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

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