Oxidation and reduction are such useful concepts that they have been extended from cases of outright electron gain or loss, to reactions involving varying degrees of electron sharing.

When copper oxide ore is reduced to metallic copper by roasting with charcoal (below), copper ions are reduced and carbon is oxidized:

2Cu2O + C ------> 4Cu + CO2

The reduction of copper is obvious; each Cu+ ion accepts one electron and becomes a copper atom.

But in what sense is carbon oxidized? It does not give up its outer electrons and become a C4+ ion in CO2.

Each carbon atom of coke initially shared its four electrons equally with other carbon atoms. It continues to share electron pairs with oxygen in CO2, but on an unequal basis.