Redox (reduction-oxidation) reactions are a family of reactions that are concerned with the transfer of electrons between reactants. Redox reactions are a matched pair – you don’t have an oxidation reaction without a reduction reaction happening at the same time. Oxidation means loss of electrons while reduction means gain of electrons. Each reaction by itself is called a “half-reaction”, simply because we need two half-reactions to form a whole reaction. We write them out like this.
Mixing magnesium powder and copper oxide together – a displacement reaction.
The copper ion has gained 2 electrons, and is reduced, the Mg atom has lost 2 electrons and is oxidised
The OILRIG rule: Oxidation is loss, reduction is gain of electrons.