Oxidation and reduction in terms of electrons

Oxidation is the loss of electrons. Reduction is the gain of electrons. A memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain.

Look at the half equations. Mg → Mg²⁺ + 2e^- shows magnesium atoms losing electrons, so magnesium is oxidised. Cu²⁺ + 2e^- → Cu shows copper ions gaining electrons, so the copper ions are reduced. In Na⁺ + e^- → Na the sodium ion gains an electron, which is reduction.

Oxidation and reduction always happen together, because electrons lost by one particle must be gained by another. This is a redox reaction. In Zn + Cu²⁺ → Zn²⁺ + Cu, zinc loses electrons and is oxidised, while the copper ions gain electrons and are reduced.