Redox as electron transfer

In 2Na + Cl₂ → 2NaCl, each sodium atom gives up one electron and each chlorine atom accepts one. The reaction can be split into two half reactions: 2Na → 2Na⁺ + 2e⁻ and Cl₂ + 2e⁻ → 2Cl⁻.

The half reaction in which electrons are lost is the oxidation half; the one in which electrons are gained is the reduction half.

Electronic definitions: oxidation is loss of electrons by a species; reduction is gain of electrons by a species.

The oxidising agent (oxidant) accepts electrons and is itself reduced. The reducing agent (reductant) donates electrons and is itself oxidised.

In 2Na + H₂ → 2NaH, hydrogen gains electrons and becomes H⁻, so hydrogen is reduced and sodium is oxidised. Hydrogen acts here as the oxidising agent, opposite to its usual role.