Key ideas: electron transfer

The idea

Electron transfer is the movement of electrons between species and explains redox reactions. Oxidation is loss of electrons; reduction is gain of electrons. In a reaction, the species that loses electrons is oxidised and acts as the reducing agent, while the species that gains electrons is reduced and acts as the oxidising agent. For example, in 2Mg + O₂ → 2MgO, magnesium atoms lose electrons to form Mg²⁺ and oxygen atoms gain electrons to form O²⁻. In displacement, Zn + Cu²⁺ → Zn²⁺ + Cu, zinc loses electrons and copper ions gain them.

What earns the marks

• States that oxidation is loss of electrons and reduction is gain of electrons, using OIL RIG or equivalent reasoning.
• Identifies the species oxidised and the species reduced in a given reaction by tracking electron loss and gain.
• Writes half-equations such as Mg → Mg²⁺ + 2e⁻ and O₂ + 4e⁻ → 2O²⁻, balancing charge and atoms.
• Explains redox in terms of electron transfer, including the roles of oxidising and reducing agents.
• Links electron transfer to observable changes, for example displacement of copper by zinc or colour change in a solution.

Common mistakes and how to put them right

• Saying oxidation is gain of oxygen only.

  Put it right: In terms of electrons, oxidation is loss of electrons, even when oxygen is not involved.

• Writing half-equations that are not charge-balanced.

  Put it right: Add electrons to the side that needs negative charge so total charge is equal on both sides.

• Confusing the oxidising agent with the species oxidised.

  Put it right: The oxidising agent gains electrons and is itself reduced.

Exam tips

• Use OIL RIG to check which species loses and which gains electrons before answering.
• Balance half-equations for both atoms and charge, then combine them so electrons cancel.
• When asked to explain a displacement, name the electron transfer and the observation, such as a pink-brown solid forming.