TL;DR:
Oxidation is loss of electrons and reduction is gain (OIL RIG). They always happen together.
Oxidation numbers keep the bookkeeping: in rusting, iron goes from 0 to +3 and oxygen from 0 to −2.
A battery separates the two halves so the electrons flow through a wire.
A new nail is grey and shiny. Leave it outside for a season and it turns orange and flaky. The metal has changed into a different substance, and the change is a transfer of electrons. The same kind of transfer runs a torch battery.
Electrons change hands
Oxidation is the loss of electrons. Reduction is the gain of electrons. The memory phrase is OIL RIG: Oxidation Is Loss, Reduction Is Gain.
The two always occur together, because electrons cannot be lost into nothing: something has to take them. A reaction with both is called a redox reaction. The substance that is oxidised is the reducing agent, since it supplies the electrons. The substance that is reduced is the oxidising agent.
Oxidation numbers: the bookkeeping
An oxidation number is the charge an atom would have if all its bonding electrons went to the more electronegative atom. It is a counting tool. A few rules cover most cases.
An atom in its pure element form is 0, such as Fe, O₂ or Zn.
A simple ion has its own charge: Na⁺ is +1, Cl⁻ is −1.
Oxygen is usually −2, and hydrogen is usually +1.
The numbers in a neutral compound add to 0, and in an ion add to the ion's charge.
Try it on the permanganate ion, MnO₄⁻. Four oxygens at −2 give −8. The total must be −1, so manganese is x − 8 = −1, which gives x = +7.
Oxidation means the oxidation number goes up. Reduction means it goes down.
Worked example: iron rusting
Iron reacts with oxygen, in the presence of water, to form iron(III) oxide. Rust is really a hydrated form of it, but the simplified equation shows the electron transfer: 4 Fe + 3 O₂ → 2 Fe₂O₃.
Iron starts as the element: oxidation number 0.
In Fe₂O₃, oxygen is −2. Three oxygens give −6, so the two irons must give +6 together, which is +3 each.
Iron went from 0 to +3: it lost 3 electrons each, so it was oxidised.
Oxygen went from 0 to −2: it gained 2 electrons per atom, so it was reduced.
Count electrons: 4 iron atoms lose
4 × 3 = 12. Three O₂ molecules are 6 atoms, and gain6 × 2 = 12. They match.
The equal numbers are the check: every electron lost by iron is gained by oxygen. Iron is the reducing agent and oxygen is the oxidising agent. Rusting needs both oxygen and water, which is why keeping iron dry or coating it with paint slows it.
Redox in a battery
Take a zinc strip in zinc sulfate solution connected to a copper strip in copper sulfate solution. Zinc is more eager to lose electrons than copper is. The overall change is Zn + Cu²⁺ → Zn²⁺ + Cu.
Zinc:
Zn → Zn²⁺ + 2e⁻. Oxidation number 0 to +2, oxidised. This electrode is the anode.Copper ions:
Cu²⁺ + 2e⁻ → Cu. Oxidation number +2 to 0, reduced. This electrode is the cathode.Check: 2 electrons lost, 2 gained.
If the two chemicals touched directly, the electrons would hop across and the energy would just warm the mixture. In a cell, the halves are separated and joined by a wire and a salt bridge, so the electrons must travel through the wire, and that flow is the current you can use. This classic cell gives about 1.1 V under standard conditions.
Batteries and steel wool: Do not connect a battery terminal to terminal with a wire, steel wool or a coin. The large current heats the metal quickly and can cause burns or fire. Never use mains electricity for any electrochemistry experiment. Leave copper and zinc salt solutions to a supervised lab, and do not drink them.
Here is one more reaction to practise on: magnesium burning in oxygen, 2 Mg + O₂ → 2 MgO. Magnesium goes from 0 to +2, so each atom loses 2 electrons: 2 × 2 = 4 lost. Each oxygen atom goes from 0 to −2, and there are 2 atoms, so 2 × 2 = 4 gained. The electron count balances, which confirms the equation.
A quick method for balancing a redox equation is to write the two half-reactions, make the electrons equal by multiplying, and then add them. For zinc and copper ions, each half-reaction already involves 2 electrons, so they add directly with no multiplying.
How to check your answer
Electrons balance. Electrons lost equal electrons gained.
Totals. The oxidation numbers in a compound add to its charge.
Direction. Oxidised means the number rises. Reduced means it falls.
Mass and charge. The equation is balanced in atoms and in charge.
Mistakes that cost marks
Mixing up the agent and the process: the reducing agent is the one that is oxidised.
Forgetting that oxidation numbers are per atom, not per formula.
Assuming oxygen is always −2. In peroxides it is −1.
Treating oxidation and reduction as separate events.
Thinking oxidation needs oxygen. Many redox reactions have none.
Where you will use it next
Redox runs corrosion, batteries, combustion and the energy chemistry of living cells, such as cellular respiration and ATP. It also connects to heat release in endothermic and exothermic reactions.
Frequently asked questions
Is burning a redox reaction?
Yes. In burning methane, carbon goes from −4 to +4 and oxygen from 0 to −2, so carbon is oxidised and oxygen is reduced.
Why does a galvanised nail resist rust?
Zinc is oxidised more readily than iron, so it corrodes first and protects the iron underneath.
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