Reduction is a gain of electrons by a reactant in any reaction.

Oxidation is a loss of electrons by a reactant in any reaction.

In a redox reaction, reduction and oxidation take place at the same time.

Ion-electron equations can be combined to produce redox equations.

An ion-electron equation can be balanced by adding appropriate numbers of water molecules, hydrogen ions and electrons, in the following steps.

1. Balance all elements that are not $O$ or $H$.
2. Balance any $O$s by adding $H_2O$.
3. Balance any $H$s by adding $H^+$.
4. Balance the charges by adding electrons — $e^-$.

An oxidising agent is a substance that accepts electrons.

A reducing agent is a substance that donates electrons.

Compounds, group ions and molecules can act as oxidising or reducing agents.

• hydrogen peroxide is a molecule that is an oxidising agent
• dichromate and permanganate ions are group ions that are strong oxidising agents in acidic solutions
• carbon monoxide is a gas that can be used as a reducing agent

Elements with low electronegativities tend to form ions by losing electrons and so act as reducing agents.

Elements with high electronegativities tend to form ions by gaining electrons and so act as oxidising agents.

In the periodic table, the strongest reducing agents are in group 1.

In the periodic table, the strongest oxidising agents are in group 7.

Oxidising and reducing agents can be identified in redox reactions, as they are the chemicals that are themselves being reduced or oxidised respectively.

The electrochemical series represents a series of reduction reactions.