Atoms are more stable when they have a full outer shell of electrons.
To get a full outer shell of electrons atoms can lose or gain electrons to form charged ions.
If an atom loses electrons it is oxidised.
If an atom gains electrons it is reduced.
Reducing agents get oxidised themselves, causing another species to be reduced.
Oxidising agents get reduced themselves, causing another species to be oxidised.
Half equations show how one species is being oxidised or reduced in a redox reaction.
Atoms can lose and gain electrons in order to have a ‘full’ outer shell of electrons. An atom that has lost or gained an electron becomes charged and is called an ion. An atom that loses an electron becomes a positive ion and has been oxidised. An atom that gains an electron becomes a negative ion and has been reduced.
Oil Rig is a great way of remembering these names. Oxidation is loss (of electrons) and Reduction is gain (of electrons).
A species that is easily reduced (gains electrons) can act as an oxidising agent and oxidise another species.
A species that is easily oxidised (loses electrons) can act as a reducing agent and reduce another species.
Half equations show how an individual ion reacts or how it is produced in terms of electron transfer. However, it is important to remember that in reality these electrons are being lost or gained by another ion or atom – half equations are only half (hence the name!) the story with redox reactions.