The earliest view of oxidation and reduction is that of adding oxygen to form an oxide (oxidation) or removing oxygen (reduction). They always occur together.

For example, in the combustion of hydrogen to form water the following reaction takes place.

2H₂ + O₂ -> 2H₂O

The hydrogen is oxidized and the oxygen is reduced.

This developed into a more general view of the process in which oxidation is the loss of electrons and reduction as the gain of electrons. These processes always occur simultaneously and are known as redox reactions (reduction - oxidation..As an example the reaction.

4Na + O₂ - >2Na₂O

The solid sodium atoms lose electrons and are therefore oxidised.   The gaseous oxygen ions gain electrons and are reduced..

The species which brings about oxidation is the oxidising agent and this is reduced as part of the reaction.  The species which brings about reduction is the reducing agent and this itself is oxidised as part of the reaction..

The number of electrons lost by an element as a result of a chemical change is known as its positive oxidation state and the total number of electrons gained by an element as a result of a chemical change is known as its negative oxidation state...

Another method of explaining the oxidation state of number is that it is the number of electrons that must be added to or subtracted from an atom within a molecule to convert it to the elemental form; i.e., in barium chloride ( BaCl₂) the oxidation number of barium is +2 and of chlorine is -1.   Many elements can exist in more than one oxidation state.

The values indicated as fixed are useful in determining the oxidation states of other elements.

Each redox couple creates its own electrical potential difference.   It is not possible to measure absolute electrode potentials but it is possible to determine relative values using a reference hydrogen electrode.

Relative electrode potentials are determined by comparison with a standard hydrogen electrode which is assigned a value of zero under standard thermodynamic conditions.  The potential difference between the standard hydrogen electrode and other redox couples in which the concentration of the active ions in solution is effectively molar at at temperature of 298 K is known as the standard electode potential (or redox) potential.....

A table of the standard electrode potentials of some elements is provided below .  It is possible to determine the electical potential across a cell with two electrodes of different elements inimmersed in an aqueous solution using the values in this table. E.g. a cell made up from a zinc and a copper electrode would generate a potential difference of (0.34V)- (-0,76) = 1,1, Volts

In the table below the half reaction is the state of equilibrium in which the reduced element is in equilibrium with the oxidised state (the element ions).   The electrons remain with the element..a potential difference exists between the element and the solution.   The values below indicate this potential difference relative to that of a hydrogen electrode.