Topic 4 - Redox

This page looks at the various definitions of oxidation and reduction (redox) in terms of the transfer of oxygen, hydrogen and electrons. It also explains the terms oxidising agent and reducing agent.  Firstly, read this section.

This page explains how to work out electron-half-reactions for oxidation and reduction processes, and then how to combine them to give the overall ionic equation for a redox reaction. This is an important skill in inorganic chemistry.

Don't worry if it seems to take you a long time in the early stages. It is a fairly slow process even with experience. Take your time and practice as much as you can.
This page explains what oxidation states (oxidation numbers) are and how to calculate them and make use of them.
Oxidation states are straightforward to work out and to use, but it is quite difficult to define what they are in any quick way.

The oxidation number of an element indicates the number of electrons lost, gained, or shared as a result of chemical bonding. The change in the oxidation state of a species lets you know if it has undergone oxidation or reduction.

Oxidation can be defined as "an increase in oxidation number".  In other words, if a species starts out at one oxidation state and ends up at a higher oxidation state it has undergone oxidation.

Conversely, Reduction can be defined as "a decrease in oxidation number".  Any species whose oxidation number is lowered during the course of a reaction has undergone reduction.


  • Na  +  Cl2 ----->  2NaCl
  • The Na starts out with an oxidation number of zero (0) and ends up having an oxidation number of 1+. It has been oxidized from a sodium atom to a positive sodium ion.
  • The Cl2 also starts out with an oxidation number of zero (0), but it ends up with an oxidation number of 1-. It, therefore, has been reduced from chlorine atoms to negative chloride ions.

The substance bringing about the oxidation of the sodium atoms is the chlorine, thus the chlorine is called an oxidizing agent. In other words, the oxidizing agent is being reduced (undergoing reduction).

The substance bringing about the reduction of the chlorine is the sodium, thus the sodium is called a reducing agent. Or in other words, the reducing agent is being oxidized (undergoing oxidation).

Oxidation is ALWAYS accompanied by reduction. Reactions in which oxidation and reduction are occurring are usually called Redox reactions.

There are several rules for assigning the oxidation number to an element. Learning these rules will simplify the task of determining the oxidation state of an element, and thus, whether it has undergone oxidation or reduction.

The oxidation number of an atom in the elemental state is zero.  Example: Cl2 and Al both are 0

The oxidation number of a monatomic ion is equal to its charge.  Example: In the compound NaCl, the sodium has an oxidation number of 1+ and the chlorine is 1-. 

The algebraic sum of the oxidation numbers in the formula of a compound is zero.  Example: the oxidation numbers in the NaCl above add up to 0

The oxidation number of hydrogen in a compound is 1+, except when hydrogen forms compounds called hydrides with active metals, and then it is 1-.  Examples: H is 1+ in H2O, but 1- in NaH (sodium hydride)

The oxidation number of oxygen in a compound is 2-, except in peroxides when it is 1-, and when combined with fluorine. Then it is 2+.  Example: In H2O the oxygen is 2-, in H2O2 it is 1-.

The algebraic sum of the oxidation numbers in the formula for a polyatomic ion is equal to the charge on that ion.  Example: in the sulfate ion, SO42-, the oxidation numbers of the sulfur and the oxygens add up to 2-. The oxygens are 2- each, and the sulfur is 6+.


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