Main Page | See live article | Alphabetical index

Oxidation number

The oxidation state or oxidation number is defined as the sum of negative and positive charges in an atom, which indirectly indicates the number of electrons it has accepted or donated. The oxidation number is a convenient conceptual approximation when working with complex electrochemical reactions that eases the tracking of electrons and helps verify that they have been conserved. This is especially useful whilst expressing complex half-reaction equations involved in oxidation/reduction reactions.

Atoms are defined as having an oxidation number of zero, meaning that they are electrically neutral. The positive protons in the nucleus balance the negative electron cloud surrounding it, there being equal numbers of both. If an atom donates an electron it has more protons than electrons and becomes positive. This ion is said to have an oxidation number of +1. Conversely if an atom accepts an electron it becomes negatively charged, gaining an oxidation number of -1. In summary, if an atom or ion donates an electron in a reaction its oxidation state is increased by one, if an element accepts an electron its oxidation state is decreased by one.

Oxidation numbers are denoted in chemical names by bracketed roman numerals placed immediately after the relevant element. For example, an iron ion, with an oxidation state of +3 is expressed as iron (III). Manganese with an oxidation state of +7 present in manganese oxide is given the name manganese (VII) oxide. The motive for placing oxidation numbers in names is only to distinguish between different compounds of the same elements. The actual charge (positive/negative) of the ion is not expressed because it is not necessary for this purpose.

In chemical formulae, the oxidation number of ions is placed in superscript after the element's symbol. For example, oxygen (II) is written as O2-. Oxidation numbers of neutral numbers are not expressed. The following formula describes the element I2 accepting two electrons to gain an oxidation number of -1.

I2 + 2e- -> 2I- 

When dealing with reactions, the following rules define oxidation number: Sometimes it is not immediately obvious what the oxidation number of ions in a molecule are from its molecular formula. For example, given Cr(OH)3, no oxidation numbers are present yet it is clear that ionic bonding is occurring.

There are a number of rules that can be used in determining an ions atomic number:

With the example, Cr(OH)3, oxygen has an oxidation number of -2 (no fluorine, O-O bonds or peroxide present), and hydrogen has a state of +1 (not a metal hydride). So, the triple hydroxide group has a charge of 3*(-2 + 1) = -3. As the compound is neutral, Cr has to have a charge of +3.
See also: Electrochemistry, valency