General Chemistry/Redox Reactions/Oxidation state

Oxidation states (or Oxidation numbers) are used to determine the bonding of the various atoms and ions within a compound. They are the addition of all the charges of the elements of a compound.

Method of notation

Oxidation states are written above the element or group of elements which they go along with.

Example:

$\begin{array}{c}0\\ Al\end{array}$

When using chemical nomenclature, the oxidation number is written in parenthesis after the name

Example:

$Aluminum(III)$

Determining oxidation state

For single atoms or ions

Since oxidation numbers are just the sum of the electrons given or gained, calculating them for single elements is easy: the oxidation state of a single element is the same as its charge.

Example:

$\begin{array}{c}3\\ A{l}^{+3}\end{array}$

Since atoms in their ground state have no charge, their oxidation state reflects this.

Example:

$\begin{array}{c}0\\ Cs\end{array}$

For larger molecules

When dealing with molecules of 2 or more elements, remember that all the individual oxidation states must add up to the charge on the whole substance

Example:

$\begin{array}{c}+1-1\\ H-Cl\end{array}$

Oxidation states do not necessarily represent the actual charges on an atom in a molecule but are simply numbers that indicate what the charges would be if that atom had gained or lost the electrons involved in the bonding. For example, $C{H}_4$ is a covalent molecule of methane - the C has no real charge nor does the H, however the molecule can be assigned a -4 oxidation state for the C and a +1 oxidation state for the H's.

More Examples

$\begin{array}{c}{H}_{2}O\end{array}$

$\begin{array}{cc}2(+1)& -2\\ H_2& O\end{array}$

Oxygen is usually given an oxidation number of -2 in its compounds. The exception is in peroxides O₂^-2 where it is given an oxidation of -1.

Hydrogen is usually given a oxidation of +1 or -1. It is +1 when it is the less electronegative element (e.g. HCl, hydrochloric acid). It is -1 when it is the more electronegative element (e.g. NaH, sodium hydride)