General Chemistry/Redox Reactions/Oxidation state

Oxidation states are used to determine the degree of oxidation or reduction that an element has undergone when bonding. The oxidation state of a compound is the sum of the oxidation states of all atoms within the compound, which equals zero unless the compound is ionic.

The oxidation state of an atom within a molecule is the charge it would have if the bonding were completely ionic, even though covalent bonds do not actually result in charged ions.

Method of notation

Oxidation states are written above the element or group of elements that they belong to (when drawing the molecule), or written with roman numerals in parenthesis when naming the elements.

Examples

$\begin{matrix} _0 \\ Al \end{matrix}$	aluminum
$\begin{matrix} _{+3} \\ Al \end{matrix}$	aluminum(III), an ion

Determining oxidation state

For single atoms or ions

Because oxidation numbers are just the sum of the electrons gained or lost, calculating them for single elements is easy.

Examples

$\begin{matrix} _1 \\ K^+ \end{matrix}$

$\begin{matrix} _0 \\ Br \end{matrix}$

$\begin{matrix} _1 & _{-1} \\ Na & Cl \end{matrix}$

Notice that the oxidation states of ionic compounds are simple to determine.

For larger molecules

Helpful Hint!
Remember that all the individual oxidation states must add up to the charge on the whole substance.

Although covalent bonds do not result in charges, oxidation states are still useful. They label the hypothetical transfer of electrons if the substance were ionic. Determining the oxidation states of atoms in a covalent molecule is very important when analyzing "redox" reactions. When substances react, they may transfer electrons when they form the products, so comparing the oxidation states of the products and reactants allows us to keep track of the electrons.

Examples

$\begin{matrix} _{+1} & _{-1} \\ H & Cl \end{matrix}$	for hydrogen chloride
$\begin{matrix} _{2(+1)} & _{-2} \\ H_2 & O \end{matrix}$	for water
$\begin{matrix} _{+3} & _{2(-2)} \\ Cl & O_2 \end{matrix}$	for the chlorite ion (notice the overall charge)

Guidelines

Determining oxidation states is not always easy, but there are many guidelines that can help. This guidelines in this table are listed in order of importance. The highest oxidation state that any element can reach is +8 in XeO₄.

Element	Usual Oxidation State
Fluorine	Fluorine, being the most electronegative element, will always have an oxidation of -1 (except when it is bonded to itself in F2, when its oxidation state is 0).
Hydrogen	Hydrogen always has an oxidation of +1, -1, or 0. It is +1 when it is bonded to a non-metal (e.g. HCl, hydrochloric acid). It is -1 when it is bonded to metal (e.g. NaH, sodium hydride). It is 0 when it is bonded to itself in H2.
Oxygen	Oxygen is usually given an oxidation number of -2 in its compounds, such as H2O. The exception is in peroxides (O2-2) where it is given an oxidation of -1. Also, in F2O oxygen is given an oxidation of +2 (because fluorine must have -1), and in O2, where it is bonded only to itself, the oxidation is 0.
Alkali Metals	The Group 1A metals always have an oxidation of +1, as in NaCl. The Group 2A metals always have an oxidation of +2, as in CaF2. There are some rare exceptions that don't need consideration.
Halogens	The other halogens (Cl, Br, I, As) usually have an oxidation of -1. When bonded to another halogen, its oxidation will be 0. However, they can also have +1, +3, +5, or +7. Looking at the family of chlorides, you can see each oxidation state (Cl2 (0), Cl- (-1), ClO- (+1), ClO2- (+3), ClO3- (+5), ClO4- (+7)).
Nitrogen	Nitrogen (and the other Group 5A elements, such as phosphorus, P) often have -3 (as in ammonia, NH3), but may have +3 (as in NI3) or +5 (as in phosphate, PO43-).
Carbon	Carbon can literally have any oxidation state (from -4, as in CH4, to +4, as in CF4). It is best to find the oxidation of other elements first.

In general, the more electronegative element has the negative number. Using a chart of electronegativities, you can determine the oxidation state of any atom within a compound.

Periodicity

Oxidation states are another periodic trend. They seem to repeat a pattern across each period.