Oxidation state

The oxidation state or oxidation number is defined as the sum of the negative and positive charges in an atom i.e., the number of electrons it has accepted or donated.

Atoms are considered to have oxidation numbers 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 neutrons 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.

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 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 formulas, the oxidation number of ions is placed in superscript after the element's symbol. For example, oxygen (II) is written as O^2-. Oxidation numbers of neutral numbers are not expressed. The following formula describes the element I₂ accepting two electrons to gain an oxidation number of -2.

I₂ + 2e^- -> 2I^-

Sometimes it is not immediately evident 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 occuring.

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

The oxidation number of (neutral) atoms equal zero.
In neutral molecules, the sum of the oxidation numbers adds up to zero.
Fluorine always has a -1 oxidation number within compounds.
Oxygen has an oxidation number of -2 in compounds, except (i) in the presence of fluorine, in which fluorine's oxidation number takes precedence; (ii) in oxygen-oxygen bonds, where one oxygen must neutralize the other's charge; (iii) in peroxide compounds, in which it takes an oxidation number of -1.
Group I ions have an oxidation number equal to +1 within compounds.
Group II have an oxidation number of +2 within compounds.
Halogens, besides fluorine, generally have -1 oxidation numbers in compounds. This rule can be broken in the presence of oxygen or other halogens, where the oxidation numbers can be positive.
Hydrogen always has an oxidation number of +1 oxidation number in compounds, except in metal hydrides.

See also: Electrochemistry