1.6 Oxidation numbers

Oxidation numbers

When reactions occur, an exchange of electrons takes place. Oxidation is the loss of electrons from an atom, while reduction is the gain of electrons by an atom. By giving elements an oxidation number, it is possible to keep track of whether that element is losing or gaining electrons during a chemical reaction. The loss of electrons in one part of the reaction must be balanced by a gain of electrons in another part of the reaction.

Oxidation number

A simplified way of understanding an oxidation number is to say that it is the charge an atom would have if it was in a compound composed of ions.

There are a number of rules that you need to know about oxidation numbers, and these are listed below. These will probably not make much sense at first, but once you have worked through some examples, you will soon start to understand!

1. Rule 1: An element always has an oxidation number of zero, since it is neutral. In the reaction $H_2+B{r}_2\to 2HBr$ , the oxidation numbers of hydrogen and bromine on the left hand side of the equation are both zero.
2. Rule 2: In most cases, an atom that is part of a molecule will have an oxidation number that has the same numerical value as its valency.
3. Rule 3: Monatomic ions have an oxidation number that is equal to the charge on the ion. The chloride ion $C{l}^{-}$ has an oxidation number of -1, and the magnesium ion $M{g}^{2+}$ has an oxidation number of +2.
4. Rule 4: In a molecule, the oxidation number for the whole molecule will be zero, unless the molecule has a charge, in which case the oxidation number is equal to the charge.
5. Rule 5: Use a table of electronegativities to determine whether an atom has a positive or a negative oxidation number. For example, in a molecule of water, oxygen has a higher electronegativity so it must be negative because it attracts electrons more strongly. It will have a negative oxidation number (-2). Hydrogen will have a positive oxidation number (+1).
6. Rule 6: An oxygen atom usually has an oxidation number of -2, although there are some cases where its oxidation number is -1.
7. Rule 7: The oxidation number of hydrogen is usually +1. There are some exceptions where its oxidation number is -1.
8. Rule 8: In most compounds, the oxidation number of the halogens is -1.