Oxidation numbers
Covalent bonds are about sharing electrons. The shared electrons will be responsible for the bonding, as it has been illustrated in the case of the hydrogen molecule.
This electron sharing, however, is usually unequal. That is because some elements have a stronger tendency to pull electrons than others. This tendency is called electronegativity.
The element in the molecule that has the higher electronegativity will have the electrons closer to itself. The other elements will be somehow deprived of electrons. Electron sharing will only be equal if all the elements in the molecule are the same, like in the case of molecules like H2 or O2.
Oxidation numbers ar about giving some idea about the distribution of electrons in a molecule.
The term oxidation, which is studied in detail in the section oxidation and reduction, means 2 things: to receive oxygen atoms or to lose electrons. The latter is more meaningful in the context of oxidation numbers, as they apply even to molecules that don't have any oxygen atoms.
Associating oxidation numbers to elements
It is no possible to give fixed oxidation numbers to elements in molecules because these will depend on the other elements present and their electronegativities. However there are a few rules that can be applied:
1) The charge of the molecules must equal the sum of all oxidation numbers in it.
2)The oxidation number of H is usually +1, but it may be -1 if combined to electron donating metals.
3)Oxidation numbers of elements before reacting is always zero. Exemplo, iron metal has oxidation number 0.
4)Oxidation numbers of ions, including metals from groups 1 and 2, are thesame as their charges.
5)The oxidation number of oxygen is usually -2, but there are some exceptions like in the case of peroxides and superoxides. It is positive only when oxygen is combined to fluorine, which is the only element that is more electronegative than oxygen.
Because the oxidation number of an element depends on its chemical environment (i.e. the elemnts around it), it may assume different values in different molecules. The first example will illustrate this point:
Examples:
Calculate the oxidation number (Nox) of sulfur (S) in SO2, SO3 and SO3-2 .
SO2 : because Nox of oxygen is -2 (there are 2 of them) and the sum of all numbers must be zero (total charge of the molecule), Nox of S is 4 .
SO3 : Nox of S is 6
SO3-2 : Nox of S is 4