Oxidation Number

The oxidation number of an element is the charge on its ion in an ionic compound or a measure of its "combining power" in a covalent compound.

The oxidation number can be worked out using a set of simple rules (learn them!):

All uncombined elements have oxidation number zero

All group 1 compounds have the group 1 element in oxidation state +1

All group 2 compounds have the group 2 element in oxidation state +2

All fluorine compounds have fluorine in oxidation state -1

Hydrogen is usually in oxidation state +1 in compounds

Oxygen is usually in oxidation state -2 in compounds

Chlorine, bromine & iodine are often in oxidation state -1 in compounds

These rules are in order of priority - if they give the usual oxidation state of an element then this will apply unless it is forced to be different by one of the rules above it.

The oxidation numbers of all the elements in a neutral molecule will add up to zero.

In an ion the oxidation numbers all add up to the charge on the ion.

If an element is in the compound, but not on our table, all the elements in the table will have their usual oxidation numbers. We can make the oxidation number of the unknown element what we like in order that all the oxidation numbers add up. A few examples should make this clear:

NaCl	Na is always +1, so Cl must be -1	+1 -1 = 0
NO₃^-	O is usually -2, so N must be +5	+5 + (-2 × 3) = -1
F₂O	F is always -1, so O must be +2 (unusual)	(-1 × 2) + 2 = 0
HCO₃^-	O is usually -2 and H is usually +1, so C must be +4	+ 1 + 4 + (-2 × 3) = -1
NaBiO₃	Na is always +1, O usually -2, so Bi is + 5	+ 1 + 5 + (-2 × 3) = 0
BaO₂	Ba is always + 2, so O must be - 1 (unusual)	+ 2 + (-1 × 2) = 0

Just to make life more complicated, it is possible to have oxidation numbers which are not whole numbers. Although they are not common, they do appear in exams from time to time. For example, KO₂. K is always +1. As there is no charge on KO₂, the two oxygens must add up to -1. This means each one is -1/2 = - 0.5. Note that the oxidation number is always given for a single atom. A common mistake is to give a total charge. In the KO₂ example, the oxidation number is -0.5 (each atom), not -1 (the total).

The best way to check your understanding is with plenty of examples. You can find them here.

If the oxidation number of an element increases it has been oxidized, if the oxidation number of an element decreases it has been reduced.