Oxidation Number

A compound or an element is said to be oxidized if oxygen is added to it or if hydrogen is removed from it. In both cases, electrons are removed from the element or compound, and this is called oxidation. The oxidation state gives us an idea of its magnitude.

Generally, the oxidation number (also called oxidation state) is an integer which can be positive, negative, or zero. In some cases, it can be a fraction, which will be discussed in later sections. Note that it can only be applied to ionic compounds.

The oxidation state of any neutral atom/element is always zero. Neutral here means the charge is zero. So, before any element reacts with something, its oxidation state is always zero.

• In the case of ions containing only one atom, or a mono atomic ion, the oxidation state equals the charge on the ion.

• The sum of oxidation numbers of all atoms in a given compound is zero (for a neutral compound), or equal to the charge on the compound (for compounds that have a charge).

• The highest possible oxidation number of an element is its group number, and the lowest possible oxidation number is its group number minus 8.

• Hydrogen and oxygen can have two oxidation states.

• Hydrogen can have an oxidation state of either $+1$ or $-1$ depending on the situation, or the oxidation state of hydrogen is $\pm 1$. The oxidation state is +1 when it reacts with a non-metal to form a compound, and it is -1 when it reacts with a metal to form a compound.

• Oxygen can have an oxidation state of either $-2$ (generally) or $-1$ $\big($only in some compounds like hydrogen peroxide $(\ce{H_2O_2})$ and some other peroxides$\big).$ Oxygen has an oxidation state of $-\frac12$ in superoxides.

• Oxygen has a positive oxidation state in $\ce{OF_2}$ because fluorine, being more electronegative, will get an oxidation number of -1.

• In compounds like $\ce{H_2S_2O_8}$, the usual calculations suggest that the oxidation state of sulphur is $+7$.

• The highest possible oxidation number is known to be $6$. In such cases, we take the oxidation number of sulphur as $6$. The reason why there is a discrepancy is revealed by writing the structure. There is a peroxide bond in the compound where the oxidation number of oxygen is $-1$.