Use of Oxidation Number

Understanding Oxidation Numbers

The concept of an oxidation number or oxidation state helps us understand how electrons are transferred in redox reactions.
In simple terms, the oxidation number of an atom in a molecule or ion is the charge it would have if all the bonding electrons were assigned to the most electronegative atom.
Oxidation numbers can be positive, negative, or zero, and they are shown with a plus or minus sign.

Assigning Oxidation Numbers

In uncombined elements, regardless of whether they exist as monatomic or polyatomic species, the oxidation number is always zero.
For simple ions, the oxidation number is equal to the ion’s charge.
Hydrogen’s oxidation number is usually +1, except in metal hydrides such as NaH where it is -1.
Oxygen’s oxidation number is usually -2, but there are exceptions like in peroxide compounds (H2O2, Na2O2), where it is -1.
Halogens usually have an oxidation number of -1, except when they are more electronegative than other atoms in a molecule.

Balancing Redox Reactions by Oxidation Numbers

Oxidation and reduction reactions can be balanced using oxidation numbers, as both must occur simultaneously in a redox reaction.
Reduction is a decrease in oxidation number, while oxidation is an increase in oxidation number.
In a balanced chemical equation, the total increase in oxidation numbers of the oxidised species must equal the total decrease in oxidation numbers of the reduced species.

Applying Oxidation Numbers

Oxidation numbers are instrumental in determining the direction of redox reactions and in the electrochemical series.
They help identify oxidising and reducing agents, the substances that lose or gain electrons in reactions, respectively.
An oxidising agent causes other substances to be oxidised by being reduced itself, whereas a reducing agent causes other substances to be reduced by being oxidised itself.