Rules for Determining Oxidation Numbers

The oxidation number, which expresses the state of an element, ion or molecule, can be used to determine the type of reaction, its rate and the nature of the products that would be formed. Redox reactions can be recognized with the aid of oxidation numbers. Before we look at the rules governing the determination of oxidation numbers, we shall first of all define or explain what an oxidation number is.

What is Oxidation Number?:– The oxidation number of an element in any particular molecule or ion is defined as the electrical charge it appears to have as determined by a set of arbitrary rules. These arbitrary rules make it possible to calculate the oxidation numbers for the elements in the reactants and products of a chemical change. The loss or gain of electrons by elements in a chemical reaction leads to change in its oxidation number.

There is a set of rules that is used for the determination of the oxidation numbers of elements and ions. These rules are thus:

1. The oxidation number of all elements in the free state is zero.(free state means uncombined with any other element.) e.g Na, K, Cl, Mg, P

2. The oxidation number of a simple ion(an ion consisting of a single element) has the same size and sign as the charge of the ion. e.g Na⁺, Mg²⁺, Al³⁺, Cl^– and S^2- have oxidation number s of 1, 2, 3, -1, and -2 respectively.

Note that where the ion consists of more than one element, its oxidation number is the algebraic sum of the oxidation numbers of all the elements in the ion. This oxidation number must have the same size and sign as the overall charge on the ion. For instance, in the hydroxide ion, OH^–, the overall charge is -1, therefore, its oxidation number is also -1. Look at illustration below to see how it is obtained:

[Oxidation No. of O] + [Oxidation No. of H] = [Oxidation No. of OH]

            (-2)               +           (+1)                =             (-1)

The oxidation numbers of NH₄⁺, NO₃, SO₄^2-, PO₄^3- and MnO₄^– are +1, -1, -2, -3 and -1 respectively. It is possible to use this rule to calculate the oxidation number of any element in the ion so long as those of the other elements are known.

3. The algebraic sum of the oxidation numbers of all the elements in a compound is zero. For instance, the compound, MgCl₂ has oxidation number of zero calculated as follows:

[Oxidation No. of Mg] +2[Oxidation No. of Cl] = [Oxidation No. of MgCl₂] = 0

           (+2) +               [2 x (-1)]                       =          + 2- 2                     = 0

The more electronegative elements will have negative oxidation numbers and the more electropositive ones will have positive oxidation numbers. This is true not only of ionic compounds, but also of covalent molecules, where each atom present is given an oxidation number equal arbitrarily to the charge it could have if the substance were composed of ions.

It is important to note that in most compounds, the oxidation number of oxygen is -2, except in peroxides where its oxidation number is -1, while the oxidation number of hydrogen in most compounds is +1, except in hydrides where its oxidation number is -1.