Q. What is Oxidation Numbers?

Ans.

Chemists find it useful to describe the oxidation states of an element by means of oxidation numbers. The oxidation number of an element is defined according to the following rules:

• The oxidation number of a free element is zero.

For example, the oxidation numbers for the free elements lithium and sulfur are both zero.

• The oxidation number of the more electropositive element in a compound is positive, and that of the more electronegative element is negative.

For example, consider sodium chloride, NaCl. The more electropositive element is sodium, and the more electronegative element is chlorine.

• The oxidation number for sodium is +1 and the oxidation number for chlorine is -1.

In their compounds, the oxidation number of oxygen is -2 and the oxidation number for hydrogen is +1 .

(Exceptions: in peroxides the oxidation number of oxygen is = -1, and in metal hydrides the oxidation number of hydrogen is -1)

• The oxidation number of a monatomic ion is equal to its ionic charge.

For example, consider magnesium chloride, MgCl₂. The ionic charge of magnesium is +2; therefore, the oxidation number of magnesium is also +2. Similarly, the ionic charge of chlorine is -1; therefore, the oxidation number of chlorine is also -1.
The sum of the oxidation numbers of all the atoms:

-in a neutral compound is zero.

-in a polyatomic ion is equal to the net charge of the ion.

For example, as water, H₂O, is a neutral compound, the sum of the oxidation numbers of the hydrogen and oxygen atoms comprising it must add up to zero.

However, the polyatomic ion permanganate, MnO4^-, has a net charge of -1. Therefore, the sum of the oxidation numbers of the individual atoms of magnesium and oxygen in this ion must add up to -1.