Define variable valency.

Hint: Valency is the measure of the combining capacity of atoms or molecules. Elements that are placed in the same group in a periodic table have the same valency. Valency can also be defined as the charge which is present on an atom if the compound was ionic.

Complete step by step answer:
Valency is the measure of the combining capacity of atoms or molecules. That is it is the measure of the number of ions or molecules with which an atom can join. Valency can also be defined as the charge which is present on an atom if the compound was ionic. Valency of an element can be both positive and negative. If the number of valence shell electrons is between zero and four then the atom will tend to lose electrons and will have positive valency similarly if valence shell electrons are between $4 - 8$ then the atom will tend to gain electrons and will have negative valency.
Some elements have different capacities to combine with different elements. This depends on the nature of reaction. This property of having different capacity to combine with different elements is called variable valency. For example in ferrous oxide valency of iron is $ + 2$, whereas in ferric oxide valency of iron is $ + 3$. Like iron there are some more elements that show this property. These elements are copper, mercury, silver, gold, tin, manganese, platinum, sulphur, lead and phosphorus. All these elements have a tendency to show variable valency.

Note:
Main reason for elements to show variable valency is the inert pair effect in p-block elements and the second reason is the small energy difference between outermost shell and second outermost shell in transition elements. For inner transition elements the reason is small energy difference between outermost shell and third outermost shell.