what is variable valency and write its cause??????? 

Normally metals donate electrons from their valence shell so as to form positively charged ions such that the charge on the ion is equal to its electropositive valency.

However, in transition elements, an atom loses electrons from the shell next to the valence shell (penultimate shell). In such a situation the element exhibits more than one valency (variable valency).

For example:

Iron has 2 electrons in its valence shell. On donating these electrons, its valency is 2⁺. But it can lose another electron from the shell next to the valence shell. The ion thus formed, has 3⁺ electropositive valency.

How to name ions with two different electropositive valency?

The suffix 'ous' is attached at the end of the name of the metal with lower valency.

Suffix 'ic' is attached at the end of the metal with higher valency.

Naming of ions

Deriving the chemical formula from valency

The chemical formula of a compound represents the composition of the molecule of the compound in terms of symbols of the elements present in it.