How are the elements that exhibit variable valency named? Explain in detail.

Hint: Valency is defined as the combining capacity of atoms and molecules. The elements present in the same group show the same kind of valency. In the ionic compound, the valency is the charge on the atom. Valency can be positive or negative. According to the nature of the reaction, elements have different capacities to combine with other different elements. Therefore, in this case, the elements show variable valency. Hence, according to the valency of the atom in the compound, the name is given to it.

Complete answer:
We know that valency of any element is its combining capacity with other atoms and molecules. The elements present in the same group show the same kind of valency. Valency can be negative or positive.
According to the nature of the reaction, an element shows different valency when it combines with other different elements. So, in this case elements show variable valency.
If an element shows two different positive valencies, in this case, the suffix “ous” is used to show the lower valency and the suffix “ic” is used to show the higher valency.
For example: Iron (Fe) exhibits $ + 2$ and $ + 3$ valencies.
For the lower valency of atom, the suffix -OUS is added – Ferrous $\left( {F{e^{2 + }}} \right)$
For the higher valency, the suffix – IC is added – Ferric $\left( {F{e^{3 + }}} \right)$

Note:
Generally d-block elements or transition elements show variable valencies because they have low lying vacant d-orbitals. The reason for the elements to exhibit variable valency is known as the inert pair effect. The reason is the low energy difference between the outermost shell and inner shell.