What is variable valency? Give three examples of elements having variable valency.

Hint: Valency is equal to the number of valence electrons present in the outermost shell of an atom. Nearly all elements have a single valence but there are elements which have variable valency. For such elements the valency is not constant; it may vary from compound to compound.

Complete answer:
Valency can also be defined as the number of electrons donated or gained by an atom to achieve a noble state electronic configuration. Generally metals donate the electrons to other atoms from the valence shell. However, in some cases elements lose electrons from the shell next to the valence shell. Therefore in such cases they can have multiple valencies. The main reason behind the elements for showing variable valency is the inert pair effect and a small energy gap between the valence shell and the next shell. Such variable valency is shown by transition elements.
For example:
$\left( 1 \right)$Copper shows variable valency in cuprous $\left( {C{u_2}O} \right)$oxide and cupric oxide$\left( {CuO} \right)$. It shows $ + 1$ valency in cuprous oxide while it shows $ + 2$ in cupric oxide.
$\left( 2 \right)$Iron shows variable valency in ferrous oxide $\left( {FeO} \right)$and ferric oxide$\left( {F{e_2}{O_3}} \right)$. It shows $ + 2$ valency in ferrous oxide while it shows $ + 3$ in ferric oxide.
$\left( 3 \right)$ Similarly mercury shows variable valency in mercurous and mercuric oxide. It shows $ + 1$ valency in mercurous oxide while it shows $ + 2$ in mercuric oxide.

Note:
Transition elements generally show variable valency because they have a d- subshell. Also the energy difference between the valence shell and d-subshell is less. Inert pair effect is shown by p-block elements. These variable valencies are responsible for different chemical properties of these elements.