What is variable valency? give an example

• The combining capacity of an element is called valency.
• Atoms tend to achieve a stable electronic configuration or to have a complete octet in their outermost shell.
• So the number of electrons lost or gained by the atom of an element to complete Its octet is called the valency of an element.
• Valency of an element depends on the number of valence electrons.
• For the elements having valence electrons 1,2, or 3 valency equals to the number of valence electrons.
• For the elements having valence electrons 4,5,6, or 7 valency equals to the number of electrons added to complete its octet.
• Some of the elements exhibit more than one valency, which is called variable valency.

Example

• Atomic number of elemental $\mathrm{Phosphorous}\left(\mathrm{P}\right)$ = 15
• Electronic configuration = $$\begin{gathered} \mathrm{K}\mathrm{L}\mathrm{M} \\ 285 \end{gathered}$$
• In order to complete the octet, one $\mathrm{Phosphorous}\left(\mathrm{P}\right)$ can take 3 electrons from another atom.
• It also extends its valency to 5 , by losing 5 electrons to achieve octet configuration.