Valency of Phosphorus in ${\mathrm{PCl}}_5 \mathrm{is}$

Phosphorus is combined with 5 chlorine atoms. Hence, the valence of Phosphorus is 5.

The valency of phosphorus in PCl₅ (phosphorus pentachloride) is 5 because phosphorus forms five bonds with chlorine atoms in this molecule.

Phosphorus belongs to Group 15 of the periodic table and has 5 valence electrons in its outer shell. Normally, phosphorus shows a valency of 3 due to the pairing of its electrons. However, in PCl₅, phosphorus undergoes an excited state, where one of its paired electrons from the 3s orbital gets promoted to the empty 3d orbital. This allows phosphorus to have five unpaired electrons available for bonding, enabling it to form five covalent bonds with chlorine atoms.

Thus, the valency of phosphorus in PCl₅ is 5, reflecting the total number of bonds it forms with chlorine in this compound