Question

Fill in the blanks
Valency of phosphorus in $PC{{l}_{3}}$ is ……….. , in $PC{{l}_{5}}$ is ………….

Answer 1

Hint: Valency of any substance is its ability to combine by sharing, donating or gaining electrons. It is determined by the outer electronic configuration, also the presence of other atoms in a molecule define the valency of a particular atom. Phosphorus is a compound of group 15, its bonding with other atoms can tell its valency in a molecule.

Complete answer:
Valency of any atom is the combining ability of that atom that results in the formation of bonds that are able to hold atoms in a molecule. The valency of any atom can be identified through its valence electronic configuration. Some atoms have variable valences due to the presence of vacant d – orbital in their electronic shell.
We have to find the valency of phosphorus in $PC{{l}_{3}}$ and $PC{{l}_{5}}$. The electronic configuration of phosphorus is $[Ne]3{{s}^{2}}3{{p}^{3}}$ this means it has a total of 5 electrons in the outer shell. There are 3 valence electrons in its outer orbital that gives it a group valence of 3. So, in $PC{{l}_{3}}$there are 3 Cl atoms that require 1 electron each to complete their octet which is provided by the 3 electrons in the 3p orbital of phosphorus, so valency of 3 in$PC{{l}_{3}}$.
Now, in $PC{{l}_{5}}$ there are 5 Cl atoms that require 5 electrons to complete stable configuration. The outer shell of phosphorus has 5 electrons, but 3 are unpaired of p orbital so, they readily form bonds, while 2 electrons of 3s orbital are paired. These electrons become unpaired and move to the vacant 3d orbital of phosphorus, so phosphorus is able to expand its valency to 5 in $PC{{l}_{5}}$ due to the vacant d orbital.
Hence, Valency of phosphorus in $PC{{l}_{3}}$ is 3, in $PC{{l}_{5}}$ is 5.

Note:
Chlorine is of group 17, it has 7 electrons in its outer shell, so it requires only 1 electron to complete its stable electronic configuration, so it has a valency of 1. Nitrogen of the group 15 is not able to have 5 valence as it does not have a vacant d orbital, but phosphorus of the same group can have 5 valence due to the vacant d orbital.