Question
Question: Fill in the blanks Valency of phosphorus in \(PC{{l}_{3}}\) is ……….. , in \(PC{{l}_{5}}\) is …………....
Fill in the blanks
Valency of phosphorus in PCl3 is ……….. , in PCl5 is ………….
Solution
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 PCl3 and PCl5. The electronic configuration of phosphorus is [Ne]3s23p3 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 PCl3there 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 inPCl3.
Now, in PCl5 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 PCl5 due to the vacant d orbital.
Hence, Valency of phosphorus in PCl3 is 3, in PCl5 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.