Question

Nitrogen form Ncl3 but phosphorus from Pcl3 and Pcl5 . Why

Answer 1

Phosphorus has 15 electrons. So its electronic configuration is:

Phosphorus is a period 3 element. Hence it has an empty d-orbital which the period 2 elements do not have.

When phosphorus forms a compound, if there is enough energy available then one electron from one of the orbitals jumps to the vacant d-orbital. This gives phosphorus extra 2 valencies. This is called expanded octet.

As phosphorus initially has valency as 3 it forms PCl3, and due to this expanded octet, PCl5 can also be formed.

*Note that the expanded octet phenomenon can occur only in period 3 elements.

As Nitrogen lies in period 2, it does not have the empty d-orbital and so does not form NCl5

Good Luck :)

Answer 2

So its electronic configuration is:Phosphorus is a period 3 element. Hence it has an empty d-orbital which the period 2 elements do not have. ... As phosphorus initially has valency as 3 it forms PCl3, and due to this expanded octet, PCl5 can also beformed.