Question

Pcl5 is known but ncl5 is not known.Why?

Answer 1

NCl5 will make Nitrogen to have 10 electrons in its outermost shell which is not possible as it doesn't have any d orbitals

Answer 2

Nitrogen is a 2nd period element which does not have vacant d-orbital whereas Phosphorus is the element of 3rd period which has vacant d-orbital. Hence Phosphorus can form bonds with 5 Chlorine atoms but Nitrogen can't.

hope this helped