Question

In case of nitrogen, NCl3 is possible but not NCl5 while in case of phosphorus, PCl3 as well as PCl5 are possible.

Answer 1

This is because phosphorus exhibits variable valency. It can lose electrons from the ultimate and penultimate shell. Nitrogen does not exhibit this. Basically phosphorus can lose 3 or 5 electrons whereas nitrogen only 3.

Answer 2

because nitrogen does not have facant orbital but phosphorous have ,so NCL5 is not possible but PCl5 is possible