Question

PF5 is known but NF5 not known why??

Answer 1

Nitrogen and phosphorus both have 5 electrons in their outermost shell. So they need 3 electron to complete their octet. So NF3 and PF3 exist and both nitrogen and phosphorus show the covalency of 3. 

But Nitrogen does not have vacant d orbitals but phosphorus has empty 3d orbital. So it can accept more electrons and can increase its covalency to 5 to make PF5.