Question

pf5 is known but Nf5 is not explain....

Answer 1

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.