(a) `PF_5` is known, but `NF_5` is not. Why ?
(b) The experimentally determined `N-F` bond length in `NF_3` is greater than the sum of single convalent radii of `N` and `F`.

Explanation

(a) Phosphorous has vacant d-orbitals in its valence, hence `P` can have covalency of five and `PF_5` exists. But, `N` due to absence of d-orbitals in its valence shell cannot have cavalency of five, hence `NF_5` is not known.
(b) Due to their smaller size and high electron density, experimentally determined `N-F` bond length is high, because of repulsion of bonded pair of both `N` and `F` atoms.