why the halides of nitrogen are unstable and reactive except NF3.
It is conditioned by the size of fluoride anion which is similar to nitrogen one. Therefore nitrogen forms stable compound with fluorine. Taking into account the huge sizes of other halides, which increase from Cl to I, the stability of N-Hal bond decreases. Also an increase in size and number of electrons for elements in the group leads to weaker interaction between valence orbitals and nucleus. Moreover, valence electrons are partially distributed over entire atom, which also makes them less available for bond formation (so-called the inert lone pair effect). These factors makes NHal3 (Hal = Cl, Br, I) unstable and very reactive.