Explain why:
Nitrogen does not form pentahalides

Hint: Nitrogen differs from the rest of the members in the Group 15 family due to its small size, High ionization energy, High electronegativity and non-availability of d orbitals.

Complete step by step answer:
Group 15 elements include Nitrogen, Phosphorous, Arsenic, Antimony, Bismuth. Nitrogen differs from the rest of the members in the Group 15 family due to its small size, High ionization energy, High electronegativity and non-availability of d orbitals.
Nitrogen has an electronic configuration of \[1{s^2}2{s^2}2{p^3}\] .

 It does not have d orbitals. While the other members of the family have d orbitals. The heavier elements of Group 15 are capable of forming pentahalides. Eg. \[Pc{l_5}\] , \[Sb{F_5}\] . But due to the absence of d orbitals in Nitrogen it can’t form pentahalide like \[NC{l_5}\] .
The maximum covalency of Nitrogen atoms is restricted to 4 (one s and three 2p). Only four orbitals are available for hybridization.

Additional information:
Due to non-availability of d orbitals in Nitrogen it can’t form dπ-dπ bonds. The heavier elements form dπ-dπ bonds. eg: R3P= or R3P= \[C{H_2}\] . Phosphorus and Arsenic form dπ-dπ bonds with transition metals. eg: \[P{\left( {C2H5} \right)_3}\] , As \[{\left( {C6H5} \right)_3}\] . Due to small size and high electronegativity Nitrogen is able to form pπ-pπ multiple bonds. The Only trihydride of Group 15 which has intermolecular hydrogen bonding is NH3.The boiling point of \[N{H_3}\] is higher than that of \[P{H_3}\] and \[As{H_3}\] .

Among the trihalides of Nitrogen, only \[N{F_3}\] is stable. The others are unstable and reactive (\[NC{l_3}\], \[NB{r_3}\] , \[N{I_3}\] ).

Note: The high ionization energy of Nitrogen is due to its stable half-filled electronic configuration. We know that ionization energy is the energy required to remove an electron from the valence shell of an atom. So, it is difficult to remove an electron from a stable configuration and it requires more energy.