Hint:Nitrogen is smaller in size. For this reason, it can accommodate only atoms that are smaller in size. trihalides are always unstable due to its large size of the halogen atom.

Why is NBr3 not stable?

The unstable nature of NCl3, NBr3 and NI3 is due to low polarity of NX bond and a large difference in the size of nitrogen and halogen atom.

Why nitrogen halides are explosive?

Nitrogen and chlorine have almost the same electronegativity, and NCl₃ reacts as if it were covalent. It has a heat of formation of +232 kJ mol⁻¹ and is a violent explosive. Nitrogen trifluoride is a compound that has interesting chemical properties.