$NF_3$ is stable. But, $NCl_3$ is explosive. Identify the reason:
A) $NCl_3$ undergoes hydrolysis whereas $NF_3$ does not.
B) $NCl_3$ decomposes on heating and $NF_3$ does not.
C) $NCl_3$ is a gaseous substance $NF_3$ is liquid.
D) $NCl_3$ undergoes sublimation while $NF_3$ does not.

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.

Complete step by step answer:

$NF_3$ is stable. But, $NCl_3$ is explosive because $NCl_3$ decomposes on heating and $NF_3$ does not. Nitrogen is smaller in size. For this reason, it can accommodate only atoms that are smaller in size, i.e., fluorine being smaller in size it can form a better bond with nitrogen. But, as chlorine is larger in size, it forms an unstable compound with nitrogen. Secondly, the better overlapping between nitrogen and fluorine bonds makes $NF_3$ a stable compound as they both belong to the same period due to which the same shell is involved in the overlap.

Additional information: Again, the oxidation state of chlorine in $NCl_3$ is +1 and that of nitrogen is -3. And, the oxidation state of fluorine is -1 and that of nitrogen is +3 in the case of $NF_3$. As halogen, in its +1 oxidation state, is a good oxidizing agent it can oxidize the nitrogen to the highly stable $N_2$ and get itself reduced to $Cl_2$. Therefore, it makes $NCl_3$ highly explosive.

Note:
We should always remember that the trihalides are always unstable due to its large size of the halogen atom. This size difference between nitrogen and a halogen atom gets larger and for this reason, the effective bonding gets weaker.