NF3 IS AN EXOTHERMIC COMPOUND WHEREAS NCl3 IS NOT WHY???
As we move down the group 17, the size of the atom increases from fluorine to chlorine. The larger difference in the size of N and Cl results in the weakness of strength of N – Cl bond. On the other hand, the difference in size of N and F is small; consequently the N – F bond is quite strong. As a result, NF3 is an exothermic compound.