JEE Main 2022 · 24 June · morning · Single correct · +4 / −1
The most stable trihalide of nitrogen is :
- (a) NF₃
- (b) NCl₃
- (c) NBr₃
- (d) NI₃
Answer and solution
Answer (a) NF₃
Solution
Chemistry ma’am · 3 steps
Nitrogen is a tiny atom, so only the tiny fluorine atom can bond to it strongly and sit around it comfortably.
- Size match. Nitrogen is very small. Fluorine is close to it in size, so the N–F overlap is good and the bond is short and strong.
- Bigger halogens. Going down to Cl, Br and I, the halogen gets much larger than nitrogen. Overlap gets poorer and the N–X bond gets weaker. Three large atoms are also crowded around one small nitrogen, which adds strain.
Trihalide N–X bond Behaviour NF₃ strongest stable gas, formation is exothermic NCl₃ weaker explosive oily liquid, formation is endothermic NBr₃ weaker still unstable, exists only at very low temperature NI₃ weakest explodes on touch (as its ammonia adduct) - So. Stability falls in the order NF₃ > NCl₃ > NBr₃ > NI₃. The most stable trihalide is NF₃, option (a).
The idea to keep. For nitrogen trihalides, stability follows N–X bond strength. A small nitrogen bonds best to a small halogen, so stability falls from NF₃ to NI₃.
Answer from the NTA final answer key. Solution by PrepWiser.