JEE Main 2023 · 29 January · morning · Single correct · +4 / −1
The bond dissociation energy is highest for
- (a) Cl₂
- (b) I₂
- (c) Br₂
- (d) F₂
Answer and solution
Answer (a) Cl₂
Solution
Chemistry ma’am · 3 steps
Down the group the bond gets weaker, but fluorine breaks the pattern, so chlorine comes out on top.
- Expected trend. Going down group 17, the atoms get bigger and the bond gets longer. Orbital overlap gets poorer, so bond dissociation energy should fall from F₂ to I₂.
- Fluorine anomaly. The F atom is very small, so the F-F bond is very short. The lone pairs on the two F atoms sit close together and repel strongly, which weakens the bond a lot.
Molecule Bond dissociation energy (approx.) Reason F₂ 158 kJ mol⁻¹ lone pair repulsion in a very short bond Cl₂ 242 kJ mol⁻¹ good overlap, little repulsion Br₂ 193 kJ mol⁻¹ larger atoms, longer bond I₂ 151 kJ mol⁻¹ largest atoms, weakest overlap - So. The order is Cl₂ > Br₂ > F₂ > I₂, so Cl₂ has the highest bond dissociation energy. The answer is (a).
The idea to keep. For halogens, remember Cl₂ > Br₂ > F₂ > I₂. Fluorine is too small: lone pair repulsion in its short bond makes F-F unusually weak.
Answer from the NTA final answer key. Solution by PrepWiser.