Chemistry · Chemical Bonding · NEET
The correct order is Cl2 > Br2 > F2 > I2. Cl2 has the strongest bond, I2 the weakest, and F2 does NOT sit at the top even though F is the smallest atom. This exact order was asked in NEET 2016.
Fluorine is a very small atom. Its three lone pairs (on each F) are held very close together in the tiny F2 molecule. These lone pairs repel each other strongly. This lone pair-lone pair repulsion weakens the F-F bond, so its bond enthalpy drops below Cl2 and Br2. That is why F2 is called anomalous.
That is the trap. Normally a smaller atom means a shorter, stronger bond. But in F2 the atoms are SO close that the lone pairs on the two atoms clash badly. This repulsion cancels the size advantage, so F2 ends up weaker than Cl2 and Br2. Size alone does not decide it here.
Iodine is the largest halogen. Its bonding electrons are far from the nuclei, so the I-I bond is long and weak. Long bonds are easy to break, so I2 has the smallest bond dissociation enthalpy of the four.
For a diatomic molecule like F2, Cl2, Br2 or I2, yes. Bond dissociation enthalpy is the energy needed to break ONE specific bond in a gaseous molecule. Since these halogens have only one bond, their bond dissociation enthalpy equals their bond enthalpy.
Remember two facts: (1) going down the group Cl2 > Br2 > I2 (bigger atom = weaker bond), and (2) F2 is the odd one that drops in BELOW Br2 due to lone pair repulsion. Put F2 between Br2 and I2. Final: Cl2 > Br2 > F2 > I2.
Which one of the following orders is correct for the bond dissociation enthalpy of halogen molecules?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Cl2 > Br2 > F2 > I2. Chlorine has the strongest bond; iodine the weakest; fluorine sits out of place due to lone pair repulsion.
Chlorine (Cl2) has the maximum bond dissociation enthalpy among the halogens, roughly 242 kJ/mol.
Because fluorine atoms are very small, the lone pairs on the two F atoms are packed close together and repel each other strongly, weakening the F-F bond.
As you go down the group the atomic size increases, the bond becomes longer, the shared electrons are held less tightly, so the bond gets weaker and breaks more easily.
Yes. NEET 2016 asked the exact order directly, and related C-X and H-X bond enthalpy orders were asked in NEET 2021, so it is a repeatedly tested idea.
