Which is the correct order of decreasing bond dissociation enthalpy?

The correct option is Cl₂ > Br₂ > F₂ > I₂.

To understand the order of decreasing bond dissociation enthalpy of halogen molecules, let us analyze the factors influencing bond strength in halogens:

1. Atomic Size and Bond Length:
As we move down the halogen group (Group 17) from fluorine to iodine, the atomic size increases. Consequently, the bond length increases in the order:
F₂ < Cl₂ < Br₂ < I₂
Generally, a shorter bond length results in a stronger bond and a higher bond dissociation enthalpy. Based on this factor alone, one would expect the bond dissociation enthalpy to decrease in the order F₂ > Cl₂ > Br₂ > I₂.

2. Lone Pair-Lone Pair Repulsion (The Exception of Fluorine):
Fluorine (F₂) has an exceptionally small atomic size. The F-F bond is very short, which brings the non-bonding valence electron pairs (lone pairs) on the two fluorine atoms extremely close to each other.
This close proximity causes strong interelectronic repulsion (lone pair-lone pair repulsion) between the valence shell electrons. This repulsion weakens the F-F covalent bond significantly, making it much easier to break than expected.

3. Relative Repulsion in Other Halogens:
For chlorine (Cl₂) and bromine (Br₂), the atomic sizes are larger, which keeps the lone pairs further apart, minimizing this repulsion. Thus, Cl₂ has the highest bond dissociation enthalpy, followed by Br₂.
For iodine (I₂), although the lone pair repulsion is minimal, the atomic size is so large and the bond length is so long that the covalent overlap is very weak, resulting in the lowest bond dissociation enthalpy among the four halogens.

4. Final Combined Order:
Due to the anomalous weakness of the F-F bond caused by lone pair repulsions, the bond dissociation enthalpy of F₂ drops below that of Cl₂ and Br₂, but remains higher than that of I₂.
The values of bond dissociation enthalpies are approximately:
Cl₂: 242.6 kJ/mol
Br₂: 192.8 kJ/mol
F₂: 158.8 kJ/mol
I₂: 151.1 kJ/mol
This yields the final decreasing order:
Cl₂ > Br₂ > F₂ > I₂