Usually, bond strength decreases as atoms get larger. However, Fluorine ($F_2$) is an anomaly.

• Weak Bond: The $F-F$ bond energy ($158.8\text{ kJ/mol}$) is unexpectedly low, even lower than $Cl-Cl$ and $Br-Br$.
• Reason: Fluorine atoms are so small that the lone pair-lone pair repulsions between the non-bonding electrons of the two atoms are extremely high. This significantly weakens the single covalent bond.
• Order of bond dissociation enthalpy: $C{l}_2>B{r}_2>F_2>I_2$.