Bone energy is highest in the molecules

$$\begin{gathered} \text{ Bond dissociation energy in }{\mathrm{KJmol}}^{-1}1 \\ \begin{array}{rl} & {\mathrm{F}}_2-158\\ & {\mathrm{Cl}}_2-244\\ & {\mathrm{Br}}_2-193\\ & {\mathrm{I}}_2-151\end{array} \\ \text{Hence, in order of decreasing bond strength:} \\ {\mathrm{Cl}}_2>{\mathrm{Br}}_2>{\mathrm{F}}_2>{\mathrm{I}}_2 \\ \text{Fluorine is an anomaly.} \end{gathered}$$

Bond strength decreases from chlorine to iodine as down the group, the atomic size becomes larger and thus the valence electron orbitals become more diffused, causing the overlap of orbitals to become less effective. Therefore the halogen-halogen bond becomes weaker from chlorine to iodine.Fluorine is an exception due to its extremely small size. The F-F bond length is so short that the lone pairs of electrons on the fluorine atoms repel each other and weakens the F-F bond.