In general, bond enthalpy or bond dissociation energy of halogen molecules (X2) decreases the group from Cl₂ to I₂. Cl₂ > Br₂ > F₂ > I₂ The decreases in bond enthalpy from Cl₂ to I₂ are due to increases in atomic size that increase the distance between two. Hence, it becomes easier to break them apart. The exceptionally low bond dissociation energy of fluorine molecule is due to very small size of fluorine atom. This brings the non-bonding electrons on fluorine neater to each other resulting in much greater lone pair-lone pair repulsion which weakens the covalent bond and lowers its dissociat
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