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Why does the iodine-iodine bond have less strength than the chlorine-chlorine bond?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — halogen Modern version of the periodic table of the elements.

The iodine-iodine (I–I) bond is weaker than the chlorine-chlorine (Cl–Cl) bond primarily due to the nature of the atomic orbitals involved in the bonding.

In halogen molecules, atoms are held together by a sigma bond formed from the overlap of p orbitals. As you move down the periodic table, the atoms increase in size, and their p orbitals become more diffuse. In the case of iodine, these orbitals are significantly more diffuse than those in chlorine, which results in less effective orbital overlap and a consequently weaker bond.1

This difference is reflected in the bond dissociation energies: the energy required to break the Cl–Cl bond is approximately 242.58 kilojoules per mole, whereas the energy required to break the I–I bond is approximately 151.09 kilojoules per mole.2