Properties of halogen atoms

Some important properties of the halogens appear in Table 6.2. Beneath the electronic configurations come the covalent radii of the bound atoms and the radii of the negative ions, X both increase down the Group. Then come the first ionization energies and electronegativities, which both decrease down the Group. 

What kind of interaction explains why for Equation 6.13 increases from chlorine to iodine  

The induced dipole-induced dipole interaction (Section 4.5) this increases as the molecules become larger and more polarizable. 

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