Classification of Elements and Periodicity in Properties — Chemistry STD 11 Science — Question

On moving down the halogen group, the atomic radius, ionic radius and the boiling point increases with an increase in the atomic number. However, ionisation potential decreases. The effect of an increase in the atomic size and the shielding effect is much more than the effect of the increase in nuclear charge.
Hence, the electron becomes less and less firmly held to the nucleus on moving down the group. Hence, the ionization potential gradually decreases on moving down the group.