As you move down a group, you will recall that the radius increases. Why do you think an increase in atomic radius would result in a lower ionization energy?

Because as we move down the group size/ radius of atom increases due to increase in shells and screening effect. The hold of nucleus on valence electrons decreases and effective nuclear charge decreases so it is easy to remove electron from greater sized atoms as compared to smaller sized aroms.

Energy required to remove electron is ionization energy so greater sized atoms have low ionization energy.

Explanation:

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