As atomic size increases down a group, what happens to the ionization energy?

As atomic size increases down a group, the ionization energy decreases due to increased distance between the nucleus and the outermost electrons, as well as increased electron shielding.

In larger atoms, the outer electrons are further away from the nucleus, meaning that the attractive force between the positively charged nucleus and the negatively charged electrons is weaker. This reduced attraction makes it easier to remove an electron from the outer shell, resulting in lower ionization energy. Additionally, as more electron shells are added down the group, there are more inner electrons that shield the outer electrons from the full effect of the nuclear charge. This shielding effect further reduces the effective nuclear charge felt by the outer electrons, contributing to the decrease in ionization energy.

Overall, the combination of increased distance from the nucleus and increased shielding leads to a significant drop in the energy required to remove an outer electron as you move down a group in the periodic table.