Final answer:
Ionization energy decreases within a group on the periodic table as you go down the group due to the increasing size of the atoms and weaker attraction from the nucleus.
Step-by-step explanation:
Within a group on the periodic table, the ionization energy decreases as you go down the group. This is because the size of the atoms increases down the group, meaning the outermost electron is farther from the nucleus and experiences weaker attraction. As a result, it is easier to remove the outermost electron, leading to a lower ionization energy.