Final answer:
Within a group on the periodic table, atomic radii increase as you move down the group due to the larger valence electron shell and principal quantum number.
Step-by-step explanation:
Within a group on the periodic table, the atomic radii increase as you move down the group. This can be explained by the fact that the valence electron shell is getting larger and there is a larger principal quantum number, so the valence shell lies farther away from the nucleus. The increase in the number of principal energy levels outweighs the increase in nuclear charge, resulting in an increase in atomic radius.