Final answer:
As you go down a column of the periodic table, the atomic radii increase because the valence electron shell is getting larger and there is a larger principal quantum number.
Step-by-step explanation:
As you go down a column of the periodic table, the atomic radii increase. This is because the valence electron shell is getting larger and there is a larger principal quantum number, so the valence shell lies physically farther away from the nucleus. This trend can be summarized as follows: