Final answer:
As you move down a group/family of the periodic table, the atomic radius increases 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 group/family of the periodic table, the atomic radius increases. 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. For example, as you move from lithium (Li) to sodium (Na) to potassium (K) in Group 1, the atomic radius increases because each successive element has an additional principal energy level.