How does atomic size change down a periodic-table group?

Atomic size increases as you move down a group in the periodic table.

This trend occurs because, with each successive period, an additional layer of electrons is added to the atom. You can visualize this as adding a new layer to an atomic "onion," where electrons occupy a new shell outside the existing closed-shell core. Because these new shells are further from the nucleus, the overall atomic radius expands.12