How does atomic radius change from top to bottom in a group in the periodic table?
Going down a Group atomic radii INCREASE. Going across a Period, left to right, atomic radii decrease.
Ideas of modern electronic structure hold that electrons reside in shells orbiting a positive nucleus. Each shell can accommodate a given number of electrons, and when the shell is full, a new shell is commenced that is conceived to lie farther away from the nucleus. So going down a Group, the valence shell is farther from the nuclear core, and this gives rise to increased atomic radii.