What atom has the largest atomic radius

Caesium

down the group atomic radius increases and along period wise atomic radius decreases

Francium.

Explanation:

Atomic radii vary in a predictable way across the periodic table. As can be see. in the figures below the atomic radius increases from top to bottom

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.