In general, atomic radius decreases across a period and increases down a group. ... Down a group, the number of energy levels (n) increases, so there is a greater distance between the nucleus and the outermost orbital. This results in a larger atomic radius.

Which atomic radius is largest?

Atomic radii vary in a predictable way across the periodic table. As can be seen in the figures below, the atomic radius increases from top to bottom in a group, and decreases from left to right across a period. Thus, helium is the smallest element, and francium is the largest.

Which element has the largest radius and why?

Francium has the largest, Helium has the lowest. Atomic radius increases as you go to the left and downward due to the attraction of electrons and the nucleus in an atom.