Answer
F < S < Si < Ge < Ca < Rb
Work Step by Step
Atomic radius increases from right to left. F is located at the right of all the above elements. Then S is located at the right of Si, hence it has a smaller radius. Ge is located below Si, hence it has a greater radius than Si. Ca is located to the left of Ge, hence it has a larger atomic radius than Ge. Rb is located below the Ca, so it has a greater radius than Ca.
