Rubidium, cesium and francium all have one valence electron. which element has the smallest atomic radius

Rubidium

is the answer

Answer: - Rubidium has smallest atomic radius.

Explanations: - Rubidium, Cesium and Francium all are the elements of first group and so they all have one valence electron. Atomic size increases as we move down in a group since a new shell is being added every time as we move down from one period to another.

Rubidium is above cesium and francium and so it's size is smallest.

If we compare the elements of same period then on moving left to right the electrons are keeps added to the same shell but the number of protons increases that increases the nuclear pull for the electrons and so the atomic size decreases.