As you go across a period, radius shrinks because you are adding protons. The added positive charge increases pull on the electron shells.
As you go down a group, radius gets larger because you are increasing shells of electrons. This increases shielding of the nucleus' positive charge, so the electrons are not pulled in as much.
Explanation:
When we move down a group then there is increase in the size of elements because there occurs increase in atomic mass of the elements.
It means that there occurs increase in number of electrons due to which there will be increase in number of new shells. As a result, size of atom or element increases due to which there is increase in atomic radius of the element.
Therefore, we can conclude that increase in number of electrons makes the atomic radius change down a column of the periodic table.
