Explain in terms of atomic structure why the radius of a cesium ion in cesium chloride is smaller than the radius of a cesium atom when both are in the ground state

Cesium is further down on the periodic table.

Cesium has more electron shells than Rubidium

Cesium has more protons than Rb.

Cs has more valence shells than Rb

It has fewer electron shells

Explanation:

Cesium is a metal and metals tend to lose their valence electrons to become positively charged cations. Loss of valence electrons in a metal also results in the loss of the total valence shell. Therefore, when cesium loses its valence electron, it also loses the whole outer shell which consequently leads to a decrease in radius.

Therefore, we may conclude that for a fewer electron shells, we would expect to have a smaller radius.