which element has an ion that is smaller than an ion of calcium?\npotassium\nstrontium\ncesium\nmagnesium

Brief Explanations:

Ionic radius trends: For ions with the same charge, ionic radius increases down a group. Calcium ($Ca^{2 +}$) and magnesium ($Mg^{2+}$) are in the same group (Group 2 - alkaline - earth metals), and magnesium is above calcium. Smaller - sized atoms in the same group form smaller - sized ions. Potassium ($K^+$) is in Group 1, strontium ($Sr^{2+}$) and cesium ($Cs^+$) are below calcium in their respective groups, so their ions are larger than calcium ion.

Answer:

magnesium