Which element has an ion that is smaller than an ion of calcium? potassium strontium cesium magnesium

Ionic radius is the radius of an atom's ion. Although neither atoms nor ions have sharp boundaries, they are sometimes treated as if they were hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice. So magnesium ion has smaller radius than calcium ion

The correct answer is magnesium ion.  

The radius of the atom's ion is known as the ionic radius. Though, neither ions nor atoms exhibit sharp boundaries, they are at certain occasions considered as if they were hard spheres with radii such that the total of ionic radii of the anion and the cation gives the distance between the ions in a crystal lattice. Thus, magnesium ion exhibits smaller radius than calcium ion.