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

Hello! The answer to your question is magnesium.

The radius of the atom's ion can be known as the ionic radius. Ions and atoms don't exhibit sharp boundaries, they are at certain points considered as being hard spheres with radii such that amount of ionic radii of the anion and the cation gives the distance between the ions in a crystal lattice. Then meaning that magnesium ion exhibits smaller radius than calcium ion.  

The element that has ion smaller than calcium ion is magnesium.

Explanation:

Atomic radius of an atom is defined as the total distance from the nucleus to the outermost shell of the atom.

From moving top to bottom, an extra shell is added around the nucleus and the outermost shell gets far away from the nucleus and hence, the distance between the nucleus and outermost shell increases. This increases the atomic radii of the atom.

From moving left to right in a period, more and more electrons get added up in the same shell and the attraction between the last electron and nucleus increases, which results in the shrinkage of size of an atom. This decreases the atomic radii of the atom.

For the given options:

Calcium is located in Group 2, Period 4 of the periodic table. So, the ion that must be smaller than the ion formed by calcium must be present to the right of this element in the same period or element lying in previous periods.

Potassium is located in Group 1, Period 4 of the periodic table.

Strontium is located in Group 2, Period 5 of the periodic table.

Cesium is located in Group 1, Period 6 of the periodic table.

Magnesium is located in Group 2, Period 3 of the periodic table.

Hence, the element that has ion smaller than calcium ion is magnesium.