Answer:
D. magnesium
Step-by-step explanation:
The chemical element that has an ion that is smaller than an ion of calcium is magnesium.
In Chemistry, the parent atom of an element is larger than the cations and as such the Ca atom is larger than the Ca2+ ion. Also, in the group 2A of the periodic table the chemical element magnesium is found above the chemical element calcium, thus, magnesium ion is smaller than the calcium ion (calcium ion is larger than the magnesium ion).
Therefore, Ca2+ is larger than Mg2+ (Ca2+ > Mg2+).
Additionally, ionic radius can be defined as the between the nucleus and the valence electrons found in the outermost shell of an ion of a chemical element. In the periodic table, the ionic radius increases down a group and as such calcium ion is larger than the magnesium ion.