Which ion has the smallest ionic radius?

The ion with the smallest ionic radius is Ca2+. When considering ionic radii, it is important to remember that ionic radius is affected by the charge of the ion and the number of electron shells.

Calcium (Ca) typically has two electrons in its outermost shell, and when it loses these two electrons to form Ca2+, it results in a smaller radius compared to its neutral atom. This is because removing electrons reduces electron-electron repulsion and allows the remaining electrons to be drawn closer to the nucleus. Furthermore, Ca2+ has a higher charge (+2) than Na+ (which has a +1 charge) and K+ (also a +1 charge). The increased positive charge means that the effective nuclear charge felt by the electrons is greater, pulling the electrons in even closer and resulting in a smaller ionic radius.

Magnesium (Mg2+) also has a +2 charge, similar to calcium, but calcium has a larger atomic number and more electron shells, allowing for greater nuclear charge and a more significant contraction of the electron cloud upon ionization. Therefore, while both Ca2+ and Mg2+ are cations with +2 charges, Ca2+ ends up being comparatively smaller because it is more effective in pulling its