The regulation of mineral ions in the cellular fluids of biological systems is performed by ion pumps that selectively transport ions of a specific size (ionic radius) and charge across cellular membranes.  Which of the following isoelectronic species is the smallest ion?A.Na+B.F−C.Mg2+D.O2−

The size of an isoelectronic species is determined by the effective nuclear charge, which is the net positive charge experienced by an electron in a multi-electron atom. The more protons there are in the nucleus, the greater the effective nuclear charge, and the smaller the ion will be because the electrons are drawn more closely to the nucleus.

Here are the atomic numbers (number of protons) for each species:

A. Na+ (Sodium ion) - Atomic number 11, but it has lost one electron, so it has 10 electrons. B. F− (Fluoride ion) - Atomic number 9, but it has gained one electron, so it has 10 electrons. C. Mg2+ (Magnesium ion) - Atomic number 12, but it has lost two electrons, so it has 10 electrons. D. O2− (Oxide ion) - Atomic number 8, but it has gained two electrons, so it has 10 electrons.

All these species are isoelectronic because they all have 10 electrons. However, the Mg2+ ion has the most protons (12), so it has the highest effective nuclear charge. Therefore, Mg2+ is the smallest ion among the given options. So, the answer is C. Mg2+.