The regulation of mineral ions in the cellular fluids of biological systems is performed by ion pumps that selectively transport ions of specific size and charge across cellular membranes. Which of the following isoelectronic species is the smallest ion?

Question

The regulation of mineral ions in the cellular fluids of biological systems is performed by ion pumps that selectively transport ions of specific size and charge across cellular membranes.  Which of the following isoelectronic species is the smallest ion?

Quizplus · Accepted Answer

11ecba2d_11c4_fe09_a3bf_157817706405_MD0008_00 Ionic radii tend to decrease in size across a period (row) of the periodic table (left to right) and increase moving down a group (column). This trend occurs for metal cations, and then resets and repeats for anions beginning near the division between metals and nonmetals, past which anions tend to preferentially form. Compared to the neutral atom of a given element, its cation will be smaller but its anion will be larger. Losing electrons to form a cation causes the remaining electrons to experience a greater effective nuclear charge (Z_eff), pulling the electrons closer to the nucleus. Conversely, gaining electrons to form an anion produces greater electronic repulsion and nuclear shielding (lesser Z_eff), which pushes electrons farther from the nucleus. Na⁺, F⁻, Mg²⁺, and O²⁻ ions are isoelectronic (have the same number of electrons), but because the number of protons is different in each ion, the electrons in each ion experience a different Z_eff. Therefore, in an isoelectronic series, ionic radii decrease with increasing atomic number. Because magnesium has the highest atomic number (greatest number of protons) in the isoelectronic series, Z_eff is greatest in this ion, making it the smallest within the given series. (Choice A) Cations are generally smaller than neutral atoms and anions in an isoelectronic series, but Na⁺ is not the smallest because it has one less proton than Mg²⁺ (lesser Z_eff to pull the electrons toward the nucleus). (Choices B and D) Anions in an isoelectronic series are generally larger than neutral atoms and cations in the series. Educational objective: Ionic radii decrease in size across a period and increase down a group on the periodic table. The use of isoelectronic series is helpful in comparing cations, anions, and neutral atoms.

The Regulation of Mineral Ions in the Cellular Fluids of Biological