If a channel of a particular cell preferentially transmits smaller ionic species across the membrane, given the following biologically relevant, isoelectronic ions, the channel is LEAST likely to transmit:

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11ecba2d_11f7_7f62_a3bf_8bfa6d1a625a_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 for metal cations 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. The ions S²⁻, Cl⁻, K⁺, and Ca²⁺ are isoelectronic (have the same number of electrons); however, 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 as the atomic number increases. Because sulfur has the lowest atomic number (fewest number of protons) in this isoelectronic series, Z_eff is smallest in S²⁻, making it the largest ion. Given that the cell channel preferentially transmits smaller ions, S²⁻ is least likely to be transmitted. (Choice B) Anions in an isoelectronic series are generally larger than neutral atoms and cations in the same series, but Cl⁻ is not the largest because S²⁻ has one less proton (lesser Z_eff to pull the electrons toward the nucleus). (Choices C and D) Cations are generally smaller than anions and neutral atoms in an isoelectronic series. Educational objective: Ionic radii decrease in size across a period and increase down a group on the periodic table. This trend occurs for metal cations and then resets and repeats for anions beginning near the division between metals and nonmetals. The use of an isoelectronic series is helpful in comparing cations, anions, and neutral atoms.

If a Channel of a Particular Cell Preferentially Transmits Smaller