Which of the following ions has the largest ionic radius?

• A. S^{2-}
• B. Ca^{2+}
• C. Cl^{-}
• D. K^{+}

Answer: (D)

Verified step by step guidance

1. Step 1: Understand that ionic radius depends on the number of electrons and the effective nuclear charge experienced by those electrons. Ions with more electrons or less positive charge generally have larger radii.
2. Step 2: Identify the electron configurations of each ion. For example, S^{2-}, Cl^{-}, Ca^{2+}, and K^{+} all have the same number of electrons as the noble gas argon (18 electrons), but different nuclear charges.
3. Step 3: Compare the nuclear charges (number of protons) for each ion: S has 16 protons, Cl has 17, Ca has 20, and K has 19. The higher the nuclear charge, the stronger the pull on electrons, resulting in a smaller ionic radius.
4. Step 4: Since all ions have the same electron configuration, the ionic radius decreases as the nuclear charge increases. Therefore, the ion with the smallest nuclear charge will have the largest ionic radius.
5. Step 5: Conclude that among the ions listed, S^{2-} has the smallest nuclear charge and thus the largest ionic radius, while Ca^{2+} has the largest nuclear charge and the smallest radius. The ion K^{+} has a nuclear charge between Ca^{2+} and Cl^{-}, so its radius is larger than Ca^{2+} but smaller than S^{2-}.