Which of the following isoelectronic ion has the lowest ionization energy? — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Step 1: The ions K^+, Cl^-, Ca^2+, and S^2- are isoelectronic, each containing 18 electrons. Step 2: For isoelectronic species, the nuclear charge (number of protons) determines the strength of electron attraction: K (19 protons), Cl (17 protons), Ca (20 protons), S (16 protons). Step 3: Since S^2- has the lowest nuclear charge (16), its nucleus exerts the weakest pull on the electron cloud, resulting in the lowest ionization energy.