Incorrect order of radius is : — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Step 1: Analyze options (a) and (b), which consist of isoelectronic species (36 electrons) where size increases as nuclear charge decreases, and (c), where size decreases with increasing oxidation state. Step 2: In option (d), Ba^2+ and Cs^+ belong to the 6th period (having a Xenon core) while Se^2- and As^3- belong to the 4th period (having a Krypton core). Due to the extra shells of Ba^2+ (~135 pm) and Cs^+ (~167 pm), they do not follow a simple isoelectronic sequence with 4th-period anions. Step 3: The order Ba^2+ < Cs^+ < Se^2- < As^3- is incorrect because Ba^2+ and Cs^+ are from a much higher period with larger base shells.