Which of the following is arranged in decreasing order of size? — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

Step 1: Determine that Mg^2+, Al^3+, and O^2- are isoelectronic species, with each possessing exactly 10 electrons. Step 2: For isoelectronic species, the ionic radius decreases as the atomic number (nuclear charge) increases, because a stronger nucleus pulls the same number of electrons more tightly. Step 3: The atomic numbers are O (8), Mg (12), and Al (13). Therefore, the correct decreasing order of size is O^2- > Mg^2+ > Al^3+.