Question

The ionic radius of Na⁺ ion is 1.02 Å. The ionic radii (in Å) of Mg²⁺ and Al³⁺, respectively, are :

• A. 0.85 and 0.99
• B. 0.72 and 0.54
• C. 0.68 and 0.72
• D. 1.05 and 0.99

Hint:

For isoelectronic ions, the radius is inversely proportional to the atomic number $Z$.

Answer 1

The Correct Option is B

Step 1: Na⁺, Mg²⁺, and Al³⁺ are **isoelectronic species** (all have 10 electrons).
Step 2: For isoelectronic species, as the nuclear charge (atomic number $Z$) increases, the nucleus pulls electrons more strongly.
Step 3: $Z$ for Na=11, Mg=12, Al=13.
Step 4: Therefore, ionic radius decreases: Na⁺>Mg²⁺>Al³⁺. The only option following this trend with values significantly smaller than 1.02 is (B).