Which of the following has the smallest ionic radius?
For isoelectronic species (O²⁻, F⁻, Na+, Mg²⁺, all with 10 electrons), radius decreases with increasing nuclear charge. Mg²⁺ (Z=12) has the smallest radius.
Ref: NCERT Class 11 Chemistry > Chapter 3: Classification of Elements and Periodicity in Properties > Topic: Electronic Configuration and Types of Elements - s p d f Blocks