Which of the following ions has the smallest radius among isoelectronic species with 18 electrons?
For isoelectronic species (K+, Ca²⁺, Sc³⁺, all with 18 electrons), radius decreases with increasing nuclear charge. Sc³⁺ (Z=21) has the smallest radius.
Ref: NCERT Class 11 Chemistry > Chapter 3: Classification of Elements and Periodicity in Properties > Topic: Electron Gain Enthalpy and Electronegativity and Valency