The species listed (Ca²⁺, K⁺, Ar, S²⁻, Cl⁻) are isoelectronic, meaning they all possess the same number of electrons (18 electrons).

1. Concept: For isoelectronic species, the radius is inversely proportional to the effective nuclear charge (atomic number, Z). As the number of protons increases, the attraction on the same number of electrons increases, pulling the electron cloud closer and reducing the size.
2. Analysis of Atomic Numbers (Z): Sulphur (S): Z = 16 Chlorine (Cl): Z = 17 Argon (Ar): Z = 18 Potassium (K): Z = 19

• Calcium (Ca): Z = 20

3. Trend:

• Largest Radius: S²⁻ (Lowest Z, weakest attraction).
• Smallest Radius: Ca²⁺ (Highest Z, strongest attraction).

4. Order: Increasing size follows decreasing atomic number: Ca²⁺ < K⁺ < Ar < Cl⁻ < S²⁻.