Which of the following species has the largest ionic radius

Analyze the question: We are asked to compare the ionic radii of four ions: S²⁻, Cl⁻, K⁺ and Ca²⁺.

Concept: These ions are isoelectronic. Each has 18 electrons, the same configuration as argon: 1s² 2s² 2p⁶ 3s² 3p⁶.

For isoelectronic species, the electrons are the same in number, so the size depends on the nuclear charge (number of protons):

• More protons pull the electron cloud in more strongly, so the radius is smaller.
• Fewer protons pull less strongly, so the radius is larger.

Step-by-step solution:

| Ion | Protons | Electrons | |---|---|---| | S²⁻ | 16 | 18 | | Cl⁻ | 17 | 18 | | K⁺ | 19 | 18 | | Ca²⁺ | 20 | 18 |

Order of nuclear charge: S (16) < Cl (17) < K (19) < Ca (20)

Order of ionic radius is the reverse: S²⁻ > Cl⁻ > K⁺ > Ca²⁺

(Approximate values: S²⁻ ≈ 184 pm, Cl⁻ ≈ 181 pm, K⁺ ≈ 138 pm, Ca²⁺ ≈ 100 pm.)

S²⁻ has the fewest protons holding the same 18 electrons, so it has the largest radius.

Answer: A. S²⁻

Exam tip: When you see ions with the same number of electrons, count the protons. The more protons, the smaller the ion. Remember: "Isoelectronic: highest atomic number = smallest size."