The correct order of the decreasing ionic radii among the following isoelectronic species is:

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Accepted Answer

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

1.  **Concept:** For isoelectronic ions, the ionic radius decreases as the nuclear charge (atomic number, Z) increases. A higher nuclear charge exerts a stronger electrostatic attraction on the same number of electrons, pulling the electron cloud closer to the nucleus .
2.  **Analysis:**
    *   **Sulphide ion (S²⁻):** Z = 16. Lowest nuclear charge, weakest attraction → **Largest Radius**.
    *   **Chloride ion (Cl⁻):** Z = 17.
    *   **Potassium ion (K⁺):** Z = 19.
    *   **Calcium ion (Ca²⁺):** Z = 20. Highest nuclear charge, strongest attraction → **Smallest Radius**.
3.  **Conclusion:** The order of decreasing ionic radii is S²⁻ > Cl⁻ > K⁺ > Ca²⁺.

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