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

1. Ca2+ > K+ > S2− > Cl−
2. Cl− > S2− > Ca2+ > K+
3. S2− > Cl− > K+ > Ca2+
4. K+ > Ca2+ > Cl− > S2−

Correct answer: S2− > Cl− > K+ > Ca2+

Solution

Isoelectronic species have the same number of electrons, so their ionic radii depend on nuclear charge. Higher nuclear charge pulls electrons closer, reducing size. S2− has the lowest nuclear charge, making it the largest, followed by Cl−, K+, and Ca2+.

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