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Among the ions O²-, P³-, Cl⁻, Na⁺, Ca²+, and K⁺, how many have an ionic radius larger than that of S²-?

1. 1
2. 2
3. 3
4. 4

Correct answer: 1

Solution

S²- has 18 electrons (Z=16). P³- also has 18 electrons but Z=15 (lower nuclear charge), giving it a larger radius than S²-. All other ions — O²-, Cl⁻, Na⁺, Ca²+, K⁺ — are either smaller or in the same isoelectronic series with higher Z (hence smaller). Therefore only P³- has a larger ionic size than S²-.

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