Cl^- is <strong>NOT</strong> isoelectronic with

  1. A. K^+
  2. B. Mg^{2+}
  3. C. S^{2-}
  4. D. P^{3-}

Correct Answer: B. Mg^{2+}

Explanation

The chloride ion (Cl^-) has 18 electrons. K^+, S^{2-}, and P^{3-} all have 18 electrons. The magnesium ion (Mg^{2+}) only has 10 electrons and is therefore not isoelectronic with it.

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