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Which one of the following sets of ions represents a collection of isoelectronic species?

$\begin{array}{1 1}(a)\;Li^+,Na^+,Mg^{2+},Ca^{2+}\\(b)\;K^+,Cl^-,Ca^{2+},Sc^{3+}\\(c)\;Ba^{2+},Sr^{2+},K^+,Ca^{2+}\\(d)\;N^{3-},O^{2-},F^-,S^{2-}\end{array}$

Each of $K^+,Cl^-,Ca^{2+},Sc^{3+}$ has 18 electrons.
Hence (b) is the correct answer.
edited Apr 2, 2014