UPSC CDS 2 2022 GK Paper

Concept:Isoelectronic species are atoms or ions that have the same number of electrons.Explanation:First, find the number of electrons in $\mathrm{Cl}^-$.Chlorine atom (atomic number 17) has 17 electrons.Gaining one electron gives $\mathrm{Cl}^-$ with $17+1 = 18$ electrons.Now count electrons for each option:$K^+$: Potassium atom (atomic number 19) has 19 electrons. Losing one electron gives $19-1 = 18$ electrons. Isoelectronic with $\mathrm{Cl}^-$.$\mathrm{Mg}^{2+}$: Magnesium atom (atomic number 12) has 12 electrons. Losing two electrons gives $12-2 = 10$ electrons. Not isoelectronic with $\mathrm{Cl}^-$.$\mathrm{S}^{2-}$: Sulfur atom (atomic number 16) has 16 electrons. Gaining two electrons gives $16+2 = 18$ electrons. Isoelectronic.$\mathrm{P}^{3-}$: Phosphorus atom (atomic number 15) has 15 electrons. Gaining three electrons gives $15+3 = 18$ electrons. Isoelectronic.Thus, only $\mathrm{Mg}^{2+}$ has a different electron count (10) from $\mathrm{Cl}^-$ (18).Answer:Option B: $\mathrm{Mg}^{2+}$