Which of the following species is NOT isoelectronic with neon?

Which of the following species is NOT isoelectronic with neon?
  1. $\mathrm{O}^{2-}$
  2. $\mathrm{Na}$
  3. $\mathrm{Mg}^{2+}$
  4. $\mathrm{Al}^{3+}$

Solution

Atoms and ions having the same number of electrons are isoelectronic. $\begin{array}{|l|l|} \hline \text{Species} & \text{No. of electrons} \\ \hline \text{Neon} & 10 \\ \hline \mathrm{O}^{2-} & 8+2=10 \\ \hline \mathrm{Na} & 11 \\ \hline \mathrm{Mg}^{2+} & 12-2=10 \\ \hline \mathrm{Al}^{3+} & 13-3=10 \\ \hline \end{array}$

Asked in: MHT CET 2023 (10 May Shift 1)

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