WAEC Chemistry 2021 Paper

Concept:Ions that have the same number of electrons and the same electron configuration are called isoelectronic species. The ion that differs in electron count is the exception.Explanation:Calculate the total number of electrons for each ion using its atomic number and ionic charge.For $\mathrm{O}^{2-}$: atomic number is $8$, so electrons $= 8 + 2 = 10$. Configuration: $1s^2\,2s^2\,2p^6$.For $\mathrm{Mg}^{2+}$: atomic number is $12$, so electrons $= 12 - 2 = 10$. Configuration: $1s^2\,2s^2\,2p^6$.For $\mathrm{Al}^{3+}$: atomic number is $13$, so electrons $= 13 - 3 = 10$. Configuration: $1s^2\,2s^2\,2p^6$.For $\mathrm{Cl}^{-}$: atomic number is $17$, so electrons $= 17 + 1 = 18$. Configuration: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6$.Thus $\mathrm{O}^{2-}$, $\mathrm{Mg}^{2+}$, and $\mathrm{Al}^{3+}$ all have the neon configuration, while $\mathrm{Cl}^{-}$ has a different configuration.Answer:$\mathrm{Cl}^{-}$