WAEC Chemistry 2012 Paper

Concept:Ions having the same number of electrons possess identical electron configurations; the ion with a different electron count is the odd one out.Explanation:The chloride ion $\mathrm{Cl}^-$ has atomic number 17 and gains one electron, so it contains $17 + 1 = 18$ electrons.Its configuration is $1s^2 2s^2 2p^6 3s^2 3p^6$, which is often written as 2, 8, 8.The oxide ion $\mathrm{O}^{2-}$ has atomic number 8 and gains two electrons, giving $8 + 2 = 10$ electrons, with configuration 2, 8.The magnesium ion $\mathrm{Mg}^{2+}$ has atomic number 12 and loses two electrons, giving $12 - 2 = 10$ electrons, with configuration 2, 8.The aluminium ion $\mathrm{Al}^{3+}$ has atomic number 13 and loses three electrons, giving $13 - 3 = 10$ electrons, with configuration 2, 8.Therefore, $\mathrm{O}^{2-}$, $\mathrm{Mg}^{2+}$, and $\mathrm{Al}^{3+}$ are isoelectronic, all with 10 electrons.Only $\mathrm{Cl}^-$ has 18 electrons, so its electron configuration differs from the others.Answer:A. $\mathrm{Cl}^-$