Magnesium reacts with an element ($X$) to form an ionic compound. If the ground state electronic configuration of ($X$) is $1s^{2}2s^{2}2p^3$, the simplest formula for this compound is

Solution & Explanation

\textbf{Answer:} (D) $\mathrm{Mg}{}_3\mathrm{X}{}_2$. \textbf{Solution:} The element $X$ has configuration $1s^{2}2s^{2}2p^3$, i.e. $5$ valence electrons, so it needs $3$ more electrons to complete its octet, giving a valency of $3$ (charge $-3$). Magnesium has a valency of $2$ (charge $+2$). Cross-combining the charges $\mathrm{Mg}{}^{2+}$ and $\mathrm{X}{}^{3-}$ gives the formula $\mathrm{Mg}{}_3\mathrm{X}{}_2$.