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:

1. Magnesium ($\text{Mg}$) is in group 2 and has a valency of $+2$.2. Element $X$ has configuration $1s^{2}2s^{2}2p^3$ (Nitrogen family). It has 5 valence electrons and needs 3 more to complete its octet. So, its valency is $-3$.3. Using the criss-cross method: ${\text{Mg}}^{2+}$ and $X^{3-}$ form ${\text{Mg}}_3{X}_2$.