Give the formulae of the stable binary compounds that would be formed by the combination of the following pairs of elements:
$(a)$ $Mg$ and $N_{2}$
$(b)$ $Li$ and $O_{2}$
$(c)$ $Al$ and $Cl_{2}$
$(d)$ $K$ and $O_{2}$

(A) To determine the formula of a binary compound,we balance the valencies of the combining elements:
$(a)$ $Mg$ has a valency of $+2$ and $N$ has a valency of $-3$. By criss-crossing the valencies,we get $Mg_{3}N_{2}$ (Magnesium nitride).
$(b)$ $Li$ has a valency of $+1$ and $O$ has a valency of $-2$. By criss-crossing the valencies,we get $Li_{2}O$ (Lithium oxide).
$(c)$ $Al$ has a valency of $+3$ and $Cl$ has a valency of $-1$. By criss-crossing the valencies,we get $AlCl_{3}$ (Aluminium chloride).
$(d)$ $K$ has a valency of $+1$ and $O$ has a valency of $-2$. By criss-crossing the valencies,we get $K_{2}O$ (Potassium oxide).