If the valency of Mg is ‘2’ and the valency of Sulphate $\left(S{O}_4\right)$ is ‘2’, then the formula of the compound:

To write chemical formulas by using ions like ${\text{Mg}}^{+2},{\text{SO}}_4^{-2}$, the general Criss-cross method is

followed. According to the Criss-cross method, the formula of the compound is ${\text{Mg}}_{\text{2}}{\left({\text{SO}}_{\text{4}}\right)}_2$.

The simplest form is ${\text{MgSO}}_4$ .