# What is the formula of "magnesium hydroxide"?

$M g {\left(O H\right)}_{2}$
Now we know that hydroxides are salts of $H {O}^{-}$, and some metal ion. Now if the parent metal has an electronic configuration of $2 : 8 : 2$, then there are 12 electrons, and the atomic number of the metal is equal to $12$. We look on the Periodic Table, and we find that $Z = 12$, for $\text{magnesium metal}$.
As a Group 2 metal, magnesium forms a $M {g}^{2 +}$ ion, and hence its hydroxide is $M g {\left(O H\right)}_{2}$.