# What is the formula for calcium hydroxide?

Calcium is from Group 2 and commonly forms the $C {a}^{2 +}$ ion. Hydroxide ion, $H {O}^{-}$, is singly, negatively charged.
To make a neutral species, we simply balance the charges. To balance the charge of the metal ion, $C {a}^{2 +}$, we need 2 hydroxide ions, $2 \times O {H}^{-}$.
We get the formula $C a {\left(O H\right)}_{2}$. Capisce?
Note that once you know the formula for calcium hydroxide, you know the formula for the hydroxides of ALL the Group 2 metals, i.e. $B a {\left(O H\right)}_{2}$, $S r {\left(O H\right)}_{2}$. Are you with me?