Question

What is the chemical formula of calcium hydroxide?

Answer 1

Calcium hydroxide is formed by one $C{a}^{2+}$ cation and two $O{H}^{-}$ anions, which implies that its chemical formula will be $Ca(OH{)}_2$.

Here's how you'd figure this out. If you look at the periodic table, you'll see that calcium is in group 2. That means that it has two valence electrons that it must give up in order to be stable.

On the other hand, oxygen is in group 6, which means it needs 2 electrons in order to be stable. It gets one electron from a hydrogen atom to form an $O{H}^{-}$ anion. Now it only needs one more electron, which it gets from the two calcium has to offer.

This means that you'd need two $O{H}^{-}$ groups to form calcium hydroxide, since each $O{H}^{-}$ group would get one electron from calcium → $Ca(OH{)}_2$.