Question

What are the oxidation numbers of each element in $Ca(OH{)}_2$?

Answer 1

As calcium, a Group $2$ alkaline earth, i.e. $C{a}^{2+}$ and $2\times H{O}^{-}$.

Now hydrogen generally takes $a+I$ oxidation number, and it does so here. And finally, we give oxygen $a-II$ oxidation number.

And so we got $Ca(+II),O(-II)$, and $H(+I)$ as the oxidation numbers.