What is the oxidation number of $\mathrm{Ca}$ in $\mathrm{CaO}$?

Oxidation number: Oxidation number denotes the oxidation state of an element in a compound ascertained according to a set of rules formulated on the basis that electron pair in a covalent bond belongs entirely to a more electronegative element.

Calculating the oxidation number of Calcium(Ca) in Calcium oxide $\left(\mathrm{CaO}\right)$:

The oxidation state of Oxygen(O) = -2

Consider the oxidation state of Calcium(Ca) = x

Now,

$$\begin{gathered} \text{x}-2=0 \\ \text{x}=+2 \end{gathered}$$

Hence, the oxidation number of Calcium in Calcium oxide is $+2$.