What is the oxidation number of $\mathrm{Mn}$ in ${\mathrm{MnO}}_2$?

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 Manganese(Mn) in ${\mathrm{MnO}}_2$:

The oxidation state of Oxygen(O) = -2

Consider the oxidation state of Manganese(Mn) = x

Now,

$$\begin{gathered} \text{x+}\left(\left(-2\right)\times 2\right)=0 \\ \text{x}=+4 \end{gathered}$$

Hence, the oxidation number of $\mathrm{Mn}$ in ${\mathrm{MnO}}_2$ is $+4$.