# Question #f98b4

The oxidation number for oxygen is always $- 2$ except in hydrogen peroxide it is $- 1$ and $O {F}_{2}$ it is +2.
Having said that, the oxidation number of Mn in $M n {O}_{4}^{_}$ is +7. It is calculated as follows:
$x + 4 \times \left(- 2\right) = - 1 \implies x = + 7$