The formula of an oxo-anion of manganese $(M n)$ in which it shows the oxidation state equal to its group number (7) is $\mathrm{MnO}_4^{-}$. $\mathrm{Mn}+(-2 \times 4) \;=-1$ $\mathrm{Mn}-8 \;=-1$ $\mathrm{Mn} \;=7$ Oxidation state of $\mathrm{Mn}$ in $\mathrm{MnO}_4^{-}$is + 7. The group number of $\mathrm{Mn}$ is also 7.