# What is MnO_4?

I think you mean $M n {O}_{4}^{-}$, $\text{permanganate ion}$. Potassium permanganate is an actual compound I could put in a bottle. $M n {O}_{4}$ is unknown.
Manganese has rich oxygen chemistry, but its maximum oxidation number is $V I {I}^{+}$ (which it expresses in permanganate). $M n {O}_{4}$ would require an oxidation number of $V I I {I}^{+}$, which is unknown.