It is given that MnO2 has a formula charge of – 2. We know that each oxygen or oxide ion has also a charge of – 2, therefore there are a total of – 4 oxide charges. So we can write:
X + (- 4) = - 2
Where X is the charge of one manganese ion, so calculating for X:
X = - 2 + 4
X = 2
So the charge of manganese is 2+