Here's how to determine the oxidation number of Mn in manganese VII oxide (Mn₂O₇):

* Oxygen's Oxidation Number: Oxygen usually has an oxidation number of -2, except in peroxides (where it's -1).

* Neutral Compound: The overall charge of a neutral compound like Mn₂O₇ is zero.

Calculation:

1. Total oxygen charge: 7 oxygen atoms * (-2) charge/atom = -14

2. Manganese charge: Since the overall charge must be zero, the two manganese atoms must have a total charge of +14 to balance the -14 charge from the oxygen.

3. Manganese oxidation number: +14 / 2 manganese atoms = +7

Therefore, the oxidation number of Mn in manganese VII oxide (Mn₂O₇) is +7.