Answer :
The oxidation number of the elements should be equal to zero or the charge of the compound when summed together. In this case, the overall charge is zero. Oxygen has a charge of -2, hence the magnesium ion should have a charge of +7 in order for the overall charge to be equal to zero
Answer;
+7
Explanation;
The sum of the oxidation numbers of all of the atoms in a neutral compound is 0. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion. In this case the overall oxidation number is Zero.
Thus; Oxidation number of oxygen is -2
Mn2O7; has 2 manganese atoms and 7 oxygen atoms;
= (2 × y) + ( 7 × -2) = 0
= 2y = 14
y = +7
Oxidation number of Mn in Mn2O7 is +7