Final answer:
The oxidation number of manganese in Mn2O7 is +7, which is found by balancing the total oxidation numbers in the compound with the known oxidation state of oxygen (-2). The correct option is 4.+7.
Step-by-step explanation:
The oxidation number of a manganese atom in Mn2O7 is +7. To determine this, we observe that oxygen typically has an oxidation number of -2. Given that there are 7 oxygen atoms, the total oxidation number for oxygen is -14.
The compound is neutral overall, so the sum of the oxidation numbers for manganese must balance this to be +14.
Since there are two manganese atoms, each must have an oxidation number of +7 to achieve this balance. This also agrees with the known chemistry of manganese compounds where Mn can exhibit various oxidation states, with +7 being the highest possible for the fourth period transition metals.