Final answer:
Each carbon in C2O42- has an oxidation number of +3, and each oxygen has an oxidation number of -2, resulting in the compound's overall -2 charge.
Step-by-step explanation:
To determine the oxidation number of each type of atom in C2O42-, we need to use the general rules for assigning oxidation states. Oxygen typically has an oxidation number of -2. Since there are four oxygen atoms, their total oxidation number sums to -8. The oxalate anion C2O42- has an overall charge of -2. For the molecule to be electrically neutral, the total oxidation number of the two carbon atoms must balance the -8 from oxygen and -2 from the charge, giving a total of +6. Therefore, each carbon atom in C2O42- must have an oxidation number of +3, as 2(+3) + 4(-2) = -2, matching the charge on the ion.