What is the oxidation number of ##C_2O_4^(2-)##?

The oxidation states of the ##C_2O_4^-2## ion are ##C^(+3)## and ##O^-2##.

##C_2O_4## oxalate is a polyatomic ion with a charge of -2.

The oxygen atoms in this molecule have a oxidation state of -2, vecause oxygen is always a -2 charge. Since there are 4 atoms of oxygen the overall charge of the oxygen atoms is 4(-2) = -8.

Since the total charge of oxalate is -2 the charge created by the carbon atoms must be +6. (-8 +6 = -2) this means that each carbon atom must have an oxidation state of +3. (+6/2 = +3)

The oxidation states of the ##C_2O_4^-2## ion are ##C^(+3)## and ##O^-2##