oxidation number of carbon in bicarbonate
Oxygen = three ions each with a charge of -2 = 3 x -2 = -6. The overall charge of the polyatomic ion is -2. This means that the oxidation number of the carbon must be +4. Total charge = +4 + -6 = -2. Here is a video which discusses this topic by discussing how to determine oxidation numbers of elements in the bicarbonate and dichromate ions. Video from: Noel Pauller.