|H2C=O + H2O —> H2C(OH)2|
We begin by noting that elemental carbon has a zero oxidation state by definition. A carbon atom bonded only to other carbon atoms, as in the structures on the right below, is therefore assigned an oxidation number.of zero. If one of the carbon substituents is replaced by a hydrogen atom this oxidation number changes by -1; whereas, if the carbon substituent is replaced by a more electronegative substituent (O, N, F, Cl, Br etc.) the oxidation number changes by +1. Further examples of this simple system are shown in the diagram.