What is the oxidation state of each element in #COH_2#?
Start by assigning formal oxidation states to the most electronegative elements, then discover the positive limits of the least electronegative elements. Elements in the middle are determined by conservation of charge.
Start with oxygen, which is more electronegative than any element except fluorine. The preferred oxidation state of O is -2.
Next is hydrogen (H), which has a preferred oxidation state of +1.
The sum of all oxidation states must equal the overall charge on the molecule (in this case, zero). If we let x represent the oxidation state of carbon (C), then
Solving for x gives
Therefore, the formal oxidation state of carbon in this molecule is zero. Note that carbon can adopt formal oxidation states from +4 (as in