For certain classes of molecules, the general formula can be simplified or rewritten more clearly. For example:
where
:a = number of carbon atoms in the compound
:b = number of hydrogen atoms in the compound
:c = number of nitrogen atoms in the compound
:f = number of halogen atoms in the compound
or
where C = number of carbons, H = number of hydrogens, X = number of halogens and N = number of nitrogens, gives an equivalent result.
In either case, oxygen and other divalent atoms do not contribute to the degree of unsaturation, as 2 − 2 = 0.