Unlike radicals in organic molecules, R cannot be hydrogen. Organic compounds can be written in such a way that anything that doesn't change before the first C-C bond is replaced with the abbreviation R (Figure 1c). When dealing with organic compounds and formulas with multiple atoms of the same element, it's easier to work with molecular formulas and average oxidation numbers (Figure 1d). Notice that changing the CH 3 group with R does not change the oxidation number of the central atom. R is an abbreviation for any group in which a carbon atom is attached to the rest of the molecule by a C-C bond. Different ways of displaying oxidation numbers of ethanol and acetic acid.