A monomer is the smallest repeating unit of a polymer. Polymers are large molecules that are made up of many repeating units, and the monomers are the building blocks of these polymers. Monomers can be either organic or inorganic, and they can be naturally occurring or synthetic. Some common examples of monomers include ethylene, which is used to make polyethylene, and styrene, which is used to make polystyrene.