2 Molecules differ in size and molecular weight as well as in structure. In a chemical reaction between molecular substances, the molecules are often broken apart into atoms or radicals that recombine to form other molecules, i.e., other substances. In other cases two or more molecules will combine to form a single larger molecule, or a large molecule will be broken up into several smaller molecules. 3 Molecules can assume many shapes and sizes. Water molecules, H 2 O, are much larger, containing an atom of oxygen as well as two of hydrogen.
More complex molecules resemble rings, chains, helices, or other forms.
Many molecules occurring in living organisms are very complex. RNA and DNA molecules resemble giant helices. By polymerization a large number of small molecules may be joined to form a single large polymer molecule. Typical polymers include synthetic resins, rubbers, and plastics. The terms atom and molecule were used interchangeably until the early 19th cent.