Columbia Encyclopedia World Factbook American Heritage Coll. A single atom is usually not referred to as a molecule, and ionic compounds such as common salt are not made up of molecules. Unlike ions , molecules carry no electrical charge. 1 Nature of Molecules Molecules are made up of two or more atoms , either of the same element or of two or more different elements, joined by one or more covalent chemical bonds . According to the kinetic-molecular theory, the molecules of a substance are in constant motion.
Substances differ according to the structure and composition of their molecules.
O. A more complex structural formula is sometimes used to show the arrangement of atoms in the molecule. 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.