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. The state (solid, liquid, or gaseous) in which matter appears depends on the speed and separation of the molecules in the matter. Substances differ according to the structure and composition of their molecules. A molecular compound is represented by its molecular formula; for example, water is represented by the formula H2O. A more complex structural formula is sometimes used to show the arrangement of atoms in the molecule.
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.
Molecules can assume many shapes and sizes. Molecules of hydrogen gas, H2, are very small; each consists of two atoms of hydrogen. Water molecules, H2O, are much larger, containing an atom of oxygen as well as two of hydrogen. The atoms in a water molecule are arranged at the corners of an isosceles triangle; the oxygen atom is located where the two equal sides meet and the angle between these sides is about 105°. A carbon dioxide molecule, CO2, is linear, with the two oxygen atoms an equal distance on either side of the carbon atom. In methane, CH4, the hydrogen atoms are arranged at the corners of a tetrahedron with the carbon atom in the center. In benzene, C6H6, the carbon atoms form a hexagonal ring with a hydrogen atom joined to each carbon atom. 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.