A molecule is the smallest unit of a substance capable of independent existence. As early as the 17th century, there were proposals that molecules were made up of atoms of elements held together either by the mutual attraction of the atoms or by the atoms fitting together in a manner similar to a jigsaw puzzle. Well into the 19th century, however, there were continuing disagreements on the molecular nature of chemical compounds. After Edward Frankland proposed in 1866 that molecules are made up of atoms held together by localized directional chemical bonds, the supporting experimental evidence led to general acceptance of the idea. In 1874, August Kekulé and the team of J. H. van't Hoff and J. A. Le Bel independently proposed a tetrahedral structure for carbon in polyatomic molecules. In the methane molecule CH₄, for instance, the carbon atom is located at the center of a tetrahedron with...