Scientific theories are labor-saving devices. If you can replace twenty facts with five rules, and five rules with one good theory, then you obviously have made life easier. Mark Twain once remarked, "There is one big advantage to telling the truth: you don't have to have such a good memory." The same can be said for constructing theories.

The greatest single achievement in chemistry after the discovery of atoms was the working out of the periodic table, by Mendeleev in Russia in 1869, and independently by Meyer in Germany later that same year. Both men realized that similar chemical behavior recurred periodically if the elements were listed by increasing atomic weight (actually atomic number). They both devised the same scheme in which the elements were arranged so as to display this common chemical behavior. Chemists no longer had to remember the properties of every element in isolation. The "twenty facts" of the opening remark had given way to a few simple rules. But where was the theory that would explain these rules and account for the rather strange structure of the periodic table? The unifying theory would have to wait for nearly another half century, until physicists began applying the new quantum mechanics to chemistry. Nevertheless, with the periodic table the first big step toward placing chemistry on a rational basis had been taken.