In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. The succinct expression as the 4n+2 rule has been attributed to von Doering (1951), although several authors were using this form at around the same time. A cyclic ring molecule follows Hückel's rule when the number of its π-electrons equals 4n+2 where n is zero or any positive integer, although clearcut examples are really only established for values of n = 0 up to about n = 6...