Hückel's rule
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 4"n" + 2 rule has been attributed to W. v. E. Doering (1951), although several authors were using this form at around the same time.