Hückel, Erich

(1896–1980) German physicist and theoretical chemist: developed molecular orbital theory of bonding in organic molecules.

Hückel’s study of physics at Göttingen was interrupted by the First World War, when he spent 2 years on aerodynamics before returning to finish his course. He worked as assistant first to the mathematician , but he did not like the work and he also wished to travel. He joined his former teacher at Zürich and began working on what is now known as the Debye–Hückel theory of electrolyte solutions. This assumes that strong electrolytes are fully dissociated into ions in solution, and calculates properties (such as electrical conductivity) for dilute solutions on this basis. Then, after a major illness, he worked on colloid chemistry, first with his father-in-law and later with F G Donnan (1879–1956) in London, and then moved to Copenhagen to work on quantum theory . The latter suggested that Hückel should try to calculate properties of the CC double bond by wave mechanics. Hückel classified the electrons making up such bonds as s (sigma) and p (pi) types on a symmetry basis and was able to make useful calculations of bond properties. Hückel molecular orbital (HMO) theory has been widely applied to organic molecules. One result is Hückel’s rule, which proposes that aromatic stability will be shown by planar monocyclic molecules in which all the cyclic atoms are part of the p-system only if the number of such p-electrons is 4 n + 2, where n is an integer. The rule has provoked much fruitful study of molecules predicted by the rule to show ‘aromaticity’. Hückel’s work on such compounds probably began with a suggestion from his elder brother Walter, an organic chemist. From 1937 Erich was professor of theoretical physics at Marburg.