**E**rich Armand Arthur Joseph Huckel was a German physicist and physical chemist. He is known for two major contributions: (a) The Debye-Huckel theory of electrolytic solutions, (b) The Huckel method of approximate molecular orbital (MO) calculations on p-electron systems. Erich Huckel was born in the Charlottenburg suburb of Berlin.

He studied physics and mathematics and received his Ph.D. in experimental physics in 1921 at the University of Gottingen. On receiving his doctorate, Erich Huckel became an assistant at Gottingen, but soon became an assistant to Peter Debye at Federal Institute of Technology (Eidgenossische Technische Hochschule) in Zurich. It was there that he and Debye developed their theory (the Debye-Huckel theory, in 1923) of electrolytic solutions, elucidating the behavior of strong electrolytes to account for their conductivity by considering interionic forces.

Debye-Huckel equation is a mathematical expression derived to elucidate certain properties of solutions of electrolytes, that is, substances present in the solutions in the form of charged particles (ions). Such solutions often behave as if the number of dissolved particles were greater or less than the number actually present; the Debye-Huckel equation takes into account the interactions between the various ions, which are the principal cause of the discrepancies between the properties of dilute solutions ofelectrolytes and those of so-called ideal solutions.

In 1928 and 1929 Huckel spent time in England and Denmark, working briefly with Niels Bohr. In 1930 Huckel received an appointment in chemicalphysics at the Technical Institute (Technische Hochschule) in Stuttgart. He worked for a while on capillary effects and wrote a book about it. Eventually his interests moved to the use of quantum mechanics to deal with organic molecules.

In 1931 he formulated the “Huckel rule” for determining whether ring molecules would show aromatic properties, and in 1937 developed his approximate MO theory (“Huckel molecular orbital theory” or simply “Huckel Theory”) for unsaturated organic molecules. He left this field of interest after a short while and published thereafter only occasionally on various other topics of physics.

However, the Huckel MO theory is still used as an approximation, though the more precise Pariser-Parr-Pople method succeeded it after its publication separately by Pariser and Parr and by Pople in 1953, for more accurate calculations. An extension of the Huckel MO theory was also developed by Roald Hoffmann for nonplanar molecules in 1963 that was widely used in organic and inorganic chemistry.

In 1937 Huckel was appointed a professor of theoretical physics at the Phillips University of Marburg, where he remained until his retirement in1962. Erich Huckel died in Marburg (Germany) in 1980 where he had been professor of physics for many years.