Arnold Johannes Wilhelm Sommerfeld
German physicist who modified the Bohr theory of the atom, suggesting that orbital electrons travelled in elliptical orbits.
The son of a physician, Sommerfeld was educated at the University of Königsberg. After teaching briefly at the universities of Göttingen, Clausthal, and Aachen he was appointed professor of physics at the University of Munich in 1906. Sommerfeld should have retired in 1936 in favour of his pupil, Werner Heisenberg. Opposition from the Nazi party to Heisenberg's appointment prolonged Sommerfeld's tenure and it was not in fact until late 1939 that he finally retired, to be succeeded not by Heisenberg but by Wilhelm Müller, a Nazi aerodynamicist without a single publication in physics to his credit. Although Sommerfeld and Heisenberg were not Jewish, they were regarded by the Nazis as Jewish sympathizers. Sommerfeld, however, survived the war and returned to his Munich chair in 1945, continuing to work at physics until he died in a car accident in 1951.
Sommerfeld's main contribution to physics concerned the model of the atom proposed by Niels Bohr in 1913. Sommerfeld argued in 1916 that the fine structure of the spectrum of the hydrogen atom could be explained if it was assumed that electrons adopt elliptical rather than the circular orbits proposed by Bohr. This involved adopting a second quantum number, referred to as the azimuthal quantum number. Sommerfeld is also remembered as the author of the influential textbook, Atombau und Spektrallinien (1919; translated as Atomic Structure and Spectral Lines, 1923).