Bohr, Niels Henrik David (nēls hănˈ rēk dāˈvēħ bōr) [key], 1885–1962, Danish physicist, one of the foremost scientists of modern physics. He studied at the Univ. of Copenhagen (Ph.D. 1911) and carried on research on the structure of the atom at Cambridge under Sir James J. Thomson and at Manchester under Lord Ernest Rutherford. In 1916, Bohr became professor of theoretical physics at the Univ. of Copenhagen, and in 1920 he was made director of the Institute of Theoretical Physics, which he was instrumental in founding.
Rutherford had discovered the nucleus of the atom in 1911, but classical theory was unable to explain the stability of the nuclear model of the atom. Bohr provided the solution to this problem in 1913, when he postulated that electrons move around the nucleus of the atom in restricted orbits and explained the manner in which the atom absorbs and emits energy. He thus combined the quantum theory with this concept of atomic structure. Much of the knowledge of modern physics was made possible by Bohr's initial revolutionary assumption that atomic processes cannot be explained by classical laws alone. Bohr was a leading figure in the continuing development of the quantum theory over the next twenty years. He received the 1922 Nobel Prize in Physics.
When Bohr visited the United States in 1938 and 1939, Bohr told American scientists of his belief, based on experiments reported by German scientists, that the uranium atom could be split into approximately equal halves. This was verified by scientists at Columbia. Bohr returned to Denmark but fled from the Nazi-occupied country in 1943. He gave valuable assistance in the atomic bomb research at Los Alamos, N.Mex., and in 1945 again returned to Denmark. His writings include The Theory of Spectra and Atomic Constitution (1922) and Atomic Theory and the Description of Nature (1934). See his collected works, ed. by León Rosenfeld (Vol. I, 1972).
His brother, Harald August Bohr, 1887–1951, a mathematician, taught (1915–30) at the College of Technology in Copenhagen and in 1930 became professor at the Univ. of Copenhagen. His most noted contribution to mathematics was his formulation of the theory of almost periodic functions. See his collected mathematical works, ed. by Erling Følner and Børge Jessen (3 vol., 1952).
See biographies by R. E. Moore (1966) and N. Blaedel (1988); studies by A. P. French and P. J. Kennedy, ed. (1987), D. R. Murdoch (1987), F. Aaserud (1989), and A. Pais (1991).