The Danish physicist Niels Henrik David Bohr, b. Oct. 7, 1885, d. Nov. 18, 1962, is known primarily for his pioneering work in the field of atomic theory. Bohr was born in Copenhagen and was educated at the University of Copenhagen at the time when Max Planck had just begun the development of Quantum Mechanics. After completing his dissertation on the electron theory of metals in 1911, Bohr went briefly to Cambridge and then on to Manchester, England. There he worked under Ernest Rutherford, who in 1911 had published the theory that the atom consisted of a central nucleus orbited by electrons. The problem with this model was that, according to classical electrodynamic theory, the electrons should radiate and therefore lose energy and spiral into the nucleus.
With the problem of atomic structure in mind, Bohr worked in Manchester relentlessly from May through July of 1912 and succeeded in obtaining a formula correctly describing the absorption of helium nuclei by other atomic nuclei. His guiding notion was that the energy lost by a helium nucleus in flying through an atom depends not on the size of the atom but rather on the distances between the nucleus and the various electrons in the atom.
In the autumn of 1912, Bohr returned to Copenhagen to become an assistant at the university and began a happy marriage that produced six sons. He pondered the results of his work at Manchester and in 1913 published a crucial trilogy of papers that made a deep impression on Albert Einstein and other scientists. Especially astonishing was that Bohr, in his explanation of atomic structure, departed from classical mechanics and made use of Planck's constant and the quantum theory. The result was a model of the atom in which radiation was emitted only when an electron jumped from one quantum orbit to another. The frequency of the light emitted by the atom was thus not related to any frequency in the atom; rather, it was connected with the difference between two energy levels within the atom. At the age of 28, Bohr had reached the summit of his career with his theory of the atom.
In 1916, Bohr was appointed professor of theoretical physics at the University of Copenhagen, and in 1922 he was awarded the Nobel Prize for physics. In the following decades he continued to work on the implications of his theory, notably putting an earlier knowledge of surface tension to use in his "droplet model" of the nucleus, which treats the nucleus as if it were a water droplet held together by its surface tension.
In 1939, one year before the German invasion of Denmark, Bohr became president of the Royal Danish Academy of Sciences and Letters and began to develop a theory of nuclear fission. In 1943 the Germans planned to arrest him and make him work in Germany on an atomic project, but Bohr fled with his family and spent the war years in the United States, where he participated in the British-American atomic bomb project at Los Alamos.
After the war Bohr returned to Denmark and periodically expressed his strong feelings regarding the duties imposed upon humanity by its possession of atomic energy. He died at his home in Denmark.
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