Bohr, Niels Henrik David (1885-1962),

Danish physicist and Nobel laureate, who made basic contributions to nuclear physics and the understanding of atomic structure.

Bohr was born in Copenhagen on October 7, 1885, the son of a physiology professor, and was educated at the University of Copenhagen, where he earned his doctorate in 1911. That same year he went to the University of Cambridge in England to study nuclear physics under J. J. Thomson, but he soon moved to the University of Manchester to work with Ernest Rutherford.

Bohr's theory of atomic structure, for which he received the Nobel Prize for Physics in 1922, was published in papers between 1913 and 1915. His work drew on Rutherford's nuclear model of the atom, in which the atom is seen as a compact nucleus surrounded by a swarm of much lighter electrons. Bohr's atomic model made use of quantum theory and the Planck constant (the ratio between quantum size and radiation frequency). The model posits that an atom emits electromagnetic radiation only when an electron in the atom jumps from one quantum level to another. This model contributed enormously to future developments of theoretical atomic physics.

In 1916 Bohr returned to the University of Copenhagen as a Professor of Physics, and in 1920 he was made Director of the university's newly formed Institute for Theoretical Physics. There Bohr developed a theory relating quantum numbers to large systems that follow classical laws, and made other major contributions to theoretical physics. His work helped lead to the concept that electrons exist in shells and that the electrons in the outermost shell determine an atom's chemical properties. He also served as a visiting professor at many universities.

In 1939, recognizing the significance of the fission experiments of the German scientists Otto Hahn and Fritz Strassmann, Bohr convinced physicists at a scientific conference in the United States of the importance of those experiments. He later demonstrated that uranium-235 is the particular isotope of uranium that undergoes nuclear fission. Bohr then returned to Denmark, where he was forced to remain after the German occupation of the country in 1940. Eventually, however, he escaped to Sweden, in peril of his life and that of his family. From Sweden the Bohrs travelled to England and eventually to the United States, where Bohr joined in the effort to develop the first atomic bomb, working at Los Alamos, New Mexico, until the first bomb's detonation in 1945. He opposed complete secrecy of the project, however, and feared the consequences of this ominous new development. He desired international control.

In 1945 Bohr returned to the University of Copenhagen, where he immediately began working to develop peaceful uses for atomic energy. He organized the first Atoms for Peace Conference in Geneva, held in 1955, and two years later he received the first Atoms for Peace Award. Bohr died in Copenhagen on November 18, 1962.