Bohr, Niels (Henrik David) (1885-1962), a Danish physicist. He was one of the pioneers in the development of modern atomic theory. Bohr was awarded the 1922 Nobel Prize in physics for his work on atomic structure and radiation, especially for a theory he developed in 1913. It states that electrons revolve around the nucleus of an atom in fixed orbits and jump from one orbit to another only when absorbing or radiating energy. Bohr was described by Albert Einstein as one of the greatest discoverers of our age in the scientific field."

Bohr was born in Copenhagen, the son of a university professor. After receiving a Ph.D. degree from the University of Copenhagen in 1911, he studied under Sir J. J. Thomson at Cambridge University in England. Then he worked with Ernest Rutherford at the University of Manchester and developed the theory for which he received the Nobel Prize. In this work, Bohr applied Max Planck's quantum theory to the model of the atom that Rutherford had proposed in 1911.

Bohr became professor of physics at the University of Copenhagen in 1916 and director of its Institute for Theoretical Physics in 1920. During the 1920's and 1930's, he made important contributions to the development of quantum, or wave, mechanics. In 1936 Bohr announced a new theory of the nucleus of the atom, laying the groundwork for the understanding of nuclear reactions.

In 1939 Bohr visited the United States and brought the news that two European physicists—Otto Hahn and Fritz Strassmann—had split a uranium atom. Bohr then spent several months working with the American physicist John A. Wheeler at the Institute for Advanced Study, Princeton, New Jersey. Together they developed a theory to explain nuclear fission and correctly suggested that a chain reaction could be obtained by using the isotope uranium 235.

Bohr returned to Denmark in 1940, but in 1943 fled from the Nazis, first to Sweden, then to England. Later in 1943 he came to the United States to take part in the work on the atomic bomb at Los Alamos, New Mexico. He returned to Denmark after the war and devoted his efforts towards developing peaceful uses for nuclear energy.