Niels Henrik David Bohr was one of the most important theoretical physicists of the 20th century. Through his own work and through his influence on younger physicists, he played a major role in formulating and interpreting quantum mechanics, the theory used to describe atomic and subatomic phenomena.
Bohr was born in Copenhagen on Oct. 7, 1885. His father was a professor of physiology. Bohr's training took place at the University of Copenhagen, where he received a doctorate in 1911 and presented a thesis in which he studied the electron theory of the structure of metals.
In 1911, Bohr went to Cambridge, England, to work with J. J. Thomson, then the leading atomic theorist. This collaboration did not work well, and after a few months Bohr made the happy decision to move to Manchester, England, to work with Ernest Rutherford. Rutherford had recently formulated the nuclear model of the atom, and Bohr used this theory in his own work, the application of quantum ideas to atomic structure and spectra. This work was carried out in Manchester and also Copenhagen, to which Bohr returned in 1912 to be married.
In Copenhagen, Bohr wrote three major papers in which he laid the foundation of a quantum theory of atoms. In this work, Bohr pointed out that atoms could not be described solely through the application of the concepts of classical physics. He proposed that an electron in an atom ordinarily resides in one of a discrete number of possible orbits, called stationary states, and each state corresponds to an angular momentum that is an integer multiple of Planck's constant divided by 2pi. Applying this assumption to the hydrogen atom, Bohr calculated the energies of the stationary states and showed that the known frequencies of light emitted by hydrogen could be expressed as the difference between two such energies. This implied that the electron (and hence atom) emits radiation only when jumping from one stationary state to another.
The success of these calculations did much to convince physicists of the importance of quantum ideas, and Bohr was awarded the Nobel Prize in physics in 1922. He had already, in 1920, been named the director of the new Institute for Theoretical Physics in Copenhagen. In the 1920s, Bohr, through his teaching and through discussions with Albert Einstein and others, displayed leadership in the quest to understand the meaning of quantum mechanics. His contributions to the formulation of Heisenberg's uncertainty principle were especially important. As a result the way in which quantum mechanics is understood by most physicists is called the Copenhagen interpretation.
In the late 1930s, Bohr's interest turned to nuclear physics, and he formulated the "liquid drop" model of the nucleus. With the aid of this model, Bohr realized, soon after the discovery of uranium fission, that it was the isotope U-235 that was fissioned by slow neutrons. This discovery was instrumental in the development of the atomic bomb. In 1943, Bohr escaped from occupied Denmark, and for the remainder of World War II he worked in the United States, contributing to the atomic bomb project. After the war he returned to his homeland and spoke out periodically about the need for responsible nuclear policy. In 1955 Bohr organized the first Atoms for Peace Conference in Geneva, Switzerland. Bohr died in Denmark on Nov. 18, 1962.
*Source: 1998 Grolier Multimedia Encylopedia