Niels Henrik David Bohr (October 7, 1885 - November 18, 1962) was a Danish physicist. He made essential contributions to understanding atom structure and quantum mechanics.
Born in Copenhagen, Denmark to Christian Bohr and Ellen Adler, Bohr got his doctorate at Copenhagen University in 1911. He then studied under Ernest Rutherford in Manchester, England. Based on Rutherford's theories, Bohr published his Bohr model about atom structure in 1913, introducing the theory of electrons travelling on orbits around the atom's nucleus, with the outer orbits holding more electrons than the inner ones, thereby determining the chemical properties of the atom. Also, an electron could drop from an outer orbit to an inner one, emitting a photon (light) of discrete energy. This became the basis for quantum theory.
In 1916, Bohr became professor at the University of Copenhagen, and director of the newly constructed "Institute of Theoretical Physics" in 1920. In 1922, he was awarded the Nobel Prize in physics for developing the Copenhagen interpretation of quantum mechanics.
Bohr also conceived the principle of complimentarity: that items could be separately analyzed as having several contradictory properties. For example, physicists currently conclude that light is both a wave and a stream of particles--two seemingly mutually exclusive properties--based on this principle. Bohr also found philosophical applications for this daringly original principle. Albert Einstein, who much preferred classical physics' clarity over the new physics of Bohr and Max Planck, and Bohr had good-natured arguments over the verity of this principle throughout their lives.
One of Bohr's most famous students was Werner Heisenberg, who became head of the German atomic bomb project. In 1941, during the German occupation of Denmark in World War II, Bohr was visted by Heisenberg in Copenhagen and apparently learned something of the German plans. In 1943 he escaped to Sweden to avoid arrest by the German police, then travelled to London. He worked at Los Alamos, New Mexico, USA, on the Manhattan Project, however his role was minor. He is quoted as saying "That is why I went to America. They didn't need my help in making the atom bomb." He was seen as a knowledgable consultant or "father confessor" on the project [1] (http://www.doug-long.com/bohr.htm). After the war he returned to Copenhagen, advocating for a peaceful use of nuclear energy. He died in Copenhagen.
The play Copenhagen, which ran on Broadway for a time, written by Michael Frayn, was about what might have happened at the 1941 meeting between Heisenberg and Bohr.
The element Bohrium is named in his honor.