Niels Henrik David Bohr (October 7, 1885 – November 18, 1962) was a Danish physicist who made essential contributions to understanding atomic structure and quantum mechanics.
Bohr's contributions to physics
Bohr's model of atomic structure.
The theory that electrons travel in orbits around the atom's nucleus, with the chemical properties of the element being largely determined by the number of electrons in the outer orbits.
The idea that an electron could drop from a higher-energy orbit to a lower one, emitting a photon (light quantum) of discrete energy (this became the basis for quantum theory).
The Copenhagen interpretation of quantum mechanics.
The principle of complementarity: that items could be separately analyzed as having several contradictory properties.
Born in Copenhagen to Christian Bohr and Ellen Adler, Bohr received his doctorate from Copenhagen University in 1911. He then studied under Ernest Rutherford in Manchester, England. Based on Rutherford's theories, Bohr published his model of atomic structure in 1913, introducing the theory of electrons travelling in orbits around the atom's nucleus, the chemical properties of the element being largely determined by the number of electrons in the outer orbits. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, emitting a photon (light quantum) of discrete energy. This became the basis for quantum theory.
In 1916, Bohr became a 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 was largely affected by the philosophy of Søren Kierkegaard of sharp sudden "quality" changes and rejection of the continuous changing. These concepts were manifested in Bohr's quantum theory.
Bohr also conceived the principle of complementarity: 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 apparently mutually exclusive properties - based on this principle. Bohr also found philosophical applications for this daringly original principle. Albert Einstein much preferred the determinism of classical physics over the probabilistic new physics of Bohr and Max Planck. He 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, a crucial figure in the development of quantum mechanics, but also head of the German atomic bomb project.
Niels Bohr and his wife Margrethe had several children, one of whom, Aage Niels Bohr, also became a very successful physicist: like his father, he won a Nobel prize.
Later years, death, and afterwards
In 1941, during the German occupation of Denmark in World War II, Bohr was visited 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, where, according to Richard Feynman, he was known by the assumed name of Nicholas Baker for security reasons. However his role in the project 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 knowledgeable consultant or "father confessor" on the project  (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 in 1962.
The element bohrium is named in his honor.
Relationship with Heisenberg
Heisenberg claimed in an interview after the war, when the author Robert Jungk was working on the book Brighter Than a Thousand Suns, that he had tried to establish a pact with Bohr such that scientists on neither side should help develop the atomic bomb. He also said that the German attempts were entirely focused on energy production, and that his circle of colleagues tried to keep it that way.
When Bohr saw this claim he disagreed wholeheartedly. He said that Heisenberg had indeed let him know in Copenhagen that he was working on an atomic bomb project, and that he thought that Germany would win the war. He dismissed the idea of any pact as an after-the-fact construction. He drafted several letters to inform Heisenberg about this but never sent any of them.
Michael Frayn's play Copenhagen, which ran on Broadway for a time, explores what might have happened at the 1941 meeting between Heisenberg and Bohr.
Books about Bohr
Niels Bohr: The Man, His Science, and the World They Changed, by Ruth Moore ISBN 0262631016
Niels Bohr's Times, In Physics, Philosophy and Polity, by Abraham Pais, ISBN 0-19-852049-2