Niels Bohr developed his atomic research through work in Britain after completing his doctorate in Copenhagen. Contact with Ernest Rutherford's laboratory was especially influential. In 1913 he proposed a model in which atoms occupied discrete energy states and emitted or absorbed radiation when changing between them. It explained important features of hydrogen's spectrum and joined Rutherford's nuclear atom to quantum ideas, although its picture of electron orbits was later replaced by fuller quantum mechanics.

Returning to Copenhagen, Bohr became a professor and founded the Institute for Theoretical Physics, opened in 1921. It attracted researchers from many countries and became a center for discussion and collaborative work. Awarded the Nobel Prize in Physics in 1922, he continued investigating atomic structure and helped shape the interpretation of the new mechanics developed during the 1920s. His principle of complementarity argued that different experimental arrangements could reveal aspects of quantum phenomena that could not be combined into one classical picture. Debates with Einstein explored the completeness and meaning of quantum theory.

Bohr also contributed to nuclear physics. His work on compound nuclei and, with John Wheeler, the theory of nuclear fission extended his influence beyond atomic spectra. The German occupation of Denmark brought danger, and in 1943 he escaped to Sweden before traveling to Britain and the United States. He participated in discussions connected to the Allied atomic bomb project, using the name Nicholas Baker. His scientific and personal standing made him valuable, while his concerns increasingly centered on the political consequences of nuclear weapons and the need for international cooperation.

After the war, Bohr argued for greater openness and international control rather than a permanent system of secrecy and rivalry. These proposals did not gain the agreement he sought from major governments. He returned to leadership of the Copenhagen institute, supported European scientific cooperation, and remained engaged in research and public discussion. He died in 1962. Bohr's career combined a major early model of the atom with institution building and the interpretation of quantum mechanics. The model's later replacement did not erase its historical importance; its success and limits helped direct the search for a theory capable of accounting more fully for atomic behavior.