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Niels Bohr. Historical portrait of the Danish physicist Niels Henrik David Bohr (1885-1962). Bohr was one of the most respected theoretical physicists of the 20th century. He is famous for his 1913 model of the hydrogen atom. This used quantized energy levels for electrons orbiting the nucleus to explain the hydrogen line spectrum. It also predicted new spectral lines, later observed, aiding the acceptance of the new quantum theory. Bohr was awarded the 1922 Nobel Prize for Physics. He participated in developing quantum theory over the following decades. In 1943 he fled Denmark for the USA, assisting in the atomic bomb research project. Photographed in Copenhagen in the 1920s. (KEYSTONE/SCIENCE PHOTO LIBRARY/SCIENCE PHOTO LIBRARY)