Niels Henrik David Bohr
1885-1962
Danish Physicist
Niels Bohr was the first to apply quantum theory in a consistent model to explain the arrangement of electrons in the atom. Bohr's model accounted for the chemical properties of the elements and for the main features in their spectra. He was awarded the Nobel Prize in Physics for this achievement in 1922.
Bohr was born in Copenhagen on October 7, 1885. After receiving his doctorate from the University of Copenhagen in 1911, he continued his education at Cambridge University under J.J. Thomson (1856-1940), the discoverer of the electron. Next he spent a year at the University of Manchester, working with Ernest Rutherford (1871-1937) just as the British physicist was discovering the atomic nucleus.
While the importance of Rutherford's work was enormous, there were a few major problems with his model of negatively charged electrons orbiting a positively charged nucleus. According to classical physics, charges orbiting in an electrostatic field should continually give off electromagnetic radiation, thus losing energy. Eventually they would spiral inwards, lacking sufficient kinetic energy to counter their attraction to the nucleus. In addition, the spectrum of light emitted from or absorbedby the atoms of an element should be a smooth continuum in this model; however, in reality the spectra showed distinct lines.
The quantum theory of the German physicist Max Planck (1858-1947) illuminated Bohr's thoughts on these problems. In 1900, Planck had put forth the idea that oscillating charges emitted and absorbed energy in discrete units called quanta. The energy of the quantum of light, or photon, was proportional to the frequency of the radiation.
In 1913, Bohr published a series of papers in which he applied quantum theory to Rutherford's atomic model. He assumed that electrons moved around atomic nuclei in certain stable orbits in which no energy was lost. Radiation was emitted or absorbed only when an electron moved from one orbit to another. The energy difference between two orbits corresponded to the energy, and thus the frequency, of the photon that was emitted or absorbed in the transition. Such transitions explained the emission and absorption lines that were seen in atomic spectra.
Bohr returned to the University of Copenhagen as a professor in 1916. Under his influence, an Institute for Theoretical Physics was established there in 1920, becoming one of the world's premier research centers. Bohr served as director of the Institute, as well as president of the Royal Danish Academy of Science. In 1922 he was awarded the Nobel Prize, and in 1932 took up residence in the "House of Honor," a mansion Denmark reserved for its most esteemed citizen. He continued his research on quantum mechanics and atomic structure throughout the 1930s, as the specter of Nazism began to cast its shadow over Europe.
Denmark was overrun by the Germans in 1940. Like many Danish intellectuals, Bohr, who was of partial Jewish descent, was involved in the resistance movement. He wrote openly about his views and attempted to protect Jewish scientists in his Institute. The situation became increasingly dangerous for him. As an atomic physicist, he risked being interned to work on Germany's weapons efforts, and he viewed with horror the prospect of the megalomaniac Hitler armed with an atomic bomb. Finally, after repeated urgings by colleagues and diplomats, he fled Denmark in 1943. In Los Alamos, New Mexico, he served as an advisor to the Manhattan Project, assisting in the U.S. atomic weapons effort. He understood it as a hedge against the Nazis and a way to deter future wars.
Niels Bohr. (Library of Congress. Reproduced with permission.)Bohr returned to Copenhagen at the end of the war in Europe. After the U.S. forced the Japanese to surrender by dropping atomic bombs on Hiroshima and Nagasaki, he devoted much of the rest of his life to promoting international cooperation and peaceful uses for atomic energy. Bohr was awarded the first Atoms for Peace award in 1957. He died in Copenhagen on November 16,1962.
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