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Bohr, Niels (Henrik David)

physics atomic nucleus quantum

[baw®] (1885–1962) Danish theoretical physicist: put forward the quantum theory of the electronic structure of atoms.

Bohr’s family was distinguished, his father was professor of physiology at Copenhagen and his younger brother Harald a gifted mathematician. Niels and Harald were both footballers to a professional standard and Niels and his son Aage (born in 1922) both won the Nobel Prize for physics, in 1922 and 1975 respectively.

After Bohr had finished his doctorate at Copenhagen (1911) he spent 8 months in Cambridge with , who was not attracted by Bohr’s ideas on atomic structure, and so he moved to joinat Manchester and spent 4 years there. Rutherford’s model of the atom (1911) envisaged electrons as spread around the central positive nucleus, but according to classical physics this system would be unstable. Bohr countered this difficulty by suggesting that the electron’s orbital angular momentum about the nucleus can only adopt multiples of a certain fixed value, ie it is quantized. Radiation is then only emitted or absorbed when an electron hops from one allowed orbit to another. On this basis Bohr calculated in 1913 what the emission and absorption spectra of atomic hydrogen should be, and found excellent agreement with the observed spectrum as described .

In 1916 Bohr returned to Copenhagen and 2 years later became the first director of its Institute of Theoretical Physics. This became the focal centre for theoretical physics for a generation, in which physicists throughout the world co-operated in developing quantum theory. In that first year Bohr established the ‘correspondence principle’: that a quantum description of microscopic physics must tend to the classical description for larger dimensions.

His ‘complementarity principle’ appeared in 1927: there is no sharp separation between atomic objects and their interaction with the instruments measuring their behaviour. This is in keeping with belief in the equivalence of wave and particle descriptions of matter; uncertainty principle and use of probability waves to describe matter also fit naturally with this principle.

Rutherford’s work had developed nuclear physics to the point by the 1930s where Bohr could apply quantum theory to the nucleus also. This was held to be of neutrons and protons coupled strongly together like molecules in a liquid drop (1936). The very variable response of nuclei to collisions with neutrons of different energies could then be explained in terms of the possible excited states of this ‘liquid drop’. By 1939 Bohr and J A Wheeler (1911–95) had a good theory of nuclear fission and were able to predict that uranium-235 would be a more appropriate isotope for fission (and, as pointed out, an atomic bomb) than uranium-238.

By the autumn of 1943 Bohr was in danger in occupied Denmark (his mother was Jewish) and he chose to escape to Sweden in a fishing boat. He was then flown to England in the bomb-bay of a Mosquito aircraft. Before he left Denmark he dissolved the heavy gold medal of his Nobel Prize in acid; the inconspicuous solution escaped detection in occupied Denmark and was later reduced to metal and the medal was recast from it. After his escape, he joined the atomic bomb programme. In 1944 he lobbied Roosevelt and Churchill on the danger inherent in atomic weapons and the need for agreements between the West and the USSR. This led to his organizing the first Atoms for Peace Conference in Geneva in 1955.

At his death in 1962 Bohr was widely acknowledged as the foremost theoretician of this century after Einstein. The Bohr model of the atom gave a good ‘fit’ with the observed spectra only for the simplest atoms (hydrogen and helium) and it was much modified later, but the concept was a milestone for physics and for chemistry. Similarly, the liquid drop model of the nucleus was to be much developed by others, and notably by Aage Bohr.

Unlike many physicists who have shaped their ideas alone, Bohr refined his ideas in discussions, which often became monologues. He was very popular with his fellow physicists, who produced a 5-yearly Journal of Jocular Physics in his honour to celebrate his birthdays.

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