Bohr, Niels Henrik David (1885–1962)
Danish physicist who was awarded the Nobel Prize for Physics in 1922 for his discovery of the structure of atoms and the radiation emanating from them. He pioneered quantum theory by showing that the nuclei of atoms are surrounded by shells of electrons, each assigned particular sets of quantum numbers according to their orbits. He explained the structure and behaviour of the nucleus, as well as the process of nuclear fission. Bohr also proposed the doctrine of complementarity, the theory that a fundamental particle is neither a wave nor a particle, because these are complementary modes of description.
Quantum theory and atomic structure
Bohr's first model of the atom was developed working in Manchester, England, with Ernest
Rutherford, who had proposed a nuclear theory of atomic structure from his work on the scattering of alpha rays in 1911. It was not, however, understood how electrons could continually orbit the nucleus without radiating energy, as classical physics demanded. In 1913, Bohr developed his theory of atomic structure by applying quantum theory to the observations of radiation emitted by atoms. Ten years earlier, Max
Planck had proposed that radiation is emitted or absorbed by atoms in discrete units, or
quanta, of energy. Bohr postulated that an atom may exist in only a certain number of stable states, each with a certain amount of energy, in which electrons orbit the nucleus without emitting or absorbing energy. He proposed that emission or absorption of energy occurs only with a transition from one stable state to another. When a transition occurs, an electron moving to a higher orbit absorbs energy and an electron moving to a lower orbit emits energy. In so doing, a set number of quanta of energy are emitted or absorbed at a particular frequency.
The liquid-droplet model
In 1939, Bohr proposed his liquid-droplet model for the nucleus, in which nuclear particles are pulled together by short-range forces, similar to the way in which molecules in a drop of liquid are attracted to one another. In the case of uranium, the extra energy produced by the absorption of a neutron causes the nuclear particles to separate into two groups of approximately the same size, thus breaking the nucleus into two smaller nuclei – a process called nuclear fission. The model was vindicated when Bohr correctly predicted the differing behaviour of nuclei of uranium-235 and uranium-238 from the fact that the numbers of neutrons in the two nuclei is odd and even respectively.
| Bohr was born and educated in Copenhagen. In 1911, he went to the UK to study at the Cambridge atomic research laboratory under J J Thomson, but moved in 1912 to Manchester to work with Rutherford. He returned to Denmark as a lecturer at the University of Copenhagen in 1913, where he developed his theory of atomic structure. He moved back to Manchester to take up a lectureship offered by Rutherford, but the authorities in Denmark enticed him back, making him a professor in 1916 and then building the Institute of Theoretical Physics, where he was director from 1920. Leading physicists from all over the world developed Bohr's work at the institute, resulting in the more sophisticated quantum mechanics that more fully explained the behaviour of electrons and other elementary particles. During World War II, he took part in work on the atomic bomb in the USA, after which he became a passionate advocate for the control of nuclear weapons. In 1952, Bohr was instrumental in creating the European Centre for Nuclear Research (CERN), now at Geneva, Switzerland. In addition to his scientific papers, Bohr published three volumes of essays: Atomic Theory and the Description of Nature (1934), Atomic Physics and Human Knowledge (1958), and Essays 1958–1962 on Atomic Physics and Human Knowledge (1963). |
