Bohr, Niels
NAME: Bohr, Niels Henrik David
ACHIEVEMENT(S): Using the quantum theory, established the structure of
the atom. Nobel Prize in physics
NICKNAME/ALIAS: None
DATE AND PLACE OF BIRTH: October 7, 1885, Copenhagen, Denmark
DATE/PLACE OF DEATH/AGE AT DEATH: November 18, 1962, Copenhagen,
Denmark. Aged 77
DISCIPLINE: Physicist
FIELD: Quantum physics
LANGUAGE: Danish, English
PARENTS: Christian Bohr; Ellen Adler
SIBLINGS: 2
SPOUSE: Margrethe Norlund
CHILDREN: Hans; Erik; Aage Niels (physicist and Nobel laureate);
Ernest
EDUCATION: Gammelholm School, Copenhagen; University of Copenhagen
MAJOR WORK: "The Structure of the Atoms and the Physical and Chemical
Properties of the Elements"
LIFE AND TIMES:
Immediately after getting his doctorate, Bohr travelled to England to continue his research in electron theory at the Cavendish Laboratory at Cambridge University. Bohr hoped that the director of the Laboratory, English physicist J.J. Thomson*, the discoverer of the electron, would appreciate his doctoral thesis, which he had painstakingly translated into English.
He was appalled to discover that Thomson had lost interest in the subject and failed to realize the importance of Bohr's thesis. Soon after, Bohr met British physicist Rutherford*, then at the University of Manchester.
In early 1912, moved to Manchester to spend three months working on Rutherford's nuclear model of the atom. Bohr began to develop a model of the atom in which electrons were disposed in rings around the nucleus -- a first step towards an explanation of atomic structure.
Rutherford had discovered the atomic nucleus in 1911 from his work on alpha rays. But his discovery created a profound problem for theorists. The existence of the nucleus meant electrons must be circling it in orbits somewhat similar to those traveled by planets around the sun; but according to classical laws of electrodynamics, an electrically-charged particle would continuously radiate energy as it travelled in such an orbit around the nucleus, and, over time, would spiral ever closer to the nucleus and eventually collide with it. As this didn’t happen, electrons orbiting the nucleus could not be behaving in accordance with classical laws.
To confront this problem, Bohr began with the question of atomic spectra. For more than a century, it had been known that light from the heating of an element produced characteristics line in the spectrum of the light seen through a prism. The specific pattern of the lines was unique for each element.
Although a great deal of research had been done on spectral lines, their relationship with atoms ( the building blocks of elements) had not been considered. Bohr came up with a possible solution to the problem: he hypothesized that certain orbits existed within an atom in which an electron could travel without radiating energy. The frequency of spectral lines corresponded to the energy released only when an electron moved from one orbit to another.
Departing radically from classical physics, Bohr postulated that: 1) any atom could exist only in a specific number of stable states, each with a definite energy value, and that the emission or absorption of energy can occur only with a transition from one stable state to another; 2) electrons normally orbit the nucleus without emitting or absorbing energy, but when a transition occurs, an electron moves to a lower or higher orbit depending on whether it emits or absorbs energy; and 3) in so doing, a set number of “quanta” (amounts) of energy are emitted or absorbed at a particular frequency.
So, with the aid of Planck's* quantum theory of electromagnetic radiation (1900, which proposed that radiation is emitted or absorbed by atoms in stepped “quanta” of energy, Bohr was able to explain why elements emit radiation at precise frequencies that give set patterns of spectral lines.
Developing his ideas further, Bohr showed that the nuclei of atoms were surrounded by shells of electrons, each assigned particular sets of “quantum” numbers according to their orbits. Bohr used his theory to determine the frequencies of spectral lines produced by elements, and succeeded brilliantly.
The theory also enabled him to explain the groups of Mendeyelev’s* Periodic Table in terms of elements with similar electron structures (an element's atomic number indicating not only its position in the periodic table, but also the number of natural units of electric charge on the nuclei of its atoms).
In 1912, Bohr returned to Copenhagen, where in 1913 he published in 1913 his theory of the "planetary atom.” But many physicists were not persuaded. They argued that it was completely at odds with classical physics, and Bohr's use of a classical model to explain the structure and behavior of the nucleus, made the theory even more illogical.
But Rutherford was totally on Bohr’s side and considered him a genius. He offered Bohr a job as lecturer in physics at Manchester. Bohr worked at Manchester from 1914 to 1916, but returned to the University of Copenhagen, where a chair of theoretical physics had been created especially for him.
Within a few years, he helped plan the University of Copenhagen's new Institute for Theoretical Physics, which opened in 1921. He served as its director for the next 40 years. The Copenhagen Institute rapidly became a center for theoretical physicists throughout the world, and such figures as Pauli*, Gamow* and Heisenberg* developed Bohr's work there, resulting in the theories of quantum and wave mechanics that more fully explain the behavior of electrons within atoms.
In 1922, the brilliant success of his work on atomic structure earned him the Nobel Prize for physics. During the 1920s and 1930s, Bohr wrote extensively about atomic structures, proposing two concepts that he considered fundamental to the "new physics".
The first was the principle of “complementarity” that stated there may be more than one true and accurate way to view natural phenomena. The best example was the wave-particle duality discovered in the 1930s, in which particles were found to have wavelike
characteristics, and waves to have particle-like properties. Bohr argued that the two parts of the duality may appear to be inconsistent, or even in conflict, but that both are necessary to obtain a complete view of particles and waves.
The second concept, the principle of “correspondence”, suggested that the laws of classical physics could be preserved in light of the new quantum physics. Bohr stated that though it is true that quantum mechanics and relativity are essential to an understanding of phenomena on the atomic scale, any conclusion drawn from these principles must not conflict with the way the everyday world behaved.
The most famous and most outspoken anti-Bohr, however, was German/American physicist Einstein*, who never accepted Bohr's claim that quantum mechanics was the “rational generalization of classical physics”, necessary for the understanding of atomic phenomena.
During the 30s, Bohr turned to a new, but related, topic: the composition of the atomic nucleus. By 1934, it had been established that the nucleus consisted of two kinds of particles, protons and neutrons. But it was unclear how those particles were arranged within the nucleus, and how the nucleus was shaped.
Bohr theorized that the nucleus could be likened to a liquid droplet, and that the forces that pull together, or attract, protons and neutrons, could be compared to the forces that operate between the molecules that make up the droplet, which was constantly oscillating and changing shape in response to its internal forces.
The most important aspect of the Bohr liquid-drop model was its later ability to explain the process of nuclear fission (the splitting of a heavy nucleus into two parts, almost equal in mass, following the absorption of a neutron, with the release of a tremendous amount of nuclear energy), discovered in 1938 by Otto Hahn, Fritz Strassmann and
Lise Meitner.
The liquid-drop model was vindicated when Bohr correctly predicted that the differing behavior of the nuclei of uranium-235 and uranium-238 was due to the fact that the number of neutrons in each nucleus was odd and even respectively.
Bohr continued on at the Copenhagen institute during the early years of the Second World War, helping many scientists escape from Nazi Germany. But in September, 1943, after receiving news that his own life was in danger, he and his family were spirited to Sweden by the Danish resistance movement.
A few days later, the British government sent an unarmed Mosquito bomber to Sweden to pick up Bohr and fly him to England. For a few months, Bohr and his son Aage helped in a British project to make a nuclear fission bomb, then moved to Los Alamos, New Mexico, with a British research team, to work on the Manhattan Project, whose aim was to build the world's first atomic bomb.
After the War, Bohr, like many other Manhattan researchers -- most notably Oppenheimer* (but in direct conflict with fellow nuclear physicist Edward Teller*) -- became activists in trying to keep atomic weapons out of the control of the military and under close civilian supervision.
In 1944, Bohr tried to persuade British prime minister Churchill* and U.S. President Franklin D. Roosevelt* of the need for international cooperation in establishing a free exchange of people and ideas so that nuclear weapons could be controlled.
Although his appeal did not succeed, Bohr continued to take the lead in promoting such efforts as the First International Conference on the Peaceful Uses of Atomic Energy, held in Geneva (1955), and in helping to create the European Council for Nuclear Research. He retired as professor of physics from the University of Copenhagen in 1955, but continued to serve as director of the Institute until his death in Copenhagen in 1962.
ASSESSMENT: His quantum mechanical model of the atom was the key step in scientific understanding of atomic structure.
EXTRA CONNECTIONS: His work related to that of Fermi*, Born* and Dirac*
He influenced Kastler*
He attended the funeral of philologist Jespersen*
ONELINER: Niels Bohr, who discovered the structure of the atom
BIBLIOGRAPHY:
EB, DSB, Collier's Encyclopaedia
Abbot, David (ed.), “Physicists”: The Biographical Dictionary of Scientists, London: Blond Educational, 1984
Newton, David E., "Niels Bohr", in Notable Twentieth Century Scientists, ed. Emily J. McMurray, New York & London: Gale Research Inc., 1995