Bohr model In September 1911, Bohr, supported by a fellowship from the
Carlsberg Foundation, travelled to England, where most of the theoretical work on the structure of atoms and molecules was being done. He met
J. J. Thomson of the
Cavendish Laboratory and
Trinity College, Cambridge. He attended lectures on
electromagnetism given by
James Jeans and
Joseph Larmor, and did some research on
cathode rays, but failed to impress Thomson. He had more success with younger physicists like the Australian
William Lawrence Bragg, and New Zealand's
Ernest Rutherford, whose 1911 small central nucleus
Rutherford model of the
atom had challenged Thomson's 1904
plum pudding model. Bohr received an invitation from Rutherford to conduct post-doctoral work at
Victoria University of Manchester, where Bohr met
George de Hevesy and
Charles Galton Darwin (whom Bohr referred to as "the grandson of the
real Darwin"). Bohr returned to Denmark in July 1912 for his wedding, and travelled around England and Scotland on his honeymoon. On his return, he became a
Privatdocent at the University of Copenhagen, giving lectures on
thermodynamics.
Martin Knudsen put Bohr's name forward for a
docent, which was approved in July 1913, and Bohr then began teaching medical students. His three papers, which later became famous as "the trilogy", were published in
Philosophical Magazine in July, September and November of that year. He adapted Rutherford's nuclear structure to
Max Planck's quantum theory and so created his
Bohr model of the atom. he advanced the theory of electrons travelling in
orbits of quantised "stationary states" around the atom's nucleus in order to stabilise the atom, but it wasn't until his 1921 paper that he showed that the chemical properties of each element were largely determined by the number of electrons in the outer orbits of its atoms. He introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a
quantum of discrete energy. This became a basis for what is now known as the
old quantum theory. of the
hydrogen atom. A negatively charged electron, confined to an
atomic orbital, orbits a small, positively charged nucleus; a quantum jump between orbits is accompanied by an emitted or absorbed amount of
electromagnetic radiation.|alt=Diagram showing electrons with circular orbits around the nucleus labelled n=1, 2 and 3. An electron drops from 3 to 2, producing radiation delta E = hv s in the 20th century:
Thomson,
Rutherford,
Bohr,
Heisenberg/Schrödinger In 1885,
Johann Balmer had come up with his
Balmer series to describe the visible
spectral lines of a
hydrogen atom: :\frac{1}{\lambda} = R_\mathrm{H}\left(\frac{1}{2^2} - \frac{1}{n^2}\right) \quad \text{for} \ n=3,4,5,... where λ is the wavelength of the absorbed or emitted light and
RH is the
Rydberg constant. Balmer's formula was corroborated by the discovery of additional spectral lines, but for thirty years, no one could explain why it worked. In the first paper of his trilogy, Bohr was able to derive it from his model: : R_Z = { 2\pi^2 m_e Z^2 e^4 \over h^3 } where
me is the electron's mass,
e is its charge,
h is the
Planck constant and
Z is the atom's
atomic number (1 for hydrogen). The model's first hurdle was the
Pickering series, lines that did not fit Balmer's formula. When challenged on this by
Alfred Fowler, Bohr replied that they were caused by
ionised helium, helium atoms with only one electron. The Bohr model was found to work for such ions. Many older physicists, like Thomson, Rayleigh and
Hendrik Lorentz, did not like the trilogy, but the younger generation, including Rutherford,
David Hilbert,
Albert Einstein,
Enrico Fermi,
Max Born and
Arnold Sommerfeld saw it as a breakthrough. Einstein called Bohr's model "the highest form of musicality in the sphere of thought." The trilogy's acceptance was entirely due to its ability to explain phenomena that stymied other models, and to predict results that were subsequently verified by experiments. Today, the Bohr model of the atom has been superseded, but is still the best known model of the atom, as it often appears in high school physics and chemistry texts. Bohr did not enjoy teaching medical students. He later admitted that he was not a good lecturer, because he needed a balance between clarity and truth, between "Klarheit und Wahrheit". He decided to return to Manchester, where Rutherford had offered him a job as a
reader in place of Darwin, whose tenure had expired. Bohr accepted. He took a leave of absence from the University of Copenhagen, which he started by taking a holiday in
Tyrol with his brother Harald and aunt
Hanna Adler. There, he visited the
University of Göttingen and the
Ludwig-Maximilians-Universität München, where he met Sommerfeld and conducted seminars on the trilogy. The First World War broke out while they were in Tyrol, greatly complicating the trip back to Denmark and Bohr's subsequent voyage with Margrethe to England, where he arrived in October 1914. They stayed until July 1916, by which time he had been appointed to the Chair of Theoretical Physics at the University of Copenhagen, a position created especially for him. His docentship was abolished at the same time, so he still had to teach physics to medical students. New professors were formally introduced to King
Christian X, who expressed his delight at meeting such a famous football player.
Institute of Theoretical Physics In April 1917, Bohr began a campaign to establish an Institute of Theoretical Physics. He gained the support of the Danish government and the Carlsberg Foundation, and sizeable contributions were also made by industry and private donors, many of them Jewish. Legislation establishing the institute was passed in November 1918. Now known as the
Niels Bohr Institute, it opened on 3 March 1921, with Bohr as its director. His family moved into an apartment on the first floor. Bohr's institute served as a focal point for researchers into
quantum mechanics and related subjects in the 1920s and 1930s, when most of the world's best-known theoretical physicists spent some time in his company. Early arrivals included
Hans Kramers from the Netherlands,
Oskar Klein from Sweden, George de Hevesy from Hungary,
Wojciech Rubinowicz from Poland, and
Svein Rosseland from Norway. Bohr became widely appreciated as their congenial host and eminent colleague. Klein and Rosseland produced the institute's first publication even before it opened. , part of the
University of Copenhagen.|alt=A block-shaped beige building with a sloped, red tiled roof The Bohr model worked well for hydrogen and ionized single-electron helium, which impressed Einstein but could not explain more complex elements. By 1919, Bohr was moving away from the idea that electrons orbited the nucleus and developed
heuristics to describe them. The
rare-earth elements posed a particular classification problem for chemists because they were so chemically similar. An important development came in 1924 with
Wolfgang Pauli's discovery of the
Pauli exclusion principle, which put Bohr's models on a firm theoretical footing. Bohr was then able to declare that the as-yet-undiscovered element 72 was not a rare-earth element but an element with chemical properties similar to those of
zirconium. (Elements had been predicted and discovered since 1871 by chemical properties), and Bohr was immediately challenged by the French chemist
Georges Urbain, who claimed to have discovered a rare-earth element 72, which he called "celtium". At the Institute in Copenhagen,
Dirk Coster and George de Hevesy took up the challenge of proving Bohr right and Urbain wrong. Starting with a clear idea of the chemical properties of the unknown element greatly simplified the search process. They went through samples from Copenhagen's Museum of Mineralogy looking for a zirconium-like element and soon found it. The element, which they named
hafnium (
hafnia being the Latin name for Copenhagen), turned out to be more common than gold. The
Bohr Festival () was a series of seven lectures given by Bohr from 12 to 22 June 1922 at the Institute of Theoretical Physics in
Göttingen. These were the Wolfskehl Lectures, funded by the Wolfskehl Foundation. Taking place in the fortnight leading up to the
Göttingen International Handel Festival, it became known as the Bohr Festival. In 1991,
Friedrich Hund suggested that
James Franck was responsible for the comparison. Modelling atomic behaviour under incident electromagnetic radiation using "virtual oscillators" at the absorption and emission frequencies, rather than the (different) apparent frequencies of the Bohr orbits, led Max Born,
Werner Heisenberg and Kramers to explore different mathematical models. They led to the development of
matrix mechanics, the first form of modern
quantum mechanics. The BKS theory also generated discussion of, and renewed attention to, difficulties in the foundations of the old quantum theory. The most provocative element of BKS – that momentum and energy would not necessarily be conserved in each interaction, but only statistically – was soon shown to be in conflict with experiments conducted by
Walther Bothe and
Hans Geiger. In light of these results, Bohr informed Darwin that "there is nothing else to do than to give our revolutionary efforts as honourable a funeral as possible".
Quantum mechanics The introduction of
spin by
George Uhlenbeck and
Samuel Goudsmit in November 1925 was a milestone. The next month, Bohr travelled to
Leiden to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate. When his train stopped in
Hamburg, he was met by Wolfgang Pauli and
Otto Stern, who asked for his opinion of the spin theory. Bohr pointed out that he had concerns about the interaction between electrons and magnetic fields. When he arrived in Leiden,
Paul Ehrenfest and Albert Einstein informed Bohr that Einstein had resolved this problem using
relativity. Bohr then had Uhlenbeck and Goudsmit incorporate this into their paper. Thus, when he met Werner Heisenberg and
Pascual Jordan in
Göttingen on the way back, he had become, in his own words, "a prophet of the electron magnet gospel". Heisenberg first came to Copenhagen in 1924, then returned to Göttingen in June 1925, shortly thereafter developing the mathematical foundations of quantum mechanics. When he showed his results to Max Born in Göttingen, Born realised that they could best be expressed using
matrices. This work attracted the attention of the British physicist
Paul Dirac, who came to Copenhagen for six months in September 1926. Austrian physicist
Erwin Schrödinger also visited in 1926. His attempt at explaining quantum physics in classical terms using wave mechanics impressed Bohr, who believed it contributed "so much to mathematical clarity and simplicity that it represents a gigantic advance over all previous forms of quantum mechanics". When Kramers left the institute in 1926 to take up a chair as professor of theoretical physics at the
Utrecht University, Bohr arranged for Heisenberg to return and take Kramers's place as a
lektor at the University of Copenhagen. Heisenberg worked in Copenhagen as a university lecturer and assistant to Bohr from 1926 to 1927. Bohr became convinced that light behaved like both waves and particles and, in 1927, experiments confirmed the
de Broglie hypothesis that matter (like electrons) also behaved like waves. He conceived the philosophical principle of
complementarity: that items could have apparently mutually exclusive properties, such as being a wave or a stream of particles, depending on the experimental framework. He felt that it was not fully understood by professional philosophers. In February 1927, Heisenberg developed the first version of the
uncertainty principle, presenting it using a
thought experiment where an electron was observed through a
gamma-ray microscope. Bohr was dissatisfied with Heisenberg's argument, since it required only that a measurement disturb properties that already existed, rather than the more radical idea that the electron's properties could not be discussed at all apart from the context they were measured in. In a paper presented at the
Como Conference in September 1927, Bohr emphasised that Heisenberg's uncertainty relations could be derived from classical considerations about the resolving power of optical instruments. Understanding the true meaning of complementarity would, Bohr believed, require "closer investigation". Einstein preferred the determinism of classical physics over the probabilistic new quantum physics to which he himself had contributed. Philosophical issues that arose from the novel aspects of quantum mechanics became widely celebrated subjects of discussion. Einstein and Bohr had
good-natured arguments over such issues throughout their lives. In 1914,
Carl Jacobsen, the heir to
Carlsberg breweries, bequeathed his mansion (the Carlsberg Honorary Residence, currently known as Carlsberg Academy) to be used for life by the Dane who had made the most prominent contribution to science, literature or the arts, as an honorary residence (). Harald Høffding had been the first occupant, and upon his death in July 1931, the Royal Danish Academy of Sciences and Letters gave Bohr occupancy. He and his family moved there in 1932. He was elected president of the Academy on 17 March 1939. By 1929, the phenomenon of
beta decay prompted Bohr to again suggest that the
law of conservation of energy be abandoned, but
Wolfgang Pauli's hypothetical
neutrino and the subsequent 1932 discovery of the
neutron provided another explanation. This prompted Bohr to create a new theory of the
compound nucleus in 1936, which explained how neutrons could be captured by the nucleus. In this model, the nucleus could be deformed like a drop of liquid. He worked on this with a new collaborator, the Danish physicist Fritz Kalckar, who died suddenly in 1938. The
discovery of nuclear fission by
Otto Hahn in December 1938 (and its theoretical explanation by
Lise Meitner) generated intense interest among physicists. Bohr brought the news to the United States where he opened the fifth
Washington Conference on Theoretical Physics with Fermi on 26 January 1939. When Bohr told
George Placzek that this resolved all the mysteries of
transuranic elements, Placzek told him that one remained: the neutron capture energies of uranium did not match those of its decay. Bohr thought about it for a few minutes and then announced to Placzek,
Léon Rosenfeld and
John Wheeler that "I have understood everything." Based on his
liquid drop model of the nucleus, Bohr concluded that it was the
uranium-235 isotope and not the more abundant
uranium-238 that was primarily responsible for fission with thermal neutrons. In April 1940,
John R. Dunning demonstrated that Bohr was correct. In the meantime, Bohr and Wheeler developed a theoretical treatment, which they published in a September 1939 paper on "The Mechanism of Nuclear Fission". == Philosophy ==