Niels Henrik David Bohr Biography

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Bohr was born in Copenhagen on October 7, 1885. He was the son of Christian Bohr, a professor of physiology at the University of Copenhagen. Niels entered the university in 1903 and studied there until he received his doctorate in 1911. He is reputed to have been not only a brilliant student, but also an accomplished soccer player.

After graduation, Bohr traveled to England to work first at the Cavendish laboratory, under Joseph J. Thomson, and later at the University of Manchester, under Ernest Rutherford. He remained at Manchester until 1916, when he returned to Copenhagen as professor of physics.

During his tenure at Manchester, Bohr developed his theory of the electronic structure of the hydrogen atom, the accomplishment for which he is most famous. In 1911 Rutherford proposed a new model of the atom, one in which all of the atom's positive charge was concentrated at its center, the nucleus. The electron s in Rutherford's model were located at relatively great distances outside the nucleus, traveling in orbits around it.

One obvious problem with Rutherford's model related to the electrons. According to classical physical theory, an accelerating charged particle ought to radiate energy. Electrons orbiting around the nucleus should, therefore, gradually lose energy and slowly spiral into the nucleus. Rutherford was unable to explain how this would not occur in his model of the atom.

Bohr saw a possible solution in the quantum theory proposed in 1900 by Max Planck. In his study of black-body radiation, Planck concluded that, at least in some cases, energy could be emitted not in continuous waves, but in discrete packages that he called quanta. The energy of one quantum depended

only on the frequency, as expressed by the equation

E = , where is a proportionality constant, now known as Planck's constant.

Bohr adapted Planck's theory to Rutherford's model of the atom by suggesting that electrons could travel around the nucleus without radiating energy provided that they remained in certain restricted orbits. Further, he proposed that an electron could move from one orbit to another by gaining or losing one or more quanta of energy.

The energy change involved in such a process can easily be calculated. If the electron goes from an orbit with energy E₁ to one with energy E₂, for example, the energy change is equal to the quantum gained or lost in the process. In other words,

E₁ - E₂ =

Solving this equation for , the frequency of energy gained or lost, gives

E₁ - E₂ = --------------

From this equation Bohr could calculate the frequency of radiation to be expected when electrons made various possible transfers in the atom.

These frequencies were well known for the hydrogen atom (and other atoms as well. In 1885, Johann Balmer (1825-1898) had devised a formula for the wavelengths (the inverse of the frequencies) of the lines in the hydrogen spectrum. Balmer had no theoretical basis for his theory, but worked entirely from experimental data.

Bohr's model provided the theoretical basis needed to explain Balmer's equations. The frequencies predicted from the Bohr model corresponded closely to those calculated from Balmer's equations.

Bohr's model was far from complete. First, he had no theoretical reason for inventing quantized orbits. They were simply a device for explaining the apparent existence of electronic orbit. The device worked brilliantly although Bohr had no theoretical reason for creating them. This problem was later solved with Louis de Broglie 's suggestion of the wave nature of the electron. The quantized orbits turn out to be regions in which standing electron waves can exist. Also, Bohr's model failed to explain spectra for atoms more complex than that of hydrogen.

In addition, Bohr's model placed electrons in circular orbits. In 1916 Arnold Sommerfeld (1868-1951) showed that the Bohr model could be made more precise by applying relativity theory. As a result of his calculations, Sommerfeld placed electrons in elliptical orbits, rather than circular orbits, outside the nucleus.

The great success of the Bohr model was its ability to solve the problem of electron motion in the atom. He showed how the introduction of newly devised quantum theory could be applied to a problem in classical physics. For his accomplishment, Bohr was awarded the Nobel Prize for physics for 1922.

Bohr thought, wrote and spoke further about the apparent conflicts between classical and quantum physics. In 1927, he introduced the idea of complimentarity in dealing with physical phenomena. According to this principle, a light wave sometimes demonstrates the properties of a wave and sometimes the properties of a particle. Light can not be completely understood, Bohr said, exclusively in terms of either property. The same can be said of electrons and other particles and of other forms of energy.

Bohr also proposed another fundamental idea, that of convergence. He said that, under certain circumstances, the laws of quantum physics gradually merge into those of classical physics. The issue is not so much a matter of conflict between the two theories, then, as it is a matter of scale at which they are appropriate.

In the 1930s, Bohr turned his attention to the field of nuclear physics. Among his accomplishments in this area was the development of the "liquid drop" model of the nucleus. Bohr suggested that the protons and neutrons that make up the atomic nucleus could be viewed as similar to the molecules that make up a tiny drop of water. The fission of the nucleus, then, can be viewed as similar to the breaking apart of a liquid droplet.

World War II brought major changes to Bohr's life. He stayed in Copenhagen for the first four years of the war, helping colleagues to escape from Denmark. Then, in 1943, he and his family fled to Sweden on a crowded fishing boat. From there, they were flown to England in an airplane with no oxygen system, a trip that nearly cost Bohr his life.

Bohr brought with him to England news of progress in nuclear fission research in Europe and soon became an important member of the United States Manhattan Project. After the war, he became chairman of the Danish Atomic Energy Commission and a founding member of the European Center for Nuclear Research (CERN) in Geneva. Throughout this period, he worked with great passion to bring under control the dangers posed by nuclear weapons.

In some ways, Bohr's greatest achievement may have been his role at the Institute for Theoretical Physics in Copenhagen. From the time he became director in 1920, the Institute became a Mecca for theoretical physicists from every part of the world. Most of the great discoveries in physics of the 1920s and 1930s can be traced to individuals who, at one time or another, worked with Bohr at the Institute.

Bohr continued to lead quite an active life. He conducted a meeting of the Danish Royal Academy of Sciences only two days before his death on November 18, 1962.

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