When a solid material is heated it becomes incandescent, that is, it emits light.  The same thing happens if gases and vapours are heated, or if their atoms or molecules are excited in an electrical discharge, but there is an important difference in the light produced in the two cases.  If light is passed through a prism or diffraction grating, it is split up into its spectrum, the light of different colours of which it is composed.

The spectrum of light emitted from incandescent solids is continuous, with bands of different colours merging into each other.  The spectra of gases and vapours, however, are in the form of discrete lines or bands. These line or band spectra will always be the same for gas atoms or molecules of a given element, and in fact proide a valuable method of identification for elements.  For example, when hydrogen is excited in a gas discharge tube it emits its well-known line spectrum. It was to explain the line spectrum of hydrogen that Niels Bohr put forward his model of the atom, which is still used, with modifications, to describe the atom today.

Niels Henrik David Bohr was born in 1885.  His father was professor of physiology at Copenhagen University.  In 1903 Niels Bohr entered Copenhagen University, and eight years later left with a doctorate degree and a keen interest in the theoretical problems of the atom.

Arriving in England in the autumn of 1911, Niels Bohr began work under J.J. Thomson in the Cavendish laboratory at Cambridge.  At this time the main group of physicists at Cambridge, in common with many physicists in the world, thought the atom to be like a sphere of positively charged electricity, with negatively charged electrons moving about inside the sphere in such numbers that the atom was electrically neutral.

Within a few months Niels Bohr moved to Manchester where Rutherford, one of the leading atomic physicists in the world, was professor of physics.  Rutherford had just produced experimental evidence that the atom has a very small heavy nucleus, which is positively charged, and which is surrounded by electrons, so Niels Bohr now tok over the theoretical problems of Rutherford’s atom in an effort to put forward an atomic model which would agree with the experimental evidence known at the time, particularly the evidence provided by the known spectra of elements.

Niels Bohr’s theory may be summarised in the following two statements:

  1. Electrons can only occupy certain orbits or shells in an atom.  Each orbit represents a definite energy for the electrons in it.
  2. Light is emitted by an atom when an electron jumps from one of its allowed orbits to another.  Since each orbit represents a definite electron energy, this electron jump (or transition) represents a definite energy ‘jump’.  This change in electron energy leads to emission of light of a definite energy or wavelength.

When his new model of the atom was applied to hydrogen, the simplest atom with one electron orbiting the nucleus, it was found to give, theoretically, exactly the same line spectrum as was known to exist experimentally.  Nine years later, in 1922, Niels Bohr received the Nobel Prize in physics for his explanation of the hydrogen line spectrum by use of his atomic model.

The importance of Niels Bohr’s revolutionary new atomic model can hardly be exaggerated, and his explanation of the hydrogen spectrum has been described as one of the greatest triumphs in physics.  From 1913, with little correction until the mid-1920’s, the Bohr model was fundamental to all thinking in terms of the atom.

In 1920 the Institute of Theoretical Physics at Copenhagen was founded with Niels Bohr as its head, and has remained in the forefront of nuclear physics theory ever since.  Leading physicists from all over the world made visits to Copenhagen to discuss with Niels Bohr the modern modifications of his theories (which are mostly mathematical, and very complicated).

Niels Bohr, the unassuming Danish physicist, who was born before radioactivity was discovered and who became a leading authority on the fundamental laws of the atom as they are now appreciated, died on November 18th, 1962.

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