Rutherford-Bohr theory of atomic structur

At a symposium on Ultra-Violet Light and X-Rays, held at the meeting of the American Association for the Advancement of Science at St. Louis in December 1919,1 I presented a set of computations of the K critical absorption frequencies based on the Rutherford-Bohr theory of atomic structure and the mechanism of radiation. The computed values equalled the observed values to within one or two per cent. In these computations the electrons were supposed to revolve in orbits which lay in planes passing through the nucleus of the atom. 

The nuclear theory of atomic structure, put forward by Rutherford, regarded the electrons as moving in orbits round the nucleus. The dynamical theory of this system was developed by Bohr, who found it necessary to supplement classical mechanics by the quantum mechanics of Planck. According to classical theory, a system consisting of an electron moving in a circular orbit round a nucleus, to which it is attracted according to Coulomb s law, would lose energy, with the result that the electron would approach and finally collide with the nucleus. Thus on the basis of classical theory, the Rutherford atom would only be stable for about io seconds, after which time the electron would have fallen into the nucleus. 

Atoms and Spectra. We started our discussion by a study of the structure of the atom based on chemical properties and on the indications of the periodic table (Chapter I), and we referred to Rutherford s original theory of the nuclear atom and to the modifications which were applied by Bohr and Sommer-feld in order to keep the chemical theories in line with contemporary physics. Since the study of the physics of atomic-structure probably owes more to spectrum analysis than to any other method of approach we must now deal briefly with the physical interpretation of spectra. 

It was a time of great excitement and ferment in science, and Rutherford s laboratory was one of the epicenters of discovery in atomic physics. The first coherent theory of the structure of the atom was just then being developed by Rutherford and his research group, which, besides Moseley, included Niels Bohr, Hans Geiger, Kasimir Eajans, and others. 

Quantum theory has provided a picture of the atom that allows an immediate approach to valency and molecular structure while furnishing a far more secure basis for the periodic arrangement of the elements than selected physical and chemical properties. This is not to denigrate the achievements of Thomson, Rutherford and Bohr in atomic structure, or of Newlands, Lothar Meyer, Mendeleeff, Bohr and others in evolving the Periodic Law, but rather to seize upon developments which their prescient work made possible. Readers will find it easier to assimilate and remember the facts embodied in the Periodic Classification when these are seen to emerge from the systematic development of electronic structure with increasing atomic number. 

The idea of energy quantization weis brought into chemistry with the application of quantum theory to the electronic structure of atoms in 1913 by the Danish physicist Niels Bohr (1885-1962, 1922 Nobel laureate in Physics). At the time, Bohr was working in the laboratory of the New Zealand physidst Ernest Rutherford (1871-1937, 1909 Nobel laureate in Chemistry) in England, a short time after the nuclear structure for the atom had been established by Rutherford and his co-workers. Classical electromagnetic theory predicted that the electrons around the nucleus. 

One consequence of the discovery of the structure of atoms through the work of Rutherford was that the concept of chemical bonding became a subject of theoretical consideration. In Bohr s theory, the most loosely bound electrons, the valence electrons, are responsible for chemical bonding. Before quantum mechanics was completed, Kossel, Lewis, and Langmuir invented a phenomenological electronic concept, the electron pair bond, corresponding to the dash used previously to indicate a covalent bond between atoms. 

The spectrum emitted by an atom presumably is related to the structure of the atom. Until 1913, attempts to relate the spectrum to a definite atomic model were unsuccessful. By 1913 it was known that the atom had a positively charged nucleus, but the nuclear model of Rutherford was unstable according to classical electromagnetic theory. This Gordian knot was cut by Niels Bohr in 1913. 

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