Bohr theory of atomic structure

Moseley photographed characteristic spectra for some 38 elements that could serve as x-ray tube targets. In two papers,37 he not only uncovered structure in the K and L spectra—he alscr established the atomic number as more fundamental than the atomic weight, and he provided brilliant support for- the Bohr theory of atomic structure. 

The carbon atom has, outside its nucleus, six electrons which, on the Bohr theory of atomic structure, were believed to be arranged in orbits at increasing distance from the nucleus. These orbits corres-... 

The Bohr theory of atomic structure allotted to each extra-nuclear electron within the atom a definite geometrical orbit and, more important, associated with each orbit a fixed total energy value. 

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. 

Bohr theory the first theory of atomic structure which involved definite internal energy levels for electrons. 

In 1920 Bohr turned his attention to the problem of atomic structure. Matters had become somewhat more complicated than they were in Mendeleev s day. By 1920, 14 elements had been discovered that did not seem to follow Mendeleev s periodic law. Called the rare earths, they had similar properties and followed one another in the table of elements they were elements 58 through 71. When Mendeleev formulated his law only two had been discovered, so they didn t seem to present any great problem. But now they presented an anomaly that no one had been able to clear up. A workable theory of atomic structure would have to explain not only why periodicities were seen in the larger part of the table of the elements but also why they disappeared when one came to the rare earths. 

On the basis of his quantum theory of atomic structure, Niels Bohr believed that, since Urbain s celtium had been obtained from the rare earths, it could not be element 72, for the latter must be quadrivalent rather than trivalent and must belong to the zirconium family. He showed that the chemical properties of an atom are determined by the number and arrangement of the electrons within it and especially by the number... 

Figure 8. A Bohr-Sommerfeld model of the xenon atom. (From H. A. Kramers and H. Horst, The Atom and the Bohr Theory of its Structure, 1924).

This was the state of our knowledge of the structure of the atom when Langmuir, the modern scientific conquistador, attempted to invade the tiny world of the atom. There was an unmistakable conflict between Bohr s theory of the hydrogen atom and the conception of Lewis. Chemists could see but little use in the Bohr atom. They wanted an atom which would explain chemical reactions. The first World War over, Langmuir undertook to reconcile the two theories by publishing his concentric shell theory of atomic structure. 

Bohr s importance can be attributed first and foremost to his model of the hydrogen atom. In a series of three papers, now called the Trilogy, Bohr laid the foundation for a quantum theory of atomic structure. The Trilogy was published in Phiksophical Magazine in 1913 and these three papers established Bohr s reputation. ... 

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. 

The experimental foundation of the quantum theory of atomic structure as put forward by Bohr, lies in the stability of the atom and in the existence of discrete energy levels and the ability of the atom to absorb and emit energy only in quanta, as demonstrated by the discontinuous nature of atomic spectra and by the critical potential measurements of Franck and Hertz. Bohr postulated that the atom could only exist in a limited number of orbits or stationary states, which were defined by the quantum condition that the angular momentum can assume only certain limited values which are given by the expressioiT ... 

The final aim of a theory of atomic structure must be to construct the whole periodic system of the elements from an atom model. Bohr had already made attempts in this direction in his earlier works. He made use of ring models, in which the individual electrons were situated at the comers of concentric regular polygons (the rings ). A considerable amount of work has been expended on the calculations of such ring systems by Bohr,4 Sommerfeld,5 Debye, Kroo,7... 

At first Bohr s model appeared very promising. It fit the hydrogen atom very well. However, when this model was applied to atoms other than hydrogen, it did not work. In fact, further experiments showed that the Bohr model is fundamentally incorrect. Although the Bohr model paved the way for later theories, it is important to realize that the current theory of atomic structure is not the same as the Bohr model. Electrons do not move around the nucleus in circular orbits like planets orbiting the sun. Surprisingly, as we shall see later in this chapter, we do not know exactly how the electrons move in an atom. 

Niels Bohr publishes a theory of atomic structure relating the electron arrangement in atoms and atomic chemical properties. 

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