Bohr’s theory of the atom

Between 1912 and 1925, Bohr s theory of the atom gave rise to a conceptual framework for the study of matter on many fronts. Applying the theory to predict the energy levels and therefore the emission frequencies of atoms more complicated than hydrogen—say, helium with two electrons, lithium with three, and so on— led to the concept of electronic shells about the nucleus, the outer shell less tightly bound, and it is these outer-shell electrons that determine the element s spectrum. It should be pointed out that the theory was not fully successful in predicting the spectral lines of the elements that are more complicated than hydrogen. 

Some important successes of classical quantum theory Bohr s theory of the atomic spectrum of hydrogen... 

The concept of electron orbitals, or the earlier notion of electron orbits, began with Niels Bohr s theory of the atom, which is examined further below. Electronic orbitals have become perhaps the most important concept in the modern explanation of the periodic system, as discussed in chapters 8-10. 

In fact the concept of electronic configuration as a causally explanatory feature has become very much the domain of chemistry or to be more precise it is the dominant paradigm in modem chemistry. Conversely, physicists are only too aware of the limitations of the electronic configuration model and they only draw upon it as a zero order approximation. Hettema and Kuipers further state that Bohr s theory of the atom, despite its level of approximation, is to be regarded as a physical theory because the explanation of the periodic table was only a spin-off from its development. But given Heilbron and Kuhn s detailed version of the historical development, it was precisely the explanation of the periodic table which provided the initial impetus for Bohr s famous theory of the atom, whereas the explanation of the hydrogen spectmm only arose later. (Heilbron and Kuhn, 1969). 

The concept of chemical periodicity is central to the study of inorganic chemistry. No other generalization rivals the periodic table of the elements in its ability to systematize and rationalize known chemical facts or to predict new ones and suggest fruitful areas for further study. Chemical periodicity and the periodic table now find their natural interpretation in the detailed electronic structure of the atom indeed, they played a major role at the turn of the century in elucidating the mysterious phenomena of radioactivity and the quantum effects which led ultimately to Bohr s theory of the hydrogen atom. Because of this central position it is perhaps not surprising that innumerable articles and books have been written on the subject since the seminal papers by Mendeleev in 1869, and some 700 forms of the periodic table (classified into 146 different types or subtypes) have been proposed. A brief historical survey of these developments is summarized in the Panel opposite. 

The discovery of the rare earth elements provide a long history of almost two hundred years of trial and error in the claims of element discovery starting before the time of Dalton s theory of the atom and determination of atomic weight values, Mendeleev s periodic table, the advent of optical spectroscopy, Bohr s theory of the electronic structure of atoms and Moseley s x-ray detection method for atomic number determination. The fact that the similarity in the chemical properties of the rare earth elements make them especially difficult to chemically isolate led to a situation where many mixtures of elements were being mistaken for elemental species. As a result, atomic weight values were not nearly as useful because the lack of separation meant that additional elements would still be present within an oxide and lead to inaccurate atomic weight values. Very pure rare earth samples did not become a reality until the mid twentieth century. 

Spectroscopy was to prove indispensable in unlocking the structure of atoms, particulary their electronic stmcture— but those developments would depend on other, later researchers. Max Planck s analysis of blackbody radiation and Bohr s theory of the hydrogen spectrum are just two examples. 

Millikan s experiment did not prove, of course, that (he charge on the cathode ray. beta ray, photoelectric, or Zeeman particle was e. But if we call all such particles electrons, and assume that they have e/m = 1.76 x Hi" coulombs/kg. and e = 1.60 x 10" coulomb (and hence m =9.1 x 10 " kg), we find that they fit very well into Bohr s theory of the hydrogen atom and successive, more comprehensive atomic theories, into Richardson s equations for thermionic emission, into Fermi s theory of beta decay, and so on. In other words, a whole web of modem theory and experiment defines the electron. The best current value of e = (1.60206 0.00003) x 10 g coulomb. 

By 1903. llie wave theory of light based oil Maxwell s equations was well established, but certain phenomena would not fit in. It seemed that emission and absorption of hght occur discontinuously. This led Einstein to (lie view that the energy is concentrated in discrete particles. It was a revolutionary idea, very hard to understand, as the successes of the wave theory were undeniable. It seemed that light had to be understood sometimes as waves, sometimes as particles, and physicists had to get used to it, The idea was incorporated into Bohr s theory of the hydrogen atom and forms an essential part of it. 

THIRTY YEARS THAT SHOOK PHYSICS The Story of Quantum Theory, George Gamow. Lucid, accessible introduction to influential theory of energy and matter. Careful explanations of Dirac s anti-particles, Bohr s model of the atom, much more. 12 plates. Numerous drawings. 240pp. 5X x 8H. 24895-X Pa. 5.95... 

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. 

Problem 1.3 According to Bohr s theory of the H atom [49], the energy (E ) at n levels of H is given as = -p/2n2 (in a.u.), where p is the reduced mass of H. The same Bohr s theory is applicable to Ps, with a reduced mass pPs of half mass of the electron or about half that of pH. Therefore, the ionization energy (I.E). for the ground state (n=l) of Ps is only about half that of H. 

What significance did Bohr s model of the atom have in the development of the modern theory of the atom ... 

Briefly describe Bohr s theory of the hydrogen atom and how it explains the appearance of an emission spectrum. How does Bohr s theory differ from concepts of classical physics ... 

Bohr s theory of the dynamical behaviour of an electron under the influence of a massive electrically charged atomic nucleus is well known. It is of interest to recall the way in which he was led [14] not only to provide a theoretical basis for... 

The first 25 years of the twentieth century were momentous for the rapid pace of change in scientists understanding of the nature of matter, (a) How did Rutherford s experiments on the scattering of a particles hy a gold foil set the stage for Bohr s theory of the hydrc en atom (b) In what ways is de Breve s hypothesis, as it applies to electrons, consistent with J. J. Thomson s conclusion that the electron has mass In what sense is it consistent with proposals preceding Thomson s work that the cathode rays are a wave phenomenon ... 

CQ tolls for the rapid pace of change in scientists understanding of the nature of matter, (a) How did Rutherford s experiments on the scattering of a particles by a gold foil set the stage for Bohr s theory of the hydrogen atom ... 

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