Bohr’s model

For nearly half a century, Mendeleev s periodic table remained an empirical compilation of the relationship of the elements. Only after the first atomic model was developed by the physicists of the early twentieth century, which took form in Bohr s model, was it possible to reconcile the involved general concepts with the specificity of the chemical elements. Bohr indeed expanded Rutherford s model of the atom, which tried to connect the chemical specificity of the elements grouped in Mendeleev s table with the behavior of electrons spinning around the nucleus. Bohr hit upon the idea that Mendeleev s periodicity could... 

Chemists were quick to appreciate Bohr s model because it provided an extremely clear and simple interpretation of chemistry. It explained the reason behind Mendeleev s table, that the position of each element in the table is nothing other than the number of electrons in the atom of the element, which, of course, represents an equal number of periodic changes in the nucleus. Each subsequent atom has one more electron, and the periodic valence changes reflect the successive filling of the orbital. Bohr s model also provided a simple basis for the electronic theory of valence. 

Using Bohr s model, one could calculate the energy difference between orbits of an electron in a hydrogen atom with Planck s equation. In the example of a system with only two possible orbits, the equation of the emitted radiation as the electron went from a higher energy state 2 to a lower one j would be - E = hf, where h is Planck s constant and/is the frequency of the emitted radiation. 

Bohr s model of a sodium atom. The lone electron in the outer shell is more energetic and less tightly bound to the nucleus than the other electrons. 

Bohr s model of the hydrogen atom. Identification of atomic number with nuclear charge number (H. Moseley). 

There are some scientists and philosophers who still claim that a model by definition "furnishes a concrete image" and "does not constitute a theory." 10 But if the model is the mathematical description, then the question of whether the model is the theory appears to become moot, since most people accept the view that rigorous mathematical deduction constitutes theory. For others, like Hesse and Kuhn, even if the model is a concrete image leading to the mathematical description, it still has explanatory or theoretical meaning, for, as Kuhn put it, "it is to Bohr s model, not to nature, that the various terms of the Schrodinger equation refer." 11 Indeed, as is especially clear from a consideration of mathematical models in social science, where social forces are modeled by functional relations or sets of mathematical entities, the mathematical model turns out to be so much simpler than the original that one immediately sees the gap between a "best theory" and the "real world." 12... 

Scientists of the nineteenth century lacked the concepts necessary to explain line spectra. Even in the first decade of the twentieth century, a suitable explanation proved elusive. This changed in 1913 when Niels Bohr, a Danish physicist and student of Rutherford, proposed a new model for the hydrogen atom. This model retained some of the features of Rutherford s model. More importantly, it was able to explain the line spectrum for hydrogen because it incorporated several new ideas about energy. As you can see in Figure 3.8, Bohr s atomic model pictures electrons in orbit around a central nucleus. Unlike Rutherford s model, however, in which electrons may move anywhere within the volume of space around the nucleus, Bohr s model imposes certain restrictions. 

There was a problem with Bohr s model, however. It successfully explained only one-electron systems. That is, it worked fine for hydrogen and for ions with only one electron, such as He, Li, and Be. Bohr s model was unable, however, to explain the emission spectra produced by atoms with two or more electrons. Either Bohr s model was a coincidence, or it was an oversimplification in need of modification. Further investigation was in order. 

It was fairly straightforward to modify Bohr s model to include the idea of energy sublevels for the hydrogen spectrum and for atoms or ions with only one electron. There was a more fundamental problem, however. The model still could not explain the spectra produced by many-electron atoms. Therefore, a simple modification of Bohr s atomic model was not enough. The many-electron problem called for a new model to explain spectra of all types of atoms. However, this was not possible until another important property of matter was discovered. 

Hinshelwood, 1951), it is evident that / cannot be greater than — 1. It is also clear that ( ) does not depend on the total value of n but on the separate values of n and /. The correspondence between n and the principal quantum number is thus obvious the energy levels follow the prediction of Bohr s model, assuming distinct values according to quantum numbers n, /, and nti, which lead to configurations s, p, and d. Equation 1.122 is then satisfied by a product of separate functions, one dependent on / and a function of angular coordinates 0 and and the other dependent on n, /, and a function of radial distance r ... 

The great success of Bohr s model of Mendeleev s periodic table of the elements and the appUcabiUty of the Ritz formula for the energy levels show that treating the electron in an atom as if it were in a Coulomb field is a reasonable approximation. 

Bohr s planetary atomic model proved to be a tremendous success. By utilizing Planck s quantum hypothesis, Bohr s model solved the mystery of atomic spectra. Despite its successes, though, Bohr s model was limited because it did not explain why energy levels in an atom are quantized. Bohr himself was quick to point out that his model was to be interpreted only as a crude beginning, and the picture of electrons whirling about the nucleus like planets about the sun was not to be taken literally (a warning to which popularizers of science paid no heed). 

So, a new model was proposed and accepted. The modern description of how electrons move around the nucleus in an atom is called the quantum mechanical model. In this model, the electrons do not follow an exact path, or orbit, around the nucleus the way they do in Bohr s model. Instead, for the new model, physicists calculated the chance of finding an electron in a certain position at any given time. The quantum mechanical model looks like a fuzzy... 

What are the concepts about Bohr s model which was not enough to explain ... 

How does the idea of probability differ from the idea behind Bohr s model of electron behavior ... 

The electron in Bohr s model has a definite path of travel, much like that of a race car on a track, whereas the electron in the probability model moves in a random manner and the best description of its location is a statistical likelihood. 

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... 

In 1913, the Danish physicist Niels Bohr developed a model of the atom that explained the hydrogen emission spectrum. In Bohr s model, electrons orbit the nucleus in the same way that Earth orbits the Sun, as shown in Figure D.2. The following three points of Bohr s theory help to explain hydrogen s emission spectrum. 

In Bohr s model, atoms have specific allowable energy levels. He called these energy levels stationary states. Each of these levels corresponds to a fixed, circular orbit around the nucleus. 

Bohr s model was revolutionary, because he proposed that the energy absorbed or emitted by an atom needed to have specific values. The energy change was quantized, rather than continuous. When something is quantized, it means that it is limited to discrete amounts or multiples of discrete amounts. Two great scientists paved the way for this surprising idea. 

So what was the problem Bohr s model worked beautifully, correctly predicting the line spectrum for... 

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