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BOHR THEORY OF THE ATOM |
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These days, the theory of electronic energy levels in atoms (and the associated spectra arising from electronic transitions between energy levels) is very well developed. Using the tools of quantum mechanics, electronic energy levels can be calculated to a good degree of accuracy. However, it is very instructive to consider a precursor to modern-day theory, developed by Neils Bohr in the early 20th century. |
The Bohr theory of the atom makes use of a mixture of classical arguments and early quantum mechanical ideas to provide an explanation of the observed electronic spectra of "simple" atoms or ions. The principal limitation of the theory is that it can only be applied to atoms or ions that have a single free electron. This means that it can be applied only to the hydrogen atom, singly ionised helium, doubly ionised lithium, etc.
However, for elements such as sodium, which have a single outer electron (and which is therefore principle determinant of the electronic spectrum of sodium), the Bohr theory can be adapted to give a reasonable explanation of the observed spectrum. This limitation arises because the electron-electron interaction is difficult to represent adequately in such a simple modelling framework. Indeed, virtually any many-body interaction can only be considered in an approximate way, unless certain simplifying assumptions are made.
The first article looks at some basic ideas required to understand the Bohr theory, in particular the assumptions that need to be made to apply the theory. Next we will apply the Bohr theory to the hydrogen atom, and show how the electronic spectrum of hydrogen can be deduced from it. The next article focuses on some corrections and additions that can be made to the simple Bohr theory, to extend its usefulness. One such example is the consideration of elliptical electron orbits around the nucleus. In the full study of quantum mechanics, it is found that electronic energy levels need to be specified by a set of quantum numbers, and the consideration of elliptic orbits provides a physical picture of these quantum numbers.
Next we will take a look at how the Bohr theory of the hydrogen atom compares with the results obtained from the full quantum mechanical theory of the hydrogen atom, and in the next article we will look at a very important idea known as the correspondence principle. Finally, we look at the Bohr theory of deuterium, and how it led to the discovery of this isotope of hydrogen.