Atoms differ from other microscopic systems by the presence of a well-defined center of force. Because of their attraction to the nucleus, the electrons are confined to a localized region of space and they move in a field which is approximately spherical. This elementary property of the atom explains the importance of angular-momentum theory to atomic physics, and it provides the physical basis of the shell model of the atom. There is, of course, an enormous amount of experimental evidence which supports this approximate description. The regularities of the chemical properties of the elements, for instance, strongly indicate that the properties of an atom are to a large extent determined by the angular-momentum properties of its outer electrons. The first direct evidence of atomic shell structure was given by optical spectroscopy and x-ray absorption data. Nowadays, modern techniques, such as x-ray photoelectron spectroscopy and laser spectroscopy, provide us with a wide variety of accurate information about the structure of the atom.
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