SHELL STRUCTURE IN ATOMS

First let me say something about shell structure in atoms. It was known since the early days of quantum mechanics that shell structure in atoms is very important. In fact, already in the 19th century, Mendeleev had demonstrated the periodic character of chemical properties with atomic number, and quantum mechanics, including the important role of the Pauli principle, gave an explanation of the closed shells in atoms. Thus configurations with 2, 10, 18, 36, 54 and 86 electrons form closed shells, and atoms with these number of electrons are known as the rare gases, being unusually stable. It takes a comparatively large energy to remove an electron from such an atom. On the other hand, atoms with one additional electron form the alkali metals, which are extremely active chemically, since the extra electron is very easy to remove. Thus there is a large difference between the ionization energies, and thus in chemical properties. Some of these questions were addressed in Goettingen in the 1920's. That was the time of the revolution in quantum mechanics, including Heisenberg's uncertainty principle. and Schroedinger's equation. The other revolution was, of course, relativity.>

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