Unlike planets orbiting the Sun, electrons cannot be at any arbitrary distance from the nucleus; they can exist only in certain specific locations called allowed orbits. This property, first explained by Danish physicist Niels Bohr in , is another result of quantum mechanics—specifically, the requirement that the angular momentum of an electron in orbit, like everything else in the quantum world, come in discrete bundles called quanta. In the Bohr atom electrons can be found only in allowed orbits, and these allowed orbits are at different energies.
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In the pattern of other quantized angular momenta , this gives total angular momentum The resulting fine structure which is observed corresponds to two possibilities for the z-component of the angular momentum. This causes an energy splitting because of the magnetic moment of the electron. Two types of experimental evidence which arose in the s suggested an additional property of the electron. One was the closely spaced splitting of the hydrogen spectral lines , called fine structure. The other was the Stern-Gerlach experiment which showed in that a beam of silver atoms directed through an inhomogeneous magnetic field would be forced into two beams.
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In the classical world, a charged, spinning object has magnetic properties that are very much like those exhibited by these elementary particles. Physicists love analogies, so they described the elementary particles too in terms of their 'spin. Instead we have learned simply to accept the observed fact that the electron is deflected by magnetic fields. If one insists on the image of a spinning object, then real paradoxes arise; unlike a tossed softball, for instance, the spin of an electron never changes, and it has only two possible orientations. In addition, the very notion that electrons and protons are solid 'objects' that can 'rotate' in space is itself difficult to sustain, given what we know about the rules of quantum mechanics. The term 'spin,' however, still remains. Bachmann of Birmingham-Southern College adds some historical background and other details: "Starting in the s, Otto Stern and Walther Gerlach of the University of Hamburg in Germany conducted a series of important atomic beam experiments.