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ORDER WAS BROUGHT to the chaos of the elements 100 years ago when Dimitri Ivanovich Mendeleyev arranged them in the periodic table. The riddle presented by the regularities in this table remained unsolved for 52 years, until Niels Bohr published his famous “Aufbauprinzip,” based on the quantum theory of the atom. Physical experimentation penetrated deeper into the structure of matter and revealed the properties of atomic nuclei. Similar periodicities were found when these nuclei were arranged according to the number of protons and neutrons. These regularities found their explanation in the shell model of the nucleus that was introduced in 1951 by J. Hans D. Jensen, Marie Goeppert‐Mayer, Otto Haxel and Hans E. Suess. In the last two decades the proton and the neutron themselves were exposed to high‐energy beams and many new short‐lived entities were discovered. Recently Murray Gell‐Mann, Kazuhiko Nishijima and Yuval Ne'eman discerned some order in the seemingly chaotic list of new “particles,” but the explanation of this order is still outstanding.
For atoms and nuclei, classification schemes provided the key to the underlying structure. Will the same be true for elementary particles?