![]() But up to 1930 proton-electron hypothesis was accepted.Īn experimental breakthrough came in 1930 with the observation by the German nuclear physicist Herbert Becker and Walther Bothe that bombardment of beryllium with alpha particles from a radioactive source produced neutral radiation which was penetrating but non-ionizing. In 1920 Rutherford proposed the existence of the third neutral particle in an atom. Since the time of Rutherford, it had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus. The proton is a baryon and is considered to be composed of two up quarks and one down quark.
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