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During the 1920's physicists came to accept the view that matter is built of only two kinds of elementary particles,electrons and protons, which they often called “negative and positive electrons.” A neutral atom of mass number A and atomic number Z was supposed to contain Aprotons, all in the nucleus, and A negative electrons, in the nucleus and the rest making up the external electron shells of the atom. Their belief that both protons and negative electrons were to be found in the nucleus arose from the observations that protons could be knocked out of light elements by alpha‐particle bombardment, while electrons emerged spontaneously (mostly from very heavy nuclei) in radioactive beta decay. Any other elementary constituent of the atom would have been considered superfluous, and to imagine that another might exist was abhorrent to the prevailing natural philosophy.
To avoid anomalies of spin and statistics Pauli suggested in 1930 that a neutral particle of small mass might accompany the electron in nuclear beta decay, calling it (until Chadwick's discovery) the neutron.