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Until 1932 physicists assumed that atomic nuclei are constructed of protons,
alpha-particles and electrons.
In 1932 Sir James Chadwick (1891-1974)
identified the neutron
by interpreting
Atomic Nucleus
correctly the results of the experiments carried out mainly by Jean Frédéric (1897-
1958) and Irène Joliot-Curie (1900-1956): neutrons, uncharged particles were
ejected out of beryllium nuclei after their bombardment with alpha-particles.
Chadwick considered
neutrons to be an electron-proton compound
and added it to
the nuclear mix.
In July 1932 Heisenberg published his
neutron-proton nuclear model
by assuming
that neutrons and protons to be the constituents of the nucleus.
His nucleus also
contained electrons, nuclear electrons, bound and unbound ones.
The assumption of the existence of nuclear electrons was completely rejected in
the late 1930’s after the introduction of the neutrino by Pauli in 1931 and the
establishment of Enrico Fermi’s (1901-1954) theory of beta-decay published in
1933.