![]() The uncharged neutron was immediately exploited as a new means to probe nuclear structure, leading to such discoveries as the creation of new radioactive elements by neutron irradiation (1934) and the fission of uranium atoms by neutrons (1938). The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick in 1932 and the determination that it was a new elementary particle, distinct from the proton. : §1.1.2 Throughout the 1920s, the nucleus was viewed as composed of combinations of protons and electrons, the two elementary particles known at the time, but that model presented several experimental and theoretical contradictions. By 1920, isotopes of chemical elements had been discovered, the atomic masses had been determined to be (approximately) integer multiples of the mass of the hydrogen atom, and the atomic number had been identified as the charge on the nucleus. In this model, atoms had their mass and positive electric charge concentrated in a very small nucleus. Early in the century, Ernest Rutherford developed a crude model of the atom, : 188 based on the gold foil experiment of Hans Geiger and Ernest Marsden. The discovery of the neutron and its properties was central to the extraordinary developments in atomic physics in the first half of the 20th century. Chadwick had discovered the neutron the year before while working at Cavendish Laboratory. James Chadwick at the 1933 Solvay Conference. ![]() Scientific background leading to the discovery of subatomic particles ![]()
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