Discovery of the neutron

The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century. The century began with Ernest Rutherford and Thomas Royds proving, in 1908, that alpha radiation consists of helium ions. and Rutherford's model for the atom in 1911, in which atoms have their mass and positive charge concentrated in a very small nucleus. The essential nature of the atomic nucleus was established with the discovery of the neutron by James Chadwick in 1932.

In the 1911 Rutherford model, the atom consisted of a small massive nucleus with positive charge surrounded by a much larger cloud of negatively charged electrons. This model had been developed from the extraordinary finding that alpha particles were on occasion scattered to high angle when passing through gold foil, indicating the alpha particles were occasionally reflecting from a small, but dense, component of atoms. Rutherford and others noted the disparity between the atomic number of an atom, or number of positive charges, and its mass computed in atomic mass units. The atomic number of an atom is usually about half its atomic mass. In 1920 Rutherford suggested that the disparity could be explained by the existence of a neutrally charged particle within the atomic nucleus. Since at the time no such particle was known to exist, yet the mass of such a particle had to be about equal to that of the proton, Rutherford considered the required particle to be a neutral double consisting of an electron closely orbiting a proton. The mass of protons is about 1800 times greater than that of electrons.

There were other motivations for the proton–electron model. As noted by Rutherford at the time, "We have strong reason for believing that the nuclei of atoms contain electrons as well as positively charged bodies...", namely, it was known that beta radiation was electrons emitted from the nucleus.

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