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 that alpha radiation is helium ions in 1908 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.