By 1919, Ernest Rutherford had established a nuclear model of the atom in which electrons orbited a central, dense nucleus composed of protons. However, this model could not account for the total mass of the atom—the nucleus was significantly heavier than the combined mass of its known protons.

This mass discrepancy was finally resolved in 1932 through the work of James Chadwick. His experiments demonstrated that the atomic nucleus contains not only positively charged protons, but also neutral subatomic particles of nearly equal mass, which we now know as neutrons.