The atom's missing puzzle piece was found in 1932

Physicists knew atomic nuclei were heavier than their protons could explain, but the extra mass seemed to come from an undetectable, chargeless particle. James Chadwick bombarded beryllium with alpha particles and showed the resulting radiation was actually a stream of neutral particles about as heavy as a proton — the neutron. It completed the basic picture of the atom.

— James Chadwick, Possible Existence of a Neutron — Nature, 1932