James Chadwick discovered the neutron, earning the 1935 Nobel Prize in Physics. His discovery solved a major problem in atomic physics: atomic nuclei had more mass than could be explained by their protons alone.
In 1932, Chadwick showed that radiation produced by bombarding beryllium with alpha particles was made of neutral particles with roughly the mass of a proton. Because the particles carried no electric charge, they could penetrate matter and nuclei more easily than charged particles.
The neutron became central to nuclear physics, including the study of isotopes, nuclear fission, and nuclear reactors. It is not the same as the neutrino, which is a much lighter elementary particle. Chadwick’s work also helped establish the modern picture of the atomic nucleus as containing protons and neutrons.