James Chadwick discovered the neutron in 1932, completing the basic proton–neutron picture of the atomic nucleus.
Chadwick bombarded beryllium with alpha particles and studied the unusually penetrating radiation produced. He showed that the radiation consisted of neutral particles with a mass close to that of the proton, rather than high-energy gamma rays as some earlier interpretations suggested.
The neutron solved several nuclear puzzles. It explained why nuclei could have more mass than their proton charge suggested and provided a way to understand isotopes as atoms with the same proton number but different neutron numbers. Because neutrons carry no electric charge, they can enter nuclei more readily than charged particles.
Chadwick received the 1935 Nobel Prize in Physics for the discovery. Neutrons are composite particles made of three valence quarks—one up quark and two down quarks—bound by the strong interaction, although that internal structure was understood later.