James Chadwick (1891 - 1974 C.E.)

In 1932, James Chadwick proved the existence of neutrons. He shot alpha particles (two protons and two neutrons bound) at a sheet of beryllium, that filtered out the electrons. The neutrons hit a sheet of paraffin wax. The ones that filtered through the paraffin wax were the protons.

Chadwick discovered that neutrons help reduce the repulsion between protons to stabilize the atom's nucleus. Neutrons always reside in the nucleus of atoms, and they are about the same size as protons. However, neutrons have no charge (a charge of 0).