James Chadwick discovered neutrons in 1932, PBS states. Neutrons are the particles in an atom's nucleus that have mass but no charge. Chadwick proved their existence by tracking particle radiation.
In the early 20th century, scientists were puzzled by the mass of atoms because the mass was consistently greater than the combined mass of the electrons and protons in the atom. Several theories were proposed to explain this discrepancy. Some thought that it was possibly due to more protons and electrons in the nucleus with charges that cancelled each other out to make them less apparent. Others proposed a possible particle with no charge. Chadwick undertook experiments tracking particle radiation from certain atoms while watching for the existence of a neutrally charged particle. His experiments were successful and proved the existence of the neutron.