Neutrons were discovered by James Chadwick in 1932.
Before Chadwick's discovery, scientists knew that atoms were made up of positively charged protons in the nucleus and negatively charged electrons orbiting around the nucleus. However, they couldn't explain why some elements had different isotopes with varying atomic masses.
Chadwick performed experiments in which he bombarded beryllium atoms with alpha particles and observed that they emitted a new type of radiation that was not deflected by electric or magnetic fields. He concluded that this radiation consisted of uncharged particles with roughly the same mass as protons, which he called neutrons.
Chadwick's discovery of the neutron helped explain why some elements had different isotopes with varying atomic masses, and it paved the way for further discoveries in nuclear physics.