Neutron

The neutron was discovered as a particle with the mass of a proton and no charge, proving crucial for nuclear reactions. For example, beryllium emitted a strange radiation which Chadwick theorized were uncharged, neutral particles that were part of the atom. These "neutrons" were difficult to detect because all of the subatomic particle detection instruments were based on electric charge.
 James Chadwick England 1932 Neutron

The neutron was discovered as a particle with the mass of a proton and no charge, proving crucial for nuclear reactions. For example, beryllium emitted a strange radiation which Chadwick theorized were uncharged, neutral particles that were part of the atom. These "neutrons" were difficult to detect because all of the subatomic particle detection instruments were based on electric charge.