James Chadwick and the Discovery of the Neutron

In 1932, Chadwick, a student of Rutherford's, was experimenting with beryllium radiation, a new type of radiation that could be observed coming out of beryllium nuclei. This radiation was able to knock single protons out of certain nuclei.

It was also known at the time, that the mass of atomic nuclei were almost always heavier than the mass of the protons alone and that isotopes of these elements occurred in single proton mass units. In other words the three isotopes of hydrogen are the mass of one proton, the mass of two protons and the mass of the three protons.

Chadwick was able to determine that the mass of beryllium radiation was nearly identical to that of a proton. In addition, he concluded that only a non-charged particle could get close enough to the positive nucleus to behave in such a way. 

Through this thought process, he reasoned that the radiation he was dealing with was the unknown part of the nucleus and, based on its lack of charge he named it the neutron.