What experiment did Chadwick do to provide the proof of a "massive" particle and what subatomic particle was it?
Chadwick provided evidence (not proof!) of the
Given Rutherford's earlier discoveries of the proton, and its relationship to the atomic number, a disparity was observed between
It was reasonable, therefore, to presume the existence of massive nuclear particles, with ZERO electric charge. And James Chadwick managed to supply the evidence for such a neutral particle. Note that this was a much trickier proposition than the discovery of the proton or the electron. While, they are massive particles, they have ZERO electric charge, and are thus difficult to detect physically. Chadwick found that the massive neutrons had the same momenta as the charged protons, and thus an equivalent mass.
In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. This conclusion arose from the disparity between an element's atomic number (protons = electrons) and its atomic mass (usually in excess of the mass of the known protons present). James Chadwick was assigned the task of tracking down evidence of Rutherford's tightly bound
See this site for experimental details.