What observation led Chadwick to conclude that there must be something besides just the proton in the nucleus of atoms?

Chadwick observed that the mass of the nucleus was greater than the combined mass of the protons it contained, which suggested the presence of another particle. Additionally, he noted that when beryllium was bombarded with alpha particles, a highly penetrating radiation was emitted that could not be attributed to protons or electrons. This led him to conclude the existence of a neutral particle, which he identified as the neutron, thereby explaining the discrepancy in mass and enhancing the understanding of atomic structure.