Final answer:
James Chadwick discovered the neutron in 1932, which contributed to the understanding of atoms by explaining the missing mass and refining our understanding of atomic structure.
Step-by-step explanation:
James Chadwick contributed to the understanding of atoms by discovering the neutron in 1932. By bombarding a beryllium target with alpha particles, he found that a highly penetrating radiation consisting of neutral particles, now known as neutrons, was produced. The discovery of the neutron provided an explanation for the missing mass in atoms and helped to refine our understanding of atomic structure.