In 1927, the significant discovery related to radioactivity was the identification of the neutron by physicist James Chadwick. This discovery was pivotal as it completed the understanding of atomic structure, leading to improvements in nuclear science and chemistry. Prior to this, scientists knew about protons and electrons, but the neutron's existence clarified why atoms had mass and improved models of atomic behavior. Chadwick's work contributed to advancements in nuclear energy and paved the way for developments in both nuclear medicine and atomic theory. This discovery had lasting implications in various scientific and technological fields.