The phenomenon of radioactivity was discovered by

The phenomenon of radioactivity was discovered by the French physicist Henri Becquerel in 1896. While investigating whether phosphorescent minerals emitted X-rays, Becquerel observed that uranium salts spontaneously emitted penetrating radiation that could expose photographic plates even in the absence of light. This spontaneous emission was a property of the uranium itself. Although Marie and Pierre Curie significantly extended this research, coined the term 'radioactivity', and discovered elements like polonium and radium, the initial discovery is credited to Becquerel [1]. For this breakthrough, Becquerel shared the 1903 Nobel Prize in Physics with the Curies. J. J. Thomson, conversely, is known for the discovery of the electron in 1897 [1].

Sources

1. [1] https://www.osti.gov/opennet/manhattan-project-history/Events/1890s-1939/solar_system.htm