Rutherford and Soddy proposed the radioactive decay theory in 1902.
Working at McGill University, Ernest Rutherford and the chemist Frederick Soddy argued that radioactive atoms were not permanently unchanging. Instead, they could spontaneously transform into different substances while releasing radiation and energy. Their theory was presented through papers published in 1902 and 1903.
Their experiments on thorium and its radioactive emanation showed that radioactive substances could have measurable, predictable rates of change. This helped establish the idea that one chemical element could transform into another—a radical challenge to the older belief that atoms were indivisible and immutable.
The 1902 date is the standard answer for the proposal of the theory, although some accounts discuss the work as developing across 1901–1903. Rutherford and Soddy’s ideas also helped introduce the quantitative study of radioactive decay, including the later concept of half-life. Their work became a foundation of nuclear physics and the modern understanding of radioactive series.