How did the transformation theory show that atoms can change into other elements?

The transformation theory, developed by Ernest Rutherford and Frederick Soddy in 1902–1903, fundamentally challenged the long-held belief that atoms were stable, indivisible, and immutable bodies.12

By studying radioactive elements like thorium, Rutherford and Soddy observed that these substances spontaneously emitted alpha or beta particles. They concluded that this emission was not merely a surface phenomenon but a process occurring within the atom itself, signifying a chemical change that transformed one element into a different, chemically distinct substance.12

This theory provided a physical explanation for radioactivity, demonstrating that the energy released originated from within the atom. Furthermore, they observed that these new radioactive products eventually decayed according to a fixed rate, leading to the formulation of the exponential decay law and the concept of half-life. This evidence proved that atoms are not permanent, but can change their identity through radioactive disintegration.21