Radioactivity was discovered by Henri Becquerel in 1869. In 1898, Marie and Pierre Curie succeeded in chemically isolating two radioactive elements, Polonium (z=84) and Radium (z= 88).
Radioactivity or radioactive decay is the spontaneous disintegration of unstable nuclides to form stable ones with the emission of radiation.
Unstable nuclides continue to disintegrate until a stable atom is formed.
Alpha (α) and beta (ϐ) particles are emitted and the gamma rays (ϒ) accompany the ejection of both alpha and beta particles.