In 1896, Henri Becquerel discovered that when a photographic plate wrapped in black paper was placed near a salt of uranium, it got affected. The same observation was repeated with other salts of uranium. This led him to conclude that the uranium salt emitted some deep penetrating radiation. Further experiments showed that the intensity of the emitted radiation depended directly on the concentration of uranium in its various salts. The emission of radiation was completely unaffected by any change in the physical and chemical conditions of the system. Becquerel concluded that the origin of the radiation was somehow rooted in the nucleus of the uranium atom. The radiation discovered by Becquerel was initially named Becquerel rays.Later on, Madam Curie and Pierre Curie discovered other substances i.e., polonium, radium, etc., which were more active than uranium.
These substances, which have the property of spontaneous emission of radiation are called radioactive substances and the process of spontaneous emission of radiation is called Radioactivity.It was later found that all elements, which have an atomic number greater than 82 are naturally radioactive.
Radioactivity is a nuclear phenomenon. It is the spontaneous emission of radiation from the nucleus.