1896, when French physicist Henri Becquerel was studying phosphorescent materials, he found that uranium salts or metallic uranium could make photographic negatives wrapped in black paper sensitive nearby, thus inferring that uranium could continuously and automatically emit some invisible and penetrating rays.

At first, people thought this radiation was similar to the newly discovered X-ray. But physicists' research shows that this kind of radiation is more complicated than X-rays. Rutherford first discovered that its decay patterns all follow exponential decay, that is, there is a half-life. Rutherford and his student Frederick Soddy first discovered that many decays would lead to nuclear disintegration, which would change atoms into another kind of atoms.

According to the existing research results, we already know that all elements with atomic number above 84 have natural radioactivity, and some elements with atomic number below this number, such as carbon and potassium, also have this property.