These materials glow in the dark after exposure to light, and Becquerel suspected that the glow produced in cathode ray tubes by X-rays might be associated with phosphorescence. Radioactivity was discovered in 1896 by scientists Henri Becquerel and Marie Skłodowska-Curie, while working with phosphorescent materials. History of discovery Pierre and Marie Curie in their Paris laboratory, before 1907 Well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes, such as potassium-40. These 34 are known as primordial nuclides. There are 28 naturally occurring chemical elements on Earth that are radioactive, consisting of 34 radionuclides (six elements have two different radionuclides) that date before the time of formation of the Solar System. When the number of protons changes, an atom of a different chemical element is created. Except for gamma decay or internal conversion from a nuclear excited state, the decay is a nuclear transmutation resulting in a daughter containing a different number of protons or neutrons (or both). The decaying nucleus is called the parent radionuclide (or parent radioisotope ), and the process produces at least one daughter nuclide. The half-lives of radioactive atoms have a huge range from nearly instantaneous to far longer than the age of the universe. However, for a significant number of identical atoms, the overall decay rate can be expressed as a decay constant or as half-life. According to quantum theory, it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed. Radioactive decay is a stochastic (i.e., random) process at the level of single atoms. The weak force is the mechanism that is responsible for beta decay, while the other two are governed by the electromagnetism and nuclear force. Three of the most common types of decay are alpha, beta, and gamma decay.

A material containing unstable nuclei is considered radioactive. Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation.

Marie curie atomic theory