Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Radioactive decay is a stochastic process at the level of single atoms, meaning it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed. However, for a significant number of identical atoms, the overall decay rate can be expressed as a decay constant or as half-life. The three most common types of decay are alpha, beta, and gamma decay.
When a radionuclide decays, it transforms into a different atom, called a decay product. The atoms keep transforming to new decay products until they reach a stable state and are no longer radioactive. The majority of radionuclides only decay once before becoming stable. Those that decay in more than one step are called series radionuclides. The series of decay products created to reach this balance is called the decay chain, and each series has its own unique decay chain.
The degree of radioactivity depends on the fraction of unstable nuclei and how frequently those nuclei decay. The effect of radioactivity also depends on the type and energy of the particles produced during nuclear decay. For example, neutrinos pass constantly through the Earth, while alpha particles are blocked by a sheet of paper. Radioactivity can cause damage in materials and in plant, animal, and human tissue.
