Radioactivity is the process in which unstable atomic nuclei become more stable by spontaneously emitting highly energetic particles and/or energy. A sample of material is said to be radioactive if some of its atomic nuclei are emitting such radiation. The radiations emitted by unstable nuclei are capable of ionizing matter and disrupting molecules, including DNA; they are therefore a biological hazard in prolonged or intense exposures.

Radioactivity is important to society for two reasons. First, it is produced in large amounts by nuclear fission in nuclear power plants, and the safe disposal of radioactive waste is a problem. Second, radioactivity is widely used as a diagnostic and therapeutic tool in many important medical applications. It is therefore both a burden and a blessing to society.

Stable and unstable nuclei

Every atomic nucleus consists of a certain number of protons, strongly bound to a certain number of neutrons...