Radioactivity, the process by which the nuclei (cores) of unstable atoms of an element emit radiation (particles of matter and rays of energy), and in so doing become atoms of other elements. It is a property of certain types of matter. Substances in which radioactivity takes place are called radioactive. The particles and energy given off by these substances are formed of nuclear radiation. In making the emissions, the nucleus of a radioactive atom is said to decay. The radiation of a radioactive substance is harmful to life. Properly used, however, this radiation is extremely useful in science, medicine, agriculture, and industry.

Some chemical elements, including uranium, radium, thorium, and polonium, are naturally radioactive. Any element that is not naturally radioactive can be made radioactive in a nuclear reactor or particle accelerator.

The principal particles emitted by radioactive substances are alpha particles and beta particles. Most of the naturally radioactive elements emit only one of these two types of particles. The emission of either alpha or beta particles is often accompanied by the emission of gamma rays. Radium and certain other elements produce all three types of radiation.

The discovery of radioactivity late in the 19th century helped lay the foundations of modern physical science. It put an end to the theory that atoms are indivisible and indestructible, and led to great advances in the knowledge of the structure of matter, and of the relationship between matter and energy. From the research inspired by the discovery of radioactivity have come nuclear weapons on the one hand, and, on the other, nuclear power and the use of radioactive substances for the benefit of mankind.