1. The spontaneous transition of an atomic nucleus to a lower energy state (radioactive decay) accompanied by the emission of an alpha particle, a beta particle, or gamma radiation. Alpha emission results in a reduction of atomic number by two and mass number by four; (negative) beta emission results in an increase of atomic number by one but no change in mass number; in gamma emission, atomic number and mass number are unchanged. Several naturally occurring isotopes are radioactive, including carbon-14 and potassium-40, which reside in the human body. Radon (strictly, radon-222) is a natural radioactive gas originating ultimately from the radioactive decay of uranium-238. Artificial radioactivity, as opposed to natural radioactivity, is a consequence of bombardment of isotopes that are not radioactive with neutrons, protons, and other subatomic particles. See half-life. 2. (Often simply activity. ) Rate of decay of a radioactive isotope. The unit of radioactivity is the curie.