Whenever unstable atomic nuclei of somr elements disintegrate spotaneously as a result of the emission of alpha particles, beta particles and gamma rays, this phenomenon is refered to as natural radioactivity.The disintegration of atomic nuecleus of an element (called th parent element) gives rise to the formation of a new element ( the doughter element ). This newly formed element can in turn emit radioactive rays and be converted into another more stable element. Such chang is know as transmution and a series of elements formed this way is refered to as decay series