A radioactive substance is a substance whose atoms have an unstable nucleus. The nucleus emits energy in the form of heat and radiation in order to become more stable. The process of emitting energy by the nucleus of a radioactive atom in order to become more stable is called radioactive decay. The length of time taken for a radioactive substance to become stable and non radioactive varies from substanese to substances.
Antoine Henri Becquerel in 1896 accidentally discovered that photographic plates wrapped in dark paper were blackened by invisible radiation emitted by uranium . He also discovered that the invisible radiation was capable of discharging electroscope. Thus he had discovered a radioactive substance.!
Radioactive decay is a random process which means that its is not possible to tell when a particular atom will give out radioactivity, However it is possible know the time taken for half of large number of unstable atoms to decay.