Radioactivity, also known as radioactive decay, is a process by which a radioactive isotope loses subatomic particles (helium nuclei or electrons) from its nucleus along with usual emission of gamma radiation, and becomes a different element. In a large collection of atoms, there is a definite time, called the half-life of the isotope, after which one-half of the total number of nuclei would have decayed. This time varies from few-millionths of a second to millions of years for different radioactive isotopes.