Isotopes of an atom can be stable or unstable (radio-isotopes). A nucleus is unstable IF it contains too many neutrons and/or protons. The unstable nuclei will try to become more stable by ejecting particles. The process of ejecting the particles is called radioactivity.
Radioactive emission of particles from the nucleus of mass is in the form of alpha particles or beta particles, sometimes accompanied by the emission of energy in the form of gamma rays.
Radioactive decay is a random and spontaneous process in which an unstable nucleus will disintegrate into a more stable configuration by the emission of alpha-particles (helium nuclei), beta-particles (electrons) and/or gamma radiation (short-wavelength electromagnetic waves).
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Henri Becquerel (1896) accidentally discovered that a double salt of uranium and potassium give off sub-atomic particles and/or radiation energies, when it is exposed to a photographic plate. Soon thereafter it was found that all uranium compounds and the metal itself were similarly radioactive.