The phenomenon in which a heavy big nucleus disintegrates spontaneously emitting powerful radiations is known as radioactivity. This is a nuclear phenomenon and is exhibited by all the nuclei having atomic number higher than 82. Physical conditions like pressure, temperature, electric field, magnetic field etc. has nothing to do with radioactivity. Henry Becquerel accidentally discovered this phenomenon. In a stable nucleus, coulomb repulsive force between protons is balanced by short range nuclear attractive force between them. If nucleus is heavy, attractive force is smaller compared to repulsive force. Hence a large nucleus is unstable and it emits alpha, beta and gamma rays till a stable non-radioactive end product is formed. The nucleus undergoing decay is called parent nucleus and nucleus formed after decay is called the daughter nucleus. End product is lead in all cases except in neptunium where end product is bismuth.
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