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Nucleus of  an unstable atom emits/loses energy - in the form of alpha particles, beta particles and gamma rays.  the element transforms into another element with  a different number of protons and neutrons. the decayed radiation contains positrons, alpha (helium nucleus), protons, electrons, etc.
this process is called nuclear transmutation.
 reason is instability of the nucleus of the element.
as the elements have higher atomic numbers, the repulsive forces among protons increases the nucleus becomes unstable.  somewhere aound the ratio  number of neutrons / num of protons = 1.5, the instability begins..