Radioactive decay is a process that occurs amongst atomic nuclei that are too large and are unstable in their naturally occuring state. The large atomic nucleus emits radiation in the form of an alpha particle, a beta particle, or gamma radiation, where it emits a part of itself, and in so doing, decays to a more stable element. A good example would be the element carbon-14, where it emits radiation to its decay daughter element nitrogen. Radioactive decay was first discovered in 1896 by Henri Becquerel in his experiments with a phosphorescent uranium salt. Elements that undergo radioactive decay have a concept known as a half-life, which is the time it takes for a sample of the radioactive material to decay half its nuclei to the more stable daughter element. Theoretically, each half-life halves the amount of radioactive material remaining and never reaches a zero point.
In heavy metal, metal with high mass, their nucleus is unstable.
now "every susbstance tends to attend stable state either by losing or gaining energy"
Now in heavy metal due to the unstability of the nucleus, the nucleus breaks down more like a decay forming to smaller nucleus this phenomenon is called as nuclar decay.
the best example is the atom bomd. whcih is made of of uranium. Now uranium is a heavy metal (radioactive). it has a unstable nucleus. it break down to forming new element with the release of high energy. These new eleemnt have more stable nucleus compare to the uranium atom.
In similar manner , Hydrogen which is very reactive have only proton in its nucleus so it is unstable. now it combine with another hydrogen atom forming helium which is stable with the release of energy(enormous). This is called as nuclear fusion reaction. This si the source of energy of our sun