When an unstable atom loses energy and charged particles (radiation) through radiation it is a phenomenon called radioactive decay, when a substantial energy is lost the atom becomes stable.There are five types of radioactive decay:
Radioactivity or radioactive decay is the process that occurs when unstable isotopes or atoms release energy by emitting radiations such as ,gamma radiations, alpha radiations and beta radiations to attain stability. Therefore, in this case decay is likely to occur when a given atom has two many neutrons in its nucleus in order to attain stability.
Note that melting and vaporization are endothermic processes in that they absorb or require energy, while freezing and condensation are exothermic process as they release energy.
The answer can be explained when you burn something cleanly (with a very hot item) or not. With a candle lots of Carbon dioxide is producted but when using a bunson burner hardly any CO2 is produced.