The process of combining lighter elements into a stable heavier nucleus is known as nuclear fusion. However, such processes can take place at reasonable rates only at very high temperatures of the order of several million degrees, which exist only in the interior of stars.
Such processes are therefore called thermonuclear reactions. Once a fusion reaction is initiated, the energy released in the process is sufficient to maintain the temperature and to keep the process going on.
For example, the hydrogen bomb is based on the fusion of hydrogen nuclei into heavier ones by the thermonuclear reactions with the release of enormous energy.
There are another class of nuclei that are unstable. They disintegrate (break up) spontaneously with the emission of electromagnetic radiation of very high energy. As such, the process of spontaneous transformation of a nucleus is known as radioactivity. Some of the unstable high atomic weight elements are radium, thorium and uranium. The emission or decay is associated with the radiation from the atomic nucleus, which is either an alpha particle (helium nucleus) or a beta particle (an electron). In many cases, gamma radiation comes out with the particle emission. The process of disintegration continues and after a number of stages of radiation, a stable nucleus is formed.