Radioactive isotope is any of several species of the same chemical element with different masses whose nuclei are unstabel and use up excess energy by immediately pouring out radiation in the form of alpha, beta, and gamma rays. The nuclei of radioactive isotopes are unstable and undergo disintergration to make atoms of various elements. Energetic subatomic particles are emitted during this breakdown. Every chemical element has one or more radioactive isotopes. It can be useful in medicine and diagnistic purposes.