Radioactivity is defined as the release of energy and matter from the nucleus of an element as a result of instability of the nucleus.
Radioactive isotopes are called radioisotopes. If an element has three isotopes, the one that is most likely to be a radioisotope is the isotope with an unstable nucleus.
An isotope with an unstable nucleus will look for a way of becoming stable by giving off a neutron, this happens in form of radiation and the nucleus is said to be radioactive.