The process of spontaneous breakdown of an atomic nucleus and the release of energy and matter from the nucleus is called Radioactive Decay. Radioactive isotopes are elements with a very unstable nucleus. They are always trying to stabilize and to do that they release energy and transmute into other elements which are stable. Transmutation can occur in nature and by artificial means.
Based on the former evidence, the answer to this question is:
Radioactive decay happens when one kind of atom becomes another kind.
Carbon-14 decays into nitrogen-14.
Radioactive elements occur in nature.