sometimes a mass will change to energy. common examples of elements that make these changes we call radioactivity are radium and uranium. an atom of uranium can lose an "alpha particle" (the atomic nucleus of helium) and become a new element with a lighter nucleus. then that atom will emit two electrons, but it will not be stable yet. it will emit a series of alpha particles and electrons until it finally becomes the element pb or what we call lead. by throwing out all these particles that have mass it has made its own mass smaller. it has also produced energy.