All unstable isotopes, both naturally occurring and artificially produced, emit radiation in their pursuit of stability. These unstable isotopes decay, some rapidly and some very slowly, with the emission of huge amounts of energy that originate from atoms' nuclei. Such radiation, or radioactivity, takes the form of particles and/or light rays.
When radiation has enough energy to eject electrons from atoms, thereby creating ions, it is called ionizing radiation. Ionizing radiation has both beneficial and destructive applications.