Topics: what is radioactive dating?
All chemical elements have radionuclides. Even the lightest element, hydrogen , has a well-known radionuclide, tritium. Elements heavier than lead , and the elements technetium and promethium , exist only as radionuclides.
On Earth, naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides.
Many of these radionuclides exist only in trace amounts in nature, including all cosmogenic nuclides. Secondary radionuclides will occur in proportion to their half-lives, so short-lived ones will be very rare. Thus polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 10 10 ).   Further radionunclides may occur in nature in virtually undetectable amounts as a result of rare events such as spontaneous fission or uncommon cosmic ray interactions.