Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Radium
All isotopes of radium have half-lives much shorter than the age of the Earth, so that any primordial radium would have decayed long ago. Radium nevertheless still occurs in the environment, as the isotopes Ra, Ra, Ra, and Ra are part of the decay chains of natural thorium and uranium isotopes; since thorium and uranium have very long half-lives, these daughters are continually being regenerated by their decay. Of these four isotopes, the longest-lived is Ra (half-life 1600 years), a decay product of natural uranium. Because of its relative longevity, Ra is the most common isotope of the element, making up about one part per trillion of the Earth's crust; essentially all natural radium is Ra. Thus, radium is found in tiny quantities in the uranium ore uraninite and various other uranium minerals, and in even tinier quantities in thorium minerals. One ton of pitchblende typically yields about one seventh of a gram of radium. One kilogram of the Earth's crust contains about 900 picograms of radium, and one liter of sea water contains about 89 femtograms of radium.