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.
Ruthenium
Fifteen other radioisotopes have been characterized with atomic weights ranging from 89.93 u (Ru) to 114.928 u (Ru). Most of these have half-lives that are less than five minutes except Ru (half-life: 1.643 hours) and Ru (half-life: 4.44 hours).