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.
Half-life
Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay "on average"". In other words, the "probability" of a radioactive atom decaying within its half-life is 50%.