Uranium-238, the most prevalent isotope of uranium, has a half-life of 4.46 billion years. This means that in 4.46 billion years, only half of the uranium would have decayed. Uranium-235 and Uranium-234 have half-lives of 704 million and 245,000 years, respectively.
Uranium-238 atoms make up 99.3 percent of all the uranium on Earth. Uranium-235 makes up 0.7 percent, and uranium-234 is found only in negligible amounts. Half the atoms of an original sample of uranium-238 becomes thorium-234 after 4.46 billion years; the other half of the sample remains uranium-238. Thorium-234 also is radioactive and is thus called a decay product.
When uranium atoms decay, they emit radioactive alpha particles. Because uranium-238 takes so long to decay, it does not emit too many alpha particles at one time and is not very radioactive. Uranium-235 and uranium-234 have shorter half-lives, which make them more radioactive.