However, in all cases carbon still has 6, and exactly 6, protons. In the same way, there are different isotopes of uranium. In both cases they have 92 protons at their nucleus, and the bulk of each isotope is composed of neutrons. Uranium-238 is a stable isotope, that is, it only rarely undergoes any form of radioactive decay. Uranium-235 is also stable but undergoes radioactive decay a little more frequently.
All atoms decay, that is, fall apart.
We can't say when an individual atom will decay, but we can use statistical techniques to say how long it will be before a lot of atoms of a particular type decay to half of their original amount. This is called the half-life of an element. Return to the Top of the Page. Fission in Uranium-235. This is a very unstable isotope and decays rapidly by splitting into lighter atoms and particles.