What is the most common isotope of uranium found in nature?

The most common isotope of uranium found in nature is uranium-238.

Uranium-238 contains 92 protons and 146 neutrons, giving it a mass number of 238. It makes up about 99.27% of natural uranium, while uranium-235 accounts for roughly 0.72% and uranium-234 for only a few parts per million.

Although uranium-235 is much more famous because it can sustain a chain reaction with slow neutrons, it is not the dominant natural isotope. Uranium-238 is not fissile in that same thermal-neutron sense, but it is fertile: after absorbing a neutron and undergoing radioactive changes, it can ultimately become plutonium-239.

Uranium-238 is also important in geology. Its half-life is about 4.46 billion years, comparable to Earth’s age, and its decay chain ends in lead-206. Uranium–lead dating uses this predictable transformation to estimate the ages of ancient rocks and minerals.