Uranium isotopes are variants of the element Uranium that have the same number of protons but different numbers of neutrons. The most common isotopes are Uranium-238 and Uranium-235. Uranium-238 makes up about 99.3% of natural uranium, while Uranium-235 is about 0.7%. These isotopes have different properties and uses. Uranium-235 is fissile, meaning it can sustain a nuclear chain reaction, making it essential for nuclear power and weapons. In contrast, Uranium-238 is fertile and can be converted into Plutonium-239, which is also used in nuclear reactors and weapons.