Final answer:
The element uranium has three naturally occurring isotopes: U-238, U-235, and a trace amount of U-234, with U-238 being the most abundant.
Step-by-step explanation:
The element uranium (U) has three naturally occurring isotopes: uranium-238 (U-238), uranium-235 (U-235), and a very small amount of uranium-234 (U-234). Uranium-238 accounts for more than 99% of natural uranium, while uranium-235 makes up less than 1% yet is the isotope that can undergo fission and is therefore used as fuel in nuclear reactors. Although both U-238 and U-235 are found in nature, they have different uses due to their different properties. Uranium in its natural form contains a mixture of these isotopes, and for nuclear fuel, the concentration of U-235 must be increased through a process known as enrichment.