U-235, E.G. is an isotope of uranium, which is a naturally occurring element found in the Earth's crust. It is the most common isotope of uranium, making up more than 99% of all naturally occurring uranium. U-235 is fissile, meaning it can be split into smaller nuclei when bombarded with neutrons, releasing energy and more neutrons in a process known as nuclear fission. This makes it a key component in nuclear power plants and nuclear weapons.