U-235, or Uranium-235, is a naturally occurring isotope of uranium. It is one of the few materials that can sustain a nuclear chain reaction, making it essential for nuclear power generation and atomic bombs. U-235 comprises about 0.7% of natural uranium, with the majority being U-238, which is not fissile. When U-235 atoms are bombarded with neutrons, they can split, releasing a significant amount of energy. This process, known as nuclear fission, is harnessed in nuclear reactors to produce electricity. Additionally, U-235 is a critical component in the development of nuclear weapons.