The heaviest naturally occurring element is uranium, specifically the isotope uranium-238, which has an atomic mass of approximately 238.05078 u (unified atomic mass units). Found in various minerals, including uraninite, uranium is a radioactive metal often used as fuel in nuclear reactors and in the manufacture of nuclear weapons. Its density also contributes to its weight, making it useful in certain applications, including counterweights and radiation shielding. The study of uranium has significant implications in both energy production and environmental science.