Uranium’s atomic number is 92. This refers to the number of protons in each nucleus of its atoms. Generally, the higher the number, the heavier the element. For decades chemists believed transuranic elements (those with atomic numbers greater than 92) could not occur in nature, only laboratories. However, in 1971, Hoffman discovered plutonium-244, atomic number 94, in rocks at the atomic weapons test site in Nevada. Plutonium, as with many other elements, exists in slightly different forms and the number 244 refers to the specific type, or isotope, of the element.