Uranium has the highest atomic number among elements found naturally in significant amounts on Earth.
Uranium is a radioactive metal with atomic number 92, meaning each uranium atom has 92 protons. It occurs naturally in rocks and minerals, usually in very small concentrations. The most abundant natural isotope is uranium-238, while uranium-235 is much less abundant but important because it can sustain a nuclear chain reaction under suitable conditions.
German chemist Martin Heinrich Klaproth discovered uranium in 1789 and named it after the planet Uranus, which had been discovered eight years earlier. The element was initially studied as a chemical curiosity; its nuclear significance became clear after the development of nuclear physics in the 20th century.
Uranium is often confused with plutonium, which has atomic number 94 and is mainly produced artificially, although trace amounts can form naturally. Thorium, atomic number 90, is also naturally occurring but is lighter than uranium. Radioactivity is a property of uranium nuclei, not a separate chemical element.