On the periodic table, uranium has the highest atomic number among elements found naturally in significant quantities.
Uranium has atomic number 92, meaning each uranium atom contains 92 protons. It is a dense, radioactive actinide metal whose symbol is U. Uranium occurs naturally in minerals such as uraninite and is distributed in small amounts through Earth’s crust.
Uranium-238 is the most abundant natural uranium isotope and has a half-life of about 4.47 billion years. Uranium-235 is much less abundant but is important because it can sustain a nuclear fission chain reaction under suitable conditions.
Elements with atomic numbers above 92 are generally called transuranium elements and are produced artificially, although tiny natural traces of some can occur through decay processes. Plutonium, atomic number 94, is therefore not the highest naturally occurring element in the usual classification.