The element with the greatest atomic mass among all naturally occurring elements is uranium (symbol U) with an atomic number of 92. Uranium is a radioactive element found in trace amounts in the Earth's crust. The most common isotope of uranium, uranium-238, has an atomic mass of approximately 238 atomic mass units (amu).
Uranium has the highest atomic mass among naturally occurring elements because it is located toward the bottom of the periodic table and belongs to the actinide series. Actinides are a group of elements that follow the lanthanide series and are characterized by their high atomic masses and radioactive properties. Due to their large atomic nuclei, actinides have high atomic masses.
Following uranium, the next element with the highest atomic mass is plutonium (symbol Pu) with an atomic number of 94. Plutonium is another radioactive element and is also a member of the actinide series. The most common isotope of plutonium, plutonium-244, has an atomic mass of approximately 244 amu.
It's worth noting that elements beyond plutonium, such as americium (symbol Am, atomic number 95), curium (symbol Cm, atomic number 96), and beyond, have even higher atomic masses. These elements are also part of the actinide series and are generally synthetic (man-made) elements with limited natural occurrence.