Oxygen is the most abundant element in Earth’s crust by number of atoms.
By mass, oxygen makes up about 46% of Earth’s crust, more than any other element. It is present in silicate minerals, oxides, and water-bearing materials. Silicon is the second-largest contributor by mass, which is why the two elements are often discussed together in geology.
The wording matters: abundance by mass and abundance by number of atoms are different measurements. Oxygen still leads by both common crustal measures, but exact percentages vary with the geological sample and the boundary used for “crust.”
Oxygen has atomic number 8 and is in group 16. In Earth’s atmosphere it occurs mainly as O₂, while in rocks it is usually chemically bonded to other elements rather than present as free gas.