a.k.a sulphur (British English; see spelling below) is a chemical element with symbol S and atomic number 16. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow crystalline solid when at room temperature. Chemically, sulfur can react as either an oxidant or reducing agent. It oxidizes most metals and several nonmetals, including carbon, which leads to its negative charge in most organosulfur compounds, but it reduces several strong oxidants, such as oxygen and fluorine.

Sulfur is one of the few elements found pure in nature. This "native" sulfur is probably of volcanic origin. Also called brimstone, it oxidizes and is responsible for the characteristic smell of many volcanoes.