Sulfur or Sulphur (See spelling differences) 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 at room temperature. Chemically, sulfur reacts with all elements except for nitrogen and the noble gases.
Elemental sulfur occurs naturally as the element (nativ sulfur), but most commonly occurs in combined forms as sulfide and sulfate minerals. Being abundant in native form, sulfur was known in ancient times, being mentioned for its uses in ancient India, ancient Greece, China, and Egypt.
Sulfur is referred to in the Bible as brimstone. Sulfur is an essential element for all life, but almost always in the form of organosulfur compounds or metal sulfides. Three amino acids (cysteine, cystine, and methionine) and two vitamins (biotin and thiamine) are organosulfur compounds. Many cofactors also contain sulfur including glutathione and thioredoxin and iron-sulfur proteins. Disulfides, S-S bonds, confer mechanical strength and insolubility of the protein keratin, found in outer skin, hair, and feathers. Elemental sulfur is not common in higher forms of life, but is both a product and an oxidant for various bacteria.