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 when at room temperature. Chemically, sulfur
can react as either an oxidant or a 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 occurs naturally as the pure element (native sulfur) and as sulfide and sulfate minerals. Elemental sulfur crystals are commonly
sought after by mineral collectors for their distinct, brightly colored polyhedron shapes. Being abundant in native form, sulfur was
known in ancient times, mentioned for its uses in ancient India, ancient Greece, China and Egypt. Fumes from burning sulfur were used as fumigants, and
sulfur-containing medicinal mixtures were used as balms and antiparasitics.
Sulfur is referred to in the Bible as brimstone (burn stone) in English, with this name still used in several nonscientific
tomes. It was needed to make the best quality of black gunpowder. In 1777, Antoine Lavoisier helped convince the scientific community
that sulfur was a basic element rather than a compound.
Elemental sulfur was
once extracted from salt domes where it sometimes occurs in
nearly pure form, but this method has been obsolete since the late 20th
century. Today, almost all elemental sulfur is produced as a byproduct of
removing sulfur-containing contaminants from natural gas and petroleum. The element's largest commercial use (after mostly
being converted to sulfuric acid) is to produce sulfate and
phosphate fertilizers, because of the relatively high requirement of
plants for sulfur and phosphorus. Sulfuric acid is also a primary industrial
chemical outside fertilizer manufacture. Other well-known uses for the element
are in matches, insecticides and fungicides. Many sulfur compounds are odoriferous, and the
smell of odorized natural gas, skunk scent, grapefruit, and garlic is due to
sulfur compounds. Hydrogen sulfide produced
by living organisms imparts the characteristic odor to rotting eggs and other
biological processes.
Sulfur is an essential element for all life, and is widely used in
biochemical processes. In metabolic reactions, sulfur compounds serve as both
fuels (electron donors) and
respiratory (oxygen-alternative) materials (electron acceptors).
Sulfur in organic form is present in the vitamins biotin and thiamine, the latter being named for the
Greek word for sulfur. Sulfur is an important part of many enzymes and in
antioxidant molecules like glutathione and thioredoxin. Organically bonded sulfur is a component of all
proteins, as the amino acids cysteine and methionine. Disulfide bonds are largely responsible for the mechanical
strength and insolubility of the protein keratin, found in outer skin, hair, and feathers, and the
element contributes to their pungent odor when burned.