Sulfur (sometimes spelt sulphur in British English) is chemical element number 16 in the periodic table, with symbol S. It is pale yellow in appearance, soft, light, and distinctly odorous. Sulfur is found naturally in free form near hot springs and volcanic regions, and also compounded. It is used for many industrial processes such as the production of sulfuric acid for batteries, the production of gunpowder, and the vulcanization of rubber. Sulfur is also found in small amounts in coal and petroleum, which produce sulfur dioxide when burned. In addition to being quite toxic, sulfur dioxide reacts with atmospheric water to produce acid rain.
The amino acids cysteine and methionine contain sulfur, as do some common enzymes, making sulfur a necessary component of all living cells. Many of the unpleasant odors of organic matter are based of sulfur-containing compounds such as hydrogen sulfide, which has the characteristic smell of rotten eggs. See the sulfur cycle.
Sulfur is traditionally spelt sulphur in British English, but IUPAC has adopted the spelling sulfur, as has the Royal Society of Chemistry Nomenclature Committee. Increasingly sulfur is being used in British English instead.