sulfur dioxide, chemical compound, SO₂, a colorless gas with a pungent, suffocating odor. It is readily soluble in cold water, sparingly soluble in hot water, and soluble in alcohol, acetic acid, and sulfuric acid. It is corrosive to organic materials and dissolves in water to form sulfurous acid, H₂SO₃. Sulfur dioxide is used in bleaching and in chemical manufacture and as a refrigerant and a food preservative, e.g., for fumigating fruit. It may be produced by reaction of sulfur with oxygen, e.g., by burning sulfur in air, and it is often produced during the roasting of sulfide ores, e.g., in zinc smelting. Sulfur dioxide is a dangerous air pollutant because of its corrosive properties; it irritates the eyes, nose, and lungs. It is produced by combustion of coal, fuel oil, and gasoline, since these fuels contain sulfur. The sulfur content of a fuel can be reduced by refining, so that less sulfur dioxide is emitted when the fuel is burned.

sulfur or sulphur, nonmetallic chemical element; symbol S; at. no. 16; at. wt. 32.06; m.p. 112.8°C; (rhombic), 119.0°C; (monoclinic), about 120°C; (amorphous); b.p. 444.674°C;; sp. gr. at 20°C;, 2.07 (rhombic), 1.957 (monoclinic), 1.92 (amorphous); valence -2, +4, or +6. Sulfur was known to the ancients; it is the brimstone of the Bible. It was first recognized as an element in 1777 by A. L. Lavoisier.

Properties and Compounds

Sulfur is found in Group 16 of the periodic table. It exhibits allotropy. Solid sulfur occurs principally in three forms, all of which are brittle, yellow in color, odorless, tasteless, and insoluble in water. Two of these solid forms are crystalline, composed of molecules containing eight sulfur atoms and having molecular weight 256.512 amu. Rhombic sulfur has orthorhombic crystalline structure and is stable below 95.5°C;; most sulfur is in this form. The monoclinic, or prismatic, form has long, needlelike, nearly transparent crystals; it is stable between 95.5°C; and its melting point but reverts to the rhombic form on standing at room temperature. Amorphous sulfur is a dark, noncrystalline, gumlike substance. It is often thought to be a supercooled liquid; it is formed by rapidly cooling molten sulfur, e.g., by pouring it into cold water. It slowly reverts to the rhombic form on standing. The crystalline forms are readily soluble in carbon disulfide, but the amorphous form is not. Many other forms of sulfur exist. Liquid sulfur is unusual in that its viscosity increases as it is heated. This property is thought to be due to the formation of long polymeric chains of sulfur molecules.

Sulfur is a chemically active element and forms many compounds, both by itself (sulfides) and in combination with other elements. It is part of many organic compounds, e.g., mercaptans (thiols) and thio compounds. It burns in air with a blue flame, forming sulfur dioxide, SO₂.

Natural Occurrence and Processing

Sulfur is widely distributed in nature. It is found in many minerals and ores, e.g., iron pyrites, galena, cinnabar, zinc blende, gypsum, barite, and epsom salts and in mineral springs and other waters. It is found uncombined in some volcanic regions and in large underground deposits in Sicily and in Texas and Louisiana. Sulfur often occurs with coal, petroleum, and natural gas. Sulfur is found in meteorities, and deposits of it may be present near the lunar crater Aristarchus. The distinctive colors of Jupiter's moon Io are believed to result from forms of molten, solid, and gaseous sulfur. Sulfur is a component of all living cells. The amino acids cysteine, methionine, homocysteine, and taurine contain sulfur as do some common enzymes; it is a component of most proteins. Some forms of bacteria use hydrogen sulfide (H₂S) in place of water in a rudimentary photosynthesislike process. Sulfur is absorbed by plants from soil as sulfate ions.

Sulfur is produced chiefly by the Frasch process, although it is also produced by the Sicilian method and by other methods. In the Sicilian method the sulfur-bearing ores are piled in a mound and ignited. The heat produced by the burning melts some of the sulfur, which is collected and cast. This sulfur is impure and is usually purified by sublimation. Sulfur is also recovered from natural gas, coal, crude oil, and other sources, e.g., the flue dusts and gases from the refining of metal sulfide ores. Elemental sulfur is obtained in several forms, including flowers of sulfur, a fine crystalline powder, and roll sulfur (cast cakes or sticks).

Uses

Elemental sulfur is used in black gunpowder, matches, and fireworks; in the vulcanization of rubber; as a fungicide, insecticide, and fumigant; in the manufacture of phosphate fertilizers; and in the treatment of certain skin diseases. The principal use of sulfur, however, is in the preparation of its compounds. The most important sulfur compound is sulfuric acid. Other important compounds include sulfur dioxide, used as a bleaching agent, disinfectant, and refrigerant; sodium bisulfite, used in paper manufacture; carbon disulfide, an important organic solvent; hydrogen sulfide, sulfur trioxide, and thionyl chloride, used as reagents in chemistry; Epsom salts (magnesium sulfate), used as a laxative, bath additive, exfoliant, and magnesium supplement in plant nutrition; the numerous other sulfate compounds; and sulfa drugs.

lime-sulfur, mixture of calcium polysulfides formed by reacting (e.g., by boiling) calcium hydroxide (slaked lime) with sulfur. It is used as an insecticide and fungicide.

Atomic Number:	Atomic Number: 16
Atomic Symbol:	Atomic Symbol: S
	Name of Element: Sulfur
Atomic Weight:	Atomic Weight: 32.06
Electron Configuration:	Electron Configuration: 2 · 8 · 6

or sulfur butterfly

Any of several species of butterflies (family Pieridae) that are found worldwide. Adults have a wingspan of 1.5–2.5 in. (35–60 mm). The colour and pattern of many species vary seasonally and between sexes, but they are generally bright yellow or orange. Some have two colour patterns; for example, Colias eurytheme is usually orange with black wing margins, but some females are white with black margins. Pupae are attached to a twig by a posterior spine and a girdle of silk. The larvae feed on clover and may seriously damage crops.

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Any of a diverse group of bacteria that are capable of metabolizing sulfur and its compounds and are important in the sulfur cycle. Members of the genus Thiobacillus, widespread in marine and terrestrial habitats, react with sulfur to produce sulfates useful to plants; in deep ground deposits they generate sulfuric acid, which dissolves metals in mines and corrodes concrete and steel. Desulfovibrio desulficans reduces sulfates in waterlogged soils and sewage to hydrogen sulfide, a gas with the common rotten-egg odour.

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Nonmetallic chemical element, chemical symbol S, atomic number 16. It is very reactive but occurs native in deposits, as well as combined in various ores (e.g., pyrite, galena, cinnabar); in coal, petroleum, and natural gas; and in the water in sulfur springs. Sulfur is the third most abundant constituent of minerals and one of the four most important basic chemical commodities. Pure sulfur, a tasteless, odourless, brittle yellow solid, occurs in several crystalline and amorphous allotropes, including brimstone and flowers of sulfur. It combines, with valence 2, 4, or 6, with nearly all other elements. Its most familiar compound is hydrogen sulfide, a poisonous gas that smells like rotten eggs. All metals except gold and platinum form sulfides, and many ores are sulfides. The oxides are sulfur dioxide and sulfur trioxide, which when dissolved in water make sulfurous acid and sulfuric acid, respectively. Several sulfur compounds with halogen elements are industrially important. Sodium sulfite (Na₂SO₃) is a reducing agent used to pulp paper and in photography. Organic compounds with sulfur include several amino acids, the sulfa drugs, and many insecticides, solvents, and substances used in making rubber and rayon.

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