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Sulfur is found in group VIa 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.
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