Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a bright yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals. It is an essential element for life and is found in two amino acids: cysteine and methionine. Its commercial uses are primarily in fertilizers, but it is also widely used in black gunpowder, matches, insecticides and fungicides. Elemental sulfur crystals are commonly sought after by mineral collectors for their brightly colored polyhedron shapes. In nonscientific contexts, it can also be referred to as brimstone.
At room temperature, sulfur is a soft, bright-yellow solid with only a faint odor, similar to that of matches (the strong "smell of sulfur" usually refers to the odor of hydrogen sulfide (H2S) or organosulfur compounds). Sulfur is an electrical insulator. It melts slightly above 100 °C and easily sublimes. Sulfur burns with a blue flame that emits sulfur dioxide, notable for its peculiar suffocating odor (this is the odor of burnt matches). Sulfur is insoluble in water, but soluble in carbon disulfide — and to a lesser extent in other non-polar organic solvents such as benzene and toluene. Sulfur in the solid state ordinarily exists as cyclic crown-shaped S8 molecules. The crystallography of sulfur is complex. Depending on the specific conditions, the sulfur allotropes form several crystal structures, with rhombic and monoclinic S8 best known.
Unlike most other liquids, molten sulfur increases in viscosity with temperatures of 200 °C (392 °F) due to the formation of polymers. The molten sulfur assumes a dark red color above this temperature. At still higher temperatures, however, the viscosity is decreased as depolymerization occurs.
Amorphous or "plastic" sulfur can be produced through the rapid cooling of molten sulfur. X-ray crystallography studies show that the amorphous form may have a helical structure with eight atoms per turn. This form is metastable at room temperature and gradually reverts to crystalline form. This process happens within a matter of hours to days but can be rapidly catalyzed.
