sulfur, 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.