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Phosphorus, symbol P, reactive non-metallic element that is important to living organisms and has many industrial uses. The atomic number of phosphorus is 15, and its atomic weight is 30.974. Phosphorus is in group 15 (Va) of the periodic table. Phosphorus was discovered about 1669 by the German alchemist Hennig Brand in the course of experiments in which he attempted to prepare gold from silver. Phosphorus exists in three main allotropic (distinctly different) forms: ordinary (or white) phosphorus, red phosphorus, and black phosphorus. Of these, only white and red phosphorus are of commercial importance. When freshly prepared, ordinary phosphorus is white, but it turns light yellow when exposed to sunlight. It is a crystalline, translucent, waxy solid, which glows faintly in moist air and is extremely poisonous. It ignites spontaneously in air at 34° C (93° F) and must be stored under water. It is insoluble in water, slightly soluble in organic solvents, and very soluble in carbon disulfide. White phosphorus melts at 44.1° C (111.4° F) and boils at 280° C (536° F). When heated to between 230° and 300° C (446° and 572° F) in the absence of air, it is converted into the red form. Red phosphorus is a microcrystalline, non-poisonous powder. It sublimates (passes from the solid state directly to the gaseous state) at 416° C (781° F) and has a specific gravity of 2.34. Black phosphorus is made by heating white phosphorus at 200° C (392° F) at very high pressure. It has a specific gravity of 2.69. Phosphorus is widely distributed in nature and ranks 11th in abundance among the elements in the crust of the earth. It does not occur in the free state but is found mostly as a phosphate, as in phosphate rock and apatite. It is also found in the combined state in all fertile soil and in many natural waters. The element is important in plant and animal physiology and is a constituent of all animal bones, in the form of calcium phosphate.
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