Phosphorus

I

Introduction

Phosphorus, symbol P, reactive nonmetallic 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 (or Va) of the periodic table (see Periodic Law).

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. The element’s name was drawn from the Greek word phosphoros, “light bearing,” because many phosphorus compounds are phosphorescent: they store light and give it off later.

II

Properties and Occurrence

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), boils at 280°C (536°F), and has a specific gravity of 1.8. White phosphorus is prepared commercially by heating calcium phosphate with sand (silicon dioxide) and coke in an electric furnace.

When heated to between 230° and 300°C (446° and 572°F) in the absence of air, white phosphorus is converted into the red form. Red phosphorus is a microcrystalline, nonpoisonous 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 ranging from 2.3 to 2.7.

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Phosphorus is widely distributed in nature and ranks 11th in abundance among the elements in Earth’s crust. 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.

III

Uses

Most compounds of phosphorus are trivalent or quinquevalent. Phosphorus combines readily with oxygen to form oxides, of which the most important are phosphorous oxide (P₂O₃) and phosphoric oxide (P₂O₅). Phosphorus oxide, a white crystalline solid, is used as a reducing agent. It is deliquescent—that is, it is dissolved by the moisture in air. The vapor is toxic. Phosphoric oxide, a white, deliquescent, amorphous solid, sublimes at 250°C (482°F). It reacts with water to form phosphoric acid and is used as a drying agent.

Phosphorus forms hydrides with hydrogen; the important hydride of phosphorus is PH₃, which is comparable to ammonia (NH₃), the hydride of nitrogen. All of the halogens combine directly with phosphorus to form halides, which are used in the preparation of halogen acids and organic compounds. The most important commercial compounds of phosphorus are phosphoric acid and the salts of phosphoric acid, called phosphates. The bulk of phosphorus-containing compounds are used as fertilizers. Phosphorus compounds are also used in clarifying sugar solutions, weighing silk, and fireproofing, and in such alloys as phosphor bronze and phosphor copper. White phosphorus is used in the making of rat poison, and red phosphorus is used in matches.

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