Phosphorus

/fos"feuhr euhs/, n.

Phosphor.

* * *

Nonmetallic chemical element, chemical symbol P, atomic number 15.

The ordinary allotrope, called white phosphorus, is a poisonous, colourless, semitransparent, soft, waxy solid that glows in the dark (see phosphorescence) and combusts spontaneously in air, producing dense white fumes of the oxide P₄O₁₀; it is used as a rodenticide and a military smokescreen. Heat or sunlight converts it to the red phosphorus allotrope, a violet-red powder that does not phosphoresce or ignite spontaneously. Much less reactive and soluble than white phosphorus, it is used in manufacturing other phosphorus compounds and in semiconductors, fertilizers, safety matches, and fireworks. Black phosphorus, made by heating the white form under pressure, is flaky like graphite. Phosphorus seldom occurs uncombined in nature. As the phosphate ion, it is abundant and widely distributed, in apatite, phosphorite, and many other minerals. Phosphorus has valence 3 or 5 in compounds, which have many uses in industry. Phosphine (PH₃) is a chemical raw material and a doping agent (deliberately added impurity) for solid-state electronics components. Organic phosphorus compounds are used as plasticizers, gasoline additives, insecticides (e.g., parathion), and nerve gases. In living organisms the role of phosphorus is essential; it is a component of DNA and RNA, ATP, and bone.

* * *

▪ chemical element

Introduction

 nonmetallic chemical element of the nitrogen family (Group Va of the periodic table).

Properties, occurrence, and uses.

      Ordinarily a colourless, semitransparent, soft, waxy solid that glows in the dark, it takes fire spontaneously upon exposure to air and forms dense white fumes of the oxide. Phosphorus was first prepared in elemental form in 1669 by a German alchemist, Hennig Brand, from a residue of evaporated urine.

      Phosphorus is present in the fluids within cells of living tissues as the phosphate ion, PO₄³⁻, one of the most important mineral constituents for cellular activity. The genes, which direct heredity and other cellular functions and are found in the nucleus of each cell, are molecules of DNA (deoxyribonucleic acid), which all contain phosphorus. Cells store the energy obtained from nutrients in molecules of adenosine triphosphate (ATP). Calcium phosphate is the principal inorganic constituent of teeth and bones.

      Not found free in nature except in a few meteorites, phosphorus occurs in compounds that are widely distributed in many rocks, minerals, plants, and animals. Ranking 12th in abundance among the elements in the Earth's crust, phosphorus constitutes approximately 0.10 percent of the crust in the form of minerals such as apatite, wavellite, and vivianite; it always occurs as the phosphate ion. The chief commercial source is phosphorite, or phosphate rock, an impure massive form of carbonate-bearing apatite.

      Elemental phosphorus is prepared industrially in electric furnaces in which phosphate rock, coke, and silica pebble are continuously charged and heated until they are chemically converted into phosphorus vapour, carbon monoxide gas, and a calcium silicate slag. The stream of gas is cooled to condense the phosphorus to the liquid and eventually to the solid form, which is stored under water to prevent spontaneous ignition.

      The element has about 10 forms (allotropes) that occur within three major categories: white, red, and black. White phosphorus has two allotropes: the alpha form, which is stable at ordinary temperatures, has a cubic crystal structure; the beta form, which is stable below −78° C (−108° F), has a hexagonal crystal structure. White phosphorus is poisonous. Exposure to sunlight or to heat converts it to red phosphorus, which neither phosphoresces nor spontaneously burns in air. Black phosphorus is flaky like graphite and is made by subjecting white phosphorus to high pressures. It is chemically the least reactive form; white is by far the most reactive. White phosphorus has been used for military purposes as a source of smoke and to fill incendiary shells and grenades. Red phosphorus is used in preparing the striking surface for safety matches.

      All naturally occurring phosphorus is the stable isotope, phosphorus-31. Radioactive phosphorus-32 has a half-life of 14.3 days; it is a useful tracer in studies of the life cycles of plants and animals.

Principal compounds.

      Phosphorus is used almost entirely in the form of compounds, usually in the oxidation states of +3, +5, and −3. Unlike nitrogen and various other members of the family, phosphorus tends to exhibit a preference for the +5 state.

      Of considerable economic significance is phosphine, or hydrogen phosphide, PH₃. This gaseous compound is produced either by the action of a strong base (or hot water) on white phosphorus or by the hydrolysis of a metal phosphide. Phosphine is used mainly as a starting material in the synthesis of various organic phosphorus compounds and as a doping agent for solid-state electronics components.

      Among the most commercially important phosphorus compounds are the oxides and acids. Much of the industrially produced white phosphorus is burned to form phosphorus pentoxide, P₄O₁₀. Sometimes called phosphoric anhydride, or diphosphorus pentoxide, this compound can be obtained in the form of a soft white powder or colourless crystalline solid. It is widely used in chemical analysis as a dehydrating agent and in organic synthesis as a condensing agent. Large quantities are treated with water to make orthophosphoric acid (H₃PO₄), commonly called phosphoric acid (q.v.), which has diverse industrial applications, including the production of phosphates, salts that contain the phosphate ion (PO₄³⁻), the hydrogen phosphate ion (HPO₄²⁻), or the dihydrogen phosphate ion (H₂PO₄⁻). Such salts are used as leavening agents in baking, as abrasives in toothpaste, and sometimes as additives to detergents. Another salt, prepared by the action of phosphoric acid on phosphate rock, is calcium dihydrogen phosphate, or superphosphate, Ca(H₂PO₄)₂, the most widely used phosphate fertilizer.

      With the halogen elements phosphorus forms various halides; PX₃ (in which X is F, Cl, Br, or I) and PX₅ (in which X is F, Cl, or Br) are the two simple series. Interestingly, the solids PCl₅ and PBr₅ contain PX₄⁺ cations and PX₆⁻ anions rather than PX₅ molecules. These halides are used to synthesize organic phosphorus chemicals. Phosphorus reacts with sulfur to form several sulfides that are utilized in the manufacture of organic chemicals and matches. It reacts with many metals and metalloids to form phosphides.

      Phosphorus atoms can bond with oxygen atoms to form ester groups. These can bond with carbon atoms, yielding a large number of organic phosphorus chemicals. These are found in many important biological processes. The phosphoglycerides, for example, are required for fermentation. The adenosine phosphates are essential in photosynthesis and for muscle action. Industrially important organic phosphorus compounds include plasticizers and gasoline additives. Certain highly toxic forms are employed in insecticides of the parathion type. Poisonous organic derivatives of phosphorus have been used as nerve gas, a key weapon of chemical warfare.

atomic number

15

atomic weight

30.9738

melting point (white)

44.1° C (111.4° F)

boiling point (white)

280° C (536° F)

density (white)

1.82 g/cm³ at 20° C (68° F)

oxidation states

−3, +3, +5

electron config.

1s²2s²2p⁶3s²3p³

* * *