Allotropic forms of phosphorus

Table 10. Properties of Allotropic Forms of Phosphorus(V) Oxide...

Several allotropic forms of phosphorus are known, the most common of which are the white, red, and black forms. Heating the white form at 400 °C for several hours produces red phosphorus, which is known to include several forms. A red form that is amorphous can be prepared by subjecting white phosphorus to ultraviolet radiation. In the thermal process, several substances (I2, S8, and Na) are known to catalyze the conversion of phosphorus to other forms. Black phosphorus consists of four identifiable forms that result when white phosphorus is subjected to heat and pressure. Phosphorus is used in large quantities in the production of phosphoric acid and other chemicals. White phosphorus has been used extensively in making incendiary devices, and red phosphorus is used in making matches. 

I. Remsen and E. H. Keiser considered that they had obtained a special allotropic form of phosphorus by suddenly cooling the vapour of phosphorus by iced-water. The product is here red phosphorus, for, as shown by A. Stock and co-workers, red phosphorus can be produced by suddenly quenching phosphorus vapour at 900 -1175. H. M. Vernon reported that rhombic phosphorus, i.e. rhombic crystals of phosphorus, can be obtained by slowly cooling liquid phosphorus. This observation, however, remains unverified ... 

Allotropic forms of phosphorus. Solid phosphorus exists in two distinct allotropic modifications and is also commonly encountered in a form consisting of a mixture of the two. White (or yellow) phosphorus is a translucent, waxlike solid which melts at 44°C, boils at about 290°C, and has a density of 1.83. When vaporized, the resulting gas consists of tetraatomic molecules (P4) up to a temperature of about 1500°C, whereupon these molecules partly dissociate into (and exist in equilibrium with) diatomic molecules (P2). White phosphorus is insoluble in water but is soluble in solvents such as ethyl ether and carbon disulfide. Great care should always be exercised in handling this form of phosphorus since it is highly flammable and very poisonous. Skin burns caused by phosphorus are exceedingly painful and very slow to heal. 

The red allotropic form of phosphorus is relatively nontoxic and, unlike white phosphorus, is not spontaneously flammable. Red phosphorus is, however, easily ignited. It is a polymeric form of phosphorus, thermally stable up to ca. 450°C. In its finally divided form, it has proved to be a powerful flame-retardant additive.18 Elemental red phosphorus is a highly efficient flame retardant, especially for oxygen-containing polymers such as polycarbonates and polyethylene terephthalate). Red phosphorus is particularly useful in glass-filled polyamide 6,6, where high processing temperature (about 280°C) excludes the use of most phosphorus compounds.19 In addition, coated red phosphorus is used to flame retard nylon electrical parts, mainly in Europe and Asia.20... 

Farr (109)y of the Tennessee Valley Authority, has compiled a resume of the physical and thermodynamic properties of the allotropic forms of phosphorus. Based on entropy calculations from low temperature heat capacity measurements, Stephenson (318) believes that red crystalline triclinic phosphorus (T.V.A. designation V) is the most stable form at room temperature. This point of view is buttressed by the x-ray work of Roth, DeWitt, and Smith (376). Consequently we have selected red phosphorus V as the reference state up to its sublimation point at 704° K. 

There are several allotropic forms of phosphorus, but only white phosphorus and red phosphorus (see Figure 8.18) are of importance. White phosphorus consists of discrete tetrahedral P4 molecules (Figure 21.9). A solid (m.p. 44.2°C), white phosphorus is insoluble in water but quite soluble in carbon disulfide (CS2) and in organic solvents such as chloroform (CHCI3). White phosphorus is a highly toxic substance. It bursts into flames spontaneously when exposed to air hence it is used in incendiary bombs and grenades ... 

Toxicolog y.—The red variety differs from the other allotropic forms of phosphorus in not being poisonous, probably owing to its insolubility, and in being little liable to cause injury by burning. 

Arnauld Paul Edmond Thenard (Paris, 6 October 1819-Talmay, Cote d Or, 8 August 1884), son of the famous L. J. Thenard, baron, wealthy landowner in the Cote d Or and Saone et Loire, and interested mainly in agricultural chemistry, worked (partly with A. Thenard) on phosphorus hydrides, on ozone, the action of a silent electric discharge on gases, and a black allotropic form of phosphorus. In the determination of ozone he used the oxidation of a solution of arsenious oxide in hydrochloric acid. 

The stable allotropic form of phosphorus is P4, in which each P atom is bonded to three other P atoms. Draw a Lewis structure of this molecule and describe its geometry. At high temperatures, P4 dissociates to form P2 molecules containing a P=P bond. Explain why P4 is more stable than P2. 

When heated under normal pressures, all allotropic forms of phosphorus will produce a vapour consisting of tetrahedral P4 molecules (Figure 4.3). The interbond angles of 60° represent a highly strained arrangement for which hybridised pd orbitals have been proposed. The tetrahedral P4-type structure is unusual but not quite unique in chemistry. Elements of the same group adopt it, that is, AS4, Sb4 and probably Bi4 (see below). The existence of isoelectronic Si, Ge ", Sn and Pb " anions has been established in recent years. 

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