Phosphorus allotropic forms

Phosphorus exists in four or more allotropic forms white (or yellow), red, and black (or violet). Ordinary phosphorus is a waxy white solid when pure it is colorless and transparent. White phosphorus has two modifications alpha and beta with a transition temperature at -3.8oC. 

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

The element phosphorus forms a variety of allotropic forms in the solid state. In the chemistry stockroom, you are likely to find red phosphorus and possibly white phosphorus (Figure B). As you can see, white phosphorus has the molecular formula P4, whereas red phosphorus might be represented as Px, where x is a very large number. The difference in properties between the two allotropes reflects the difference in their bonding patterns, molecular versus network covalent ... 

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. 

The previous conflicting investigations may now be rationalized. Red phosphorus is known to thermally convert to white phosphorus, which will burn in air. If white phosphorus is formed, a fire is expected and no flame retardant activity will be observed. On the other hand, if the phosphorus reacts with the polymer as in Scheme 1, then thermal stabilization is expected. The efficacy of red phosphorus seems to be closely related to the efficiency of mixing of the additive and the polymer, when they are well-mixed the phosphorus will react with the polymer and lead to flame retardant activity, if the mixing is poor then the phosphorus will be converted to the white allotrope and burning will result. Since all of the work reported herein was carried out in sealed tubes under vacuum, the phosphorus must react and lead to stabilization of the polymer against molecular weight loss and fuel production, i.e. thermal stabilization. 

Although phosphorus is in group 15 with some other metalloids, it is usually classed as a nonmetal since it resembles nitrogen somewhat, the element above it in group 15. Both are essential to the biochemical field as vital elements to support life. Phosphorus has 10 known allotropic forms. This is an unusually high number for any element. A system of categorizing the allotropes by three colors has made it easier to keep track of them. These three colors are white, red, and black phosphorus. 

White phosphorus has a white waxy appearance that turns slightly yellow with age and impurities. There are two allotropic forms of white phosphorus. The alpha (a) form has a cubic crystal structure, and the beta (P) form has a hexagonal crystalline structure. White phosphorus is extremely reactive and will spontaneously burst into flame when exposed to air at a temperature of about 35°C. It must be kept under water. But this property of spontaneous combustion has made it useful for military applications. 

Elemental phosphorus in solid phase exists in three major allotropic forms ... 

Energy also is involved in transitions from one allotropic form to another, or from one crystal form to another. To change a mole of red phosphorus to yellow phosphorus, we must supply 4.22 kilocalories (AHT = +4.22 kcal/mole), and when 1 mole of yellow silicon disulfide changes to white silicon disulfide, 3.11 kcal is liberated (AHT = -3.11 kcal/mole). 

Phosphorus exists in two common allotropic forms white phosphorus and red phosphorus (Figure 19.9). White phosphorus, the form produced in the industrial synthesis, is a toxic, waxy, white solid that contains discrete tetrahedral P4 molecules. Red phosphorus, by contrast, is essentially nontoxic and has a polymeric structure. 

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 ... 

E. Cohen and K. Inouye 3 observed no sign of the formation of an allotropic form of yellow phosphorus at low temp. but P. W. Bridgman observed the formation of an allotropic modification of ordinary yellow phosphorus during some experiments on the efiect of high press, on ordinary phosphorus. This variety is formed when yellow phosphorus at 60° is compressed under 11,000 kgrms. per... 

Elemental phosphorus itself exists in several polymeric forms. If the white allotropic form, which consists of P4 tetrahedral molecules, is put under high pressure, preferably at elevated temperatures, it can be catalytically converted to other modifications.41 It first becomes red, then violet, then black as the degree of polymerization increases. These materials are very difficult to characterize because of branching and the formation of cyclics. In the extreme limit, the structure approaches that of graphite, and shows good electrical conductivity.42 No evidence exists at all for the formation of high-molecular-weight, linear chains of elemental phosphorus. 

There are three allotropic forms of elemental phosphorus white, red, and black phosphorus. At room temperature, pure white phosphorus is a tetrahedral crystal with a molecular formula of P4. In the pure form, white phosphorus is an ivory-colored, waxy solid. The commercial product is 99.9% pure and may have a slightly yellow color. In the literature, the commercial product is often referred to as yellow phosphorus. In this chapter, the terms white phosphorus and phosphorus are used to refer to P4, which includes white and yellow phosphorus. 

Elemental phosphorus exists in several allotropic forms (Van Wazer 1982). The best known and most important commercially is the a-white phosphorus whose properties are given in Table 3-2. Commercial white phosphorus is 99.9% pure, with a slight yellow color caused by traces of red phosphorus impurities. Hence, white phosphorus also is known as yellow phosphorus. When a-white phosphorus is cooled below -79.6°C, P-white phosphorus forms. Other important solid allotropes of phosphorus are red and black phosphorus (Van Wazer 1982). 

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. 

Elemental phosphorus is known in several allotropic forms. All forms melt to give the same liquid which consists of tetrahedral P4 molecules, as shown in Fig. 15.3.1(a). The same molecular entity exists in the gas phase, the P-Pbond length being 221 pm. At high temperature (> 800°C) and low pressure P4 is in equilibrium with P2 molecules, in which the fePbond length is 189.5 pm. 

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... 

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