Allotropes of phosphorus

Phosphorus (like C and S) exists in many allotropic modifications which reflect the variety of ways of achieving catenation. At least five crystalline polymorphs are known and there are also several amorphous or vitreous forms (see Fig. 12.3). All forms, however, melt to give the same liquid which consists of symmetrical P4 tetrahedral molecules, P-P 225 pm. The same molecular form exists in the gas phase (P-P 221pm), but at high temperatures (above 800°C) and low pressures P4 is in equilibrium with the diatomic form P=P (189.5 pm). At atmospheric pressure, dissociation of P4 into 2P2 reaches 50% at 1800°C and dissociation of P2 into 2P reaches 50% at 2800°. 

The commonest form of phosphorus, and the one which is usually formed by condensation from the gaseous or liquid states, is the waxy, cubic, white form o -P4 (d 1.8232 gcm at 20°C). This, paradoxically, is also the most volatile and reactive solid form and thermodynamically the least stable. It is the slow phosphorescent oxidation of the vapour above these crystals that gives white phosphorus its most characteristic property. Indeed, the emission of yellow-green light from the oxidation of P4 is one of the earliest recorded examples of chemiluminescence, though the details of the reaction 

CORBRIDGE, The Structural Chemistry of Phosphorus, Elsevier, Amsterdam, 1974, 542 pp. 

A typical modem phosphorus fumace (12 m diameter) can produce some 4 toniKs per hour and is rated at 60-70 MW (i.e. 140000A at SOOV). Three electrodes, each weighing 60 tonnes, lead in the current. The amounts of raw material required to make 1 tonne of white phosphorus depend on their purity but are typically 8 tonnes of phosphate rock. 2 tonnes of silica, 1.5 tonnes of coke, and 0.4 tonnes of electrode carbon. The phos rfKmis vapour is driven off from the top of the fumace together with the CO and some H2 it is passed through a hot electrostatic precipitator to remove dust and then condensed by water sprays at about 70 (P4 melts at 44.T). The byproduct CO is used for supplementary heating. 

World capacity for the production of elemental P is 1.5 million tonnes per year. Some figures for 1984 are as follows  

For a century after its discovery the only source of phosphorus was urine. The present process of heating phosphate rock with sand and coke was proposed by E. Aubertin and L. Boblique in 1867 and improved by J. B. Readman who introduced the use of an electric furnace. The reactions occurring are still not fully understood, but the overall process can be represented by the idealized equation  

The presence of silica to form slag which is vital to large-scale production was perceptively introduced by Robert Boyle in his very early experiments. Two apparently acceptable mechanisms have been proposed and it is possible that both may be occurring. In the first, the rock is thought to react with molten silica to form slag and P4O10 which is then reduced by the carbon  

In the second possible mechanism, the rock is considered to be directly reduced by CO and the CaO so formed then reacts with the silica to form slag  

Whatever the details, the process is clearly energy intensive and, even at 90% efficiency, requires 15MWh per tonne of phosphorus (see Panel). 

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One key point to note is that many of the representations used in chemistry teaching are a combination of symbols and models that is they involve both purely symbolic features and other forms of representation that involve less arbitrary features. For example, a figure such as Fig. 4.2 is a representation of one molecule (i.e. at the sub-microscopic level) of an allotrope of phosphorus. 

The allotropes of phosphorus may be identified from their physical properties. White phosphorus can be identified from its chemiluminescence (a pale... 

There are three common allotropes of phosphorus. White phosphorus consists of tetrahedral P4 molecules, whereas red phosphorus may be described as interlocking chains of phosphorus atoms linked by P P bonds. The most... 

Four allotropes of phosphorus are known, the hexagonal /(-white, stable only below —77°C, the cubic a-white trap 44.1°C), the violet, and the black (which is thermodynamically the most stable). The a-white form is usually taken as the standard state. The violet is obtained by continued heating at 500°C of a solution of phosphorus in lead. When a-white phosphorus is heated to 250 C in the absence of air, a red variety (rap 590CC) is obtained which is believed to consist of a mixture of the a-white and violet allotropes, although the studies of the violet component in the mixture have shown that at least four polymorphic forms of red (violet) phosphorus exist. 

Describe the structures of the white and red allotropes of phosphorus, and explain why white phosphorus is so reactive. 

A review of the alleged allotropes of phosphorus reduces their number to four, namely, the a- and/3-forms of yellow phosphorus, red or violet phosphorus, and black phosphorus. Most of the work of various investigators has been directed towards elucidating the nature of red phosphorus, and of the transformation of yellow to red phosphorus and conversely. Red phosphorus was formerly considered to be amorphous, and it was often called amorphous phosphorus. The term amorphous, however, here referred more to the general appearance of the powder rather than to its minute structure. J. W. Retgers 5 showed that the particles of ordinary red phosphorus are rhombohedral crystals, which are well developed in those of W. Hittorf s violet phosphorus. All four varieties are therefore crystalline. J. W. Terwen has reviewed this subject in a general way and M. Copisarow discussed the theory of allotropy,... 

We have discussed the structure of crystalline N2 in Section 4.4.1. Several allotropes of phosphorus are well known. In the gas phase there are P4 tetrahedral molecules, and this condenses as white (also described as yellow) P containing P4 molecules in the solid. There are two forms of white P, but the detailed structures are not known. There are inconsistencies in the structural reports. A monoclinic form of P contains cage-like P,s and P9 groups linked by pairs of P atoms to form... 

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). 

Heating of white phosphorus in the absence of air gives red phosphorus, an amorphous material that exists in a variety of polymeric modifications. Still another al-lotrope, black phosphoms, is the most thermodynamically stable form it can be obtained from white phosphorus by heating at very high pressures. Black phosphorus converts to other forms at still higher pressures. Examples of these structures are shown in Figure 8-23. The interested reader can find more detailed information on allotropes of phosphorus in other sources." ... 

FIGURE 8-23 Allotropes of Phosphorus. (Reproduced with permission from N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press,... 

Phosphorus Two of the three solid allotropes of phosphorus are shown in Figure 7-13. White phosphorus bursts into flames in air and must be stored in water. Red phosphorus is less reactive. It forms when white phosphorus is heated in the absence of air. Red phosphorus is used on the striking surface of matchboxes. The third allotrope, black phosphorus, is produced when either red or white phosphorus is heated under high pressure. 

Three allotropes of phosphorus are known, with molecular weights of 62.0, 31.0, and 124.0 amu, respectively. Write the molecular formula for each allotrope. 

White phosphorus is known to exist as a P4 molecule wliich is in a tetrahedral configuration containing an atom of phosphorus and an unshared pair of electrons at each apex. Therefore, this allotrope of phosphorus should be subject to easy attack by electrophilic reagents. It is somewhat surprising that only one such reaction has been reported When a solution of wliite phosphorus in carbon disulfide and one molar equivalent of AICI3 at -10 °C was treated witli... 

The white, red, and black allotropes of phosphorus have different properties. Note the differences in their structures. 

White and red phosphorus are two common allotropes of phosphorus. Notice that the white phosphorus is photographed under a liquid because this form of phosphorus, which has the formula P4, reacts spontaneously with oxygen in the air. Red phosphorus is used in making matches. You can read about it in Everyday Chemistry. 

Figure 14.12 Two allotropes of phosphorus. A, White phosphorus exists as individual P4 moleoules, with the P—P bonds forming the edges of a tetrahedron. B, The reaotivity of P4 is due in part to the bond strain that arises fro m the 60° bond angle. Note how overlap of the 3p orbitals is decreased beoause they do not meet direotly end to...

There are three common allotropes of phosphorus and several other modifications of these, some of which have indefinite structures. The common forms are ... 

Phosphorus penta sulphide was discovered by Berzelius in 1843 and the red allotrope of phosphorus was prepared by Von Schrotter in 1848. 

The red allotrope of phosphorus is an even better cali-brant than elemental sulfur in many respects. Red phosphorus is a linear polymer (linearly interconnected units of P4 tetrahedra [20]) of considerable chemical stability. Particularly, it is resistant to aerial oxidation. In addition, natural phosphorus is monoisotopic ( P [100%]). Consequently, each peak in its mass spectrum is devoid of satellite peaks due to isotopologs. Peaks are evenly spaced at 31 mJz units apart, and the mass range of the peaks obtained is more extensive compared with that from sulfur (Figure 50.7). Similar to the behavior of sulfur, red phosphorus also ionizes well in both ioniza-... 

A Figure 22.39 White and red allotropes of phosphorus. White phosphorus is very reactive and is normally stored under water to protect it from oxygen. Red phosphorus is much less reactive than white phosphoms, and it is not necessary to store it under water. 

Two allotropes of phosphorus. White phosphorus (atrore) ignites and hums rapidly when exposed to oxygen in the air, 0 it is stored under water. Red phosphoras below) reacts wtth air much more slowly, so tt can he stored in contact wtth air. 

For the elusive nitrogen triazide species Nio (also see Fig. 6), the cis structure (C3) was calculated to be the most energetically favorable form but significantly less stable than the D2d symmetric bispentazole (see above). We close this section by noting that none of these novel and still hypothetical polynitrogen species corresponds to any of the known allotropes of phosphorus. 

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