Phosphorus, allotropy

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

In 1841 Jons Jakob BerzeUius (1779—1884), who introduced the term allotropy, transformed white phosphorus to red phosphorous. In 1865 Johann Wilhehn Hittorf (1824—1914) was the first to produce metalhc phosphorus. Brand, however, was given credit for the discovery of phosphorus. 

Arsenic exhibits allotropy, which is characteristic of non-metals the usual, more stable, metallic form resembles the typical metals in appearance and in being a fairly good conductor of electricity. Under atmospheric pressure it begins to volatilise at about 450° C. and passes into a vapour containing complex molecules, As4, which at higher temperatures dissociate to As2 this complexity is not unusual in non-metals. The yellow allotrope, which is stable at low temperatures, resembles white phosphorus in being soluble in carbon disulphide—a property which emphasises the non-metallic character of this variety. The reactivity of the allotropes, as in the case of phosphorus, differs considerably. 

Phosphorus is one of the most remarkable of the many remarkable substances known to the chemist. The curious method of its discovery, the universality of its distribution, its intimate connection with the phenomena of animal and vegetable life, its extraordinary physical properties and chemical activity, its abnormal molecular constitution, the protean ease of its allotropie transformation, all combine to make up a history which abundantly justifies its old appellation of the phosphorus miraibilia.—T. E. Thorpe. 

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,... 

Phosphorus exhibits allotropy, and formerly was thought to exist in two forms, yellow and red. In addition, several other varieties, scarlet, violet, metallic and black phosphorus, were discovered later, some of which are perhaps not to be regarded as true allotropes. Their properties will be considered partly in the present chapter, and partly in Chapter III. 

An examination of the behaviour of red and violet phosphorus (and indeed all solid forms) in the light of this theory leads to the conclusion that they are mixtures, with the difference that while violet phosphorus is capable of behaving in a unary manner, red phosphorus is not. Violet phosphorus is a mixture, because when it is heated to 360° C. in a vacuum, and the vapour is thus rapidly removed, the vapour pressure falls.2 The inner equilibrium has not in these circumstances time to adjust itself to the loss of the volatile Pa molecules, the residue becomes poorer in this kind and therefore has a lower vapour pressure. The production of red phosphorus below 400° C. may be explained partly by an increase in the proportion of Pa molecules in the solid solution of the pseudo-components, but principally by a delay in the establishment of the equilibrium, which leads to the production of solid solutions still richer in Pa, which are not in equilibrium but which constitute the ordinary red phosphorus. This therefore is not an allotropie modification, if such a modification is defined as a substance which can exist in inner equilibrium and which is able to behave in a unary manner. 

Phosphate rock Microcosmicsalt Disodium phosphate (/) Illustrate allotropy by phosphorus. 

A number of facts lead to the view that in the case of phosphorus as in the case of sulphur, a condition of dynamic allotropy exists (Cohen and Olie, Z. pkysikaL Chem., 1910, 71, I Stock and Stamm, Ber., 1913, 46, 3497 Smits and Bokhorst, Z, pkysikaL Chem, 1916, 91, 249). The equilibrium relations would therefore be those of a pseudo-binary system or of a system of perhaps even higher order. For a discussion of the phosphorus systems from this point of view, see Smits, FersL K, Akad. Wetensch, Amsterdam, 1912, 21, 753 1914, 22, 1145 Smits and Bokhorst, Z, physikal. Chem., 1916, 91, 249,... 

A discussion of the phosphorus equilibria from the point of view of the existence of dynamic allotropy in the liquid as well as in the solid phase is given by Smits, Z. physikal. Chem., 19 ii, 76, 439. For an experimental investigation of the relationships, see Smits and de Leeuw, Z. physikal. Chem.. 1911, 77, 367 Smits and Bokhorst, Z. physikal. Chem., 1916, 91, 249. See also Marckwald and Helm-holz. Z. anorgan. Chem., 1922, 124, 81. 

Phosphorus as Pseudo-binary System.—For the purpose of illustrating the behaviour of a one-component system as interpreted by means of the theory of allotropy put forward by Smits, a brief discussion of the behaviour of phosphorus may be given. ... 

Phosphorus exhibits complicated allotropy eleven forms have been reported, of which at least five are crystalline. Crystalline white phosphorus contains tetrahedral P4 molecules (Figure 14.3 a) in which the P—P distances (221 pm) are consistent with single bonds (r ov = 110 pm). White phosphorus is defined as the standard state of the element, but is actually... 

The allotropy of carbon, oxygen, phosphorus, and sulfur results from the versatility of their covalent bonding. Carbon occurs as diamond and as graphite (Fig. 21.1). Diamond is extremely hard, in consequence of its stable network covalent structure, which is entirely o--bonded. Graphite is relatively soft, in part because of the ease with which its TT-bonded atomic layers can slip past one another. At ordinary temperatures and pressures, both forms are quite unreactive, and graphite is the form with lower free energy (more stable) by about 0.7 kcal/mole. 

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