Unary substance

It has already been shown (p. 33) that red phosphorus probably is not a unary substance, and that the difference between the vapour pressures of red and violet phosphorus below about 400° C. are probably due to the non-equilibrium conditions in the red form. Even in the case of the more uniform violet modification, however, time is required for the establishment of equilibria with vapour, and the values of the pressures even up to 500° C. are affected by an uncertainty on this account. Condensation of vapour proceeds in general more slowly than vaporisation, and especially is this the case where there is a great difference between the molecular complexities of the vapour and of the solid. 

Note —A unary substance is one whose molecules are all identical physically as well as chemically A pseudo-binary substance is one whose molecules are chemically the same (as regards ultimate analysis), but nevertheless may be divided into two sorts differing from one another in respect of physical properties, and there exists an equilibrium between the two sorts... 

In the systems hitherto discussed (except sulphur and phosphorus), the components behaved, or were regarded as behaving, as strictly unary substances that is, the molecules of each component in all the phases in which it occurred were identical both physically and chemically. Each component formed only one molecular species, and the number of molecular species was, therefore, equal to the number of the components. The systems were purely unary (one-component) or purely binary (two-component). 

If the bonding atoms belong to the same element, the substance is called simple substance = elementary substance = unary substance (A ). 

In the unary substances case, the atoms, being identical, have equal radii and electronegativity, so their organization in solid state depends only by the distribution of the external electrons the present discussion follows (Chiriac-Putz-Chiriac, 2005). 

The unary substances of these elements have in the nodes of the network small molecules. The number of the neighboring atoms in a such stable molecule is determined by the Humme-Rotiiery rule = the crystallochemical rule = 8-N rule, where N is the number of the group where the element is placed. 

This arrangement was deduced in other way for no. e7no. AO ratio as previously exposed the unary substance is, in this case, the... 

The unary substances of the elements of this category form the lattice that have molecular nodes while the van der Waals interactions are exercised between them. 

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

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. 

But there are not a few systems in which the number of molecular species is greater than the number of components that is, substances which have the same chemical composition (but which may be isomeric forms) may give rise to different molecular species, between which, in the liquid or vapour state, a condition of equilibrium can exist. This fact may alter very markedly the behaviour of a system. Although, therefore, a system may appear to be unary, so far as chemical composition is concerned, it may, as a matter of fact, behave in some respects as a binary system. It forms a pseudo-binary system. The behaviour of these systems, as we shall see, depends largely on the rate at which the internal equilibrium between the different molecular species in the liquid or vapour phase is established. In the present chapter some of the more important aspects of these pseudobinary systems will be considered. 

It has already been indicated (p. 53), that in the case of sulphur we have a substance which can give rise to different molecular species which, in the liquid state, form an equilibrium mixture. For this reason, sulphur will behave not as a purely unary system, but in a manner similar to that of dynamic isomerides discussed in this chapter. We shall therefore discuss briefly the more important equilibrium relations of sulphur from this point of view. 

Perhaps the simplest and easiest type of phase diagram to understand is that for a one-component system, in which composition is held constant (i.e., the phase diagram is for a pure substance) this means that pressure and temperature are the variables. This one-component phase diagram (or unary phase diagram, sometimes also called a pressure-temperature [or P-T diagram) is represented as a two-dimensional plot of... 

Further, according to Ricci, although a single substance must behave as a one-component system, unary behaviour does not guarantee that a material is a single substance (9, 165). Finally, the phase rule approach doesn t always make a clear-cut distinction between unary and binary systems or between compound and solution (125). 

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