Page:EB1911 - Volume 27.djvu/875

Rh the estimate we are able to form, on the one hand, of the functions of hydrogen, on the other of those of elements such as chlorine, oxygen and nitrogen, it seems probable that in the hydrides of these elements the extra attractive power is exercised entirely by the element which enters into combination with the hydrogen—in other words, that chlorine in hydrogen chloride, oxygen in hydrone and nitrogen in ammonia are each possessed of considerable residual ailinity. The great question at issue has been and still is—What is the nature of this residual affinity and how is it exercised? This is the question raised by Kekulé and left by him as a legacy to be decided upon. When hydrogen chloride and ammonia enter into combination to form ammonium chloride, for example, do they combine in some special manner, molecularly, so that each molecule retains its individuality as a radicle in the new compound; or is a redistribution effected, so that the several atoms become arranged around the one which exercises the dominant influence much as they are in the parent compound ammonia? In the former case, two orders of affinity would come into operation; in the latter, only one. The general opinion has always been in favour of the latter view.

The discovery that compounds of sulphur containing four different monad radicles together with a single sulphur atom, such as the chloride, S(CH3)(C2H5)(CH2·CO2H)Cl, are optically active may be said to have set the question at rest, as optical activity is only to be expected in the case of a compound of asymmetric structure having the four radicles separately associated with and arranged around the sulphur atom. If it be granted that sulphur can thus function as a tetrad, it may equally be admitted that nitrogen can function as a pentad element in the ammonium compounds.

The discussion has entered on another stage, however, now that Barlow and Pope have been successful in subjecting the problem to geometric treatment by correlating crystalline form with chemical constitution. The fundamental conception upon which the relationship is based is that each atom present in a compound occupies a distinct portion of space by virtue of an influence which it exerts uniformly in every direction. A crystalline structure is regarded as a close-packed, homogeneous assemblage of the spheres of influence of the component atoms. According to this view, valency acquires volume significance. For example, the hydrogen atom being represented by a sphere of unit volume, that of the tetrad carbon atom is represented by one of four times this unit volume; the monad elements—chlorine, bromine and iodine—are supposed, in like manner, to occupy approximately unit spheres of influence. Whilst they are prepared to admit that the spheres of atomic influence of the univalent elements, for example, are not quite the same -moreover, that the volume ratios of the spheres of influence of various elements may alter slightly under changes of condition-Barlow and Pope contend that the relative magnitudes are only slightly affected in passing from compound to compound. In their view, however, the absolute magnitudes of the spheres of influence often change considerably.

For example, taking the spheres of atomic influence of carbon as of volume 4 and those of hydrogen, chlorine and bromine as of volume 1, they find that benzene, C6H6, hexachlorobenzene, C6Cl6, and hexabromobenzene, C6Br6, present an almost identical spatial arrangement of the spheres of atomic influence. This could not be the case if the atoms of carbon, hydrogen, chlorine and bromine appropriated respectively the volumes 11.0, 5.5, 22.8 and 27.8—the so-called atomic volumes deduced by Kopp. Barlow and Pope therefore consider that, both in benzene of molecular volume 77.4 and in a derivative such as tetrabromobenzene of molecular volume 130.2, the sphere of influence of the carbon atom is about four times as large as that of either hydrogen or bromine; on displacing the hydrogen atoms by bromine atoms, however, the volumes of the carbon atoms in the benzene molecule and of the remaining hydrogen atoms expand proportionally in the ratio of 77.4 : 130.2. This remarkable conclusion is a most helpful addition to the doctrine of valency. The relative fundamental valency volume, according to Barlow and Pope, is a constant—when compounds of a “ higher type ” are produced, greater number of atoms become arranged about the centralizing atom but the relative valency volumes do not change. They have shown that if an atom of valency 1 be inserted into the space already occupied by an atom of valency m, a gap is produced which must be filled up by another atom of valency 1 if the close packing is to be restored without remarshalling, thus accounting for the progression of valency by two units. Ammonium chloride, for example, is to be regarded as formed by the insertion into the ammonia assemblage of a chlorine atom of volume 1 and of an atom of hydrogen of volume 1, the nitrogen atom retaining its fundamental valency 3. This geometric conception affords a justification of Kekulé's conception of fixed valency; at the same time it gives expression to the view he advocated that a distinction was to be drawn between atomic and molecular compounds; but it also supports the contention of Kekulé's opponents that in the two classes of compound the atoms must be regarded equally as arranged about a centralizing atom. The two points of view are therefore brought into harmony. But the problem is by no means solved—other modes of arrangement than those pictured must also be possible. To take the case of a solution of ammonia, for example: it is generally admitted that only a very small proportion is present as the hydroxide NH4·OH; far the greater part must be held in solution in some other form, either as H3N=OH2 or in the form of more complex molecules of the polymethylene type. These may be regarded as Kekulé's molecular compounds and as the forerunners of the “ more organized ” compounds in which the atoms are centralized in the crystal structure. It has not been found necessary hitherto to attribute spheres of atomic influence of different relative volumes to the same element under different conditions—that is to say, elements such—as sulphur and nitrogen always exhibit the fundamental valencies 2 and 3, respectively; moreover, in the case of per- and proto-metallic salts all known facts accord with the assumption of one and only one fundamental valency of the metal. One other conclusion of interest which Barlow and Pope are inclined to draw may be referred to, namely, that although silicon apparently functions as a tetrad element, its relative valency volume is probably only 2; they even question whether any element other than carbon has a valency volume four times that of hydrogen. It may well be that the peculiar stability of carbon compounds is to be sought in this peculiarity.

The Barlow-Pope hypothesis, however, affords a purely static representation of the facts: we are still unable to apply dynamic considerations to the explanation of valency. From the time of Faraday onwards, chemists have been willing to regard chemical affinity as electrical in its origin; on this account, the atomic-charge hypothesis advocated by Helmholtz has been most favourably received: but this hypothesis does not in any way enable us to understand the many qualitative peculiarities which are apparent when the reciprocal affinities of various elements are taken into account; moreover, it affords no explanation of the apparent variations in valency which are so frequently manifest; and it affords no satisfactory explanation of the fact that many compounds of like radicles, such as the elementary gases hydrogen, nitrogen and chlorine, for example, are among the most stable compounds known more stable than many compounds consisting of elements of opposite polarity. Attempts have been made of late to apply the electronic hypothesis-these attempts, however, have involved little more than a paraphrase of current static views and they are in no way helpful in the directions in which help is most needed. It is no way surprising, however, that we should know so little of the origin of a property that may be said to be the fundamental property of matter—if we could explain it, we could explain most things; what we have reason to be surprised at is that it should have been possible to develop so consistent a doctrine as that now at our disposal.

It is scarcely necessary to point out that the sketch above given is but a bare outline of the subject, one in which attention