1911 Encyclopædia Britannica/Valency

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VALENCY. The doctrine of valency, in chemistry, may be defined as the doctrine of the combining power of the atoms or elementary radicles of which compound molecules consist. The conception that each elementary atom has a definite atom-fixing power, enunciated by Frankland in 1852, is the foundation of the system of rational or structural formulae which now plays so great a part in chemical science. Frankland dealt more particularly with the valency of the metallic elements, in which he was specially interested at the time; but in conjunction with his co-worker Kolbe, he subsequently applied it to compounds of carbon. At that time (1852–56), the application of Avogadro's theorem to the determination of atomic weights was not yet recognized; it was only when Cannizzaro[1] made this clear that it became possible to develop the doctrine of valency upon a consistent basis. Kekulé, whose services in this field rank with those of Frankland, was the first to develop the consequences of the conception that carbon is a quadrivalent element and to apply it in a logical manner to the explanation of the structure of carbon compounds generally; his paper published in 1858, “ On the Constitution and Metamorphoses of Chemical Compounds and on the Chemical Nature of Carbon,” is admittedly the foundation of the modern theory of the structure of these compounds.

An admirable though brief summary of the historical development of the doctrine of valency is to be found in the lecture delivered in 1898 by Professor Japp in memory of Kekulé (Journ. Chem. Soc. 73, p. 97). Several discoveries have since been made which have an important bearing on the doctrine.

Frankland held that each element has a certain maximum valency but may manifest one or more subordinate valencies, the affinities in abeyance in cases in which only the lower valency is manifest satisfying each other mutually. By a logical extension of this view, elements have been divided into those of odd and those of even valency; apart from a few exceptional compounds, elements are to be reckoned as belonging either to the one or to the other of these two classes.

Kekulé always maintained that valency could not vary and in discussing this question Professor Japp goes so far as to say: “ Of all the doctrines which we owe to Kekulé, that of fixed valency is probably the one that has met with least acceptance even among chemists of his own school. At the present day it is, so far as I am aware, without supporters.” But he adds, “ Yet Kekulé held it to the last.” And such a fact cannot be overlooked: that Kekulé went too far in asserting that valency could not vary is probably true; the essential feature in his objection—that in many cases valency was overestimated by the Franklin school—cannot be so easily disposed of.

He saw clearly that structure is the determining factor to be taken into account in all such discussions; he also considered that it was necessary always to make use of univalent or monad elements in determining valency; moreover, that the only compounds on which valid arguments could be based were those which could be volatilized without undergoing decomposition—a condition that must be fulfilled if the molecular weight of a compound is to be placed beyond question. He therefore objected to the use of compounds such as ammonium chloride and phosphorus pentachloride as criteria of valency, as they undergo decomposition when volatilized. This objection has been somewhat robbed of its force by Brereton Baker's observation that decomposition can be prevented if the utmost care be taken to exclude moisture. In objecting to the use of such compounds, however, Kekulé took the further important step of dividing compounds into two classes—that of atomic compounds, such as ammonia and hydrogen chloride, in which the components are held together by atomic affinities; and that of molecular compounds, such as ammonium chloride, containing atomic compounds held together by molecular affinities: but Kekulé never gave any very clear explanation of the difference. Notwithstanding Brereton Baker's observations, the question remains with us to-day, the only difference being that we have substituted the more precise term “residual affinity” for Kekulé's term “ molecular affinity.”

Hydrogen is the one element which at present can be affirmed to be of unvarying valency: as no compound of determinable molecular weight is known in which a single atom of this element can be supposed to be present in the molecule in association with more than a single atom of another element, the hydrogen atom may be regarded as a consistent univalent or monad radicle. As the element of unit valency, hydrogen is, therefore, the one fit atomic measure to be used in ascertaining valency; unfortunately, it cannot always be applied, as so few elements form volatile hydrides. Hydrocarbon radicles such as methyl, CH3, however, are so entirely comparable with the hydrogen radicle that they form equally efficient standards; as many elements form volatile methides, some assistance may be obtained by the use of such radicles. But in all other cases the difficulty becomes very great; indeed, it is doubtful if a trustworthy standard can then be found—we are still forced, in fact, to recognize the wisdom of Kekulé's contentions. The greatest difficulty of all that we have to meet is due to the fact that valency is a dependent variable in the case of many if not of most elements, the degree in which it is manifest depending on the reciprocal affinities of the associating elements, as well as on environmental conditions.

Among univalent elements, carbon is the only one that appears to have a determinate maximum valency; this is manifest in methane, CH4, the simplest hydride the element forms, the first parent of the mighty host of compounds numbering thousands upon thousands which are the subject-matter of organic chemistry. Carbon, it is well known, is distinguished from all other elements by forming a great variety of compounds with hydrogen—the hydrocarbons; from these, in turn, other series of compounds are formed by the displacement of hydrogen atoms in the hydrocarbons by various radicles. The chemistry of the carbon compounds is, in fact, the chemistry of substitution compounds; no other element can be said to give rise to substitution compounds. It is because of this fact—because of the simple relationship obtaining between the various series of hydrocarbons and between these and their substitution compounds—that we are able to deduce structural formulae for carbon compounds with a degree of certainty not attainable in the case of any other element; and we are consequently able to infer the valency of carbon with a degree of definiteness that cannot be approached in any other case. Several of the simpler derivatives of carbon exhibit peculiarities which may be referred to as of particular interest. as showing how dinicult it is to arrive at any understanding of the manner in which valency is exercised. Apparently the compound represented by the symbol CH; cannot exist, all attempts to isolate it having failed, the hydrocarbon ethylene, formed by the union of two such groups, being obtained in its place. This would be in no way surprising were it not that the corresponding oxygenated compound, carbon monoxide, CO, has no tendency whatever to undergo polymerization under ordinary conditions and is, ” in fact, speaking generally, a remarkably inert substance, although in certain cases it forms compounds without difficulty—yet always in a very quiet manner. A single atom of oxygen apparently has the power, if not of satisfying, at least of stilling the needs of the carbon atom. One other case which makes the behaviour of carbon monoxide still more exceptional may be referred to, that of the analogous sulphur compound carbon monsulphide, CS, recently discovered by Sir James Dewar and Mr H. O. Jones. This compound is so unstable, so active, that it polymerizes with explosive violence at temperatures slightly above that at which liquid air boils. Such illustrations afford clear proof that, as before mentioned, valency is a reciprocal function—that it is impossible to regard the units of affinity of the atoms of different elements as of equivalent value and capable of satisfying each other mutually.

There is no reason to suppose that an uneven number of affinities can be active in the carbon atom; in devising structural formulae, it is therefore always considered necessary to account for the disposition of the four units of affinity, the four valencies, of the carbon atom. In 1900 some excitement was aroused by the discovery by Gomberg of a remarkable hydrocarbon formed by the withdrawal of the chlorine atom from chlorotriphenylmethane, C(C6H5)3Cl: at first it was contended that this was a compound of triad carbon, triphenylmethyl; it is now generally admitted, however, that such cannot well be the case and that one of the phenyl groups becomes altered in structure and converted into a dyad radicle (see Triphenylmethyl).

The homologies of methane—the hydrocarbons of the paraffin or CnH2n+2 series, in which the carbon atoms are associated by single affinities, their remaining affinities being engaged by hydrogen atoms—behave chemically as saturated compounds and are apparently incapable of entering into combination with other molecules. But it is important to guard against the assumption that they are actually saturated in any absolute sense. Even gases such as helium and argon, destitute as they appear to be of all chemical activity, must be credited with the possession of some measure of affinity—as they can be liquefied; moreover, as Sir James Dewar has shown, when helium is liquefied in contact with charcoal a not inconsiderable amount of heat is liberated beyond that given out in the mere liquefaction of the gas. The argument may be extended to hydrogen and the paraffin's and it may even be supposed that the amount of residual affinity increases gradually as the series is ascended this would account for the fact that their activity, the readiness with which they are attacked, increases slightly as the series is ascended. In any case, it cannot well be supposed that carbon and hydrogen mutually satisfy each other even in the paraffins.

The manner in which the valencies of the carbon atom are disposed of in the case of unsaturated hydrocarbons—that is to say, those containing a lower proportion of hydrogen than is indicated by the formula CnH2n+2—has given rise to much discussion, the subject being one which affords an opportunity for great difference of opinion. In ethylene, C2H4, each carbon atom is attached to only two hydrogen atoms, as two affinities of each atom are therefore free to enter reciprocally into combination. These atoms certainly do not combine twice over in the way in which the two atoms of carbon in ethane, H3C·CH3, enter into combination—if they did, ethylene should be a saturated compound, whereas actually it behaves as an eminently unsaturated substance. It was contended by Julius Thomsen, on the basis of determinations of the heat of combustion of the hydrocarbons, that the two carbon atoms in ethylene are less firmly united in ethylene than are those in ethane; moreover, that in acetylene, C2H2, in which there are three affinities at the disposal of 'each of the two carbon atoms, the union is even less firm than in ethylene. The argument on which these conclusions are founded has been called in question and the data are clearly insufficient to justify their acceptance; moreover, the stability of acetylene at high temperatures, also the readiness with which ethylene is often formed and with which ethenoid compounds revert to the parafiin type may be cited as arguments against them.

In dealing with such a problem, it is necessary to take into account the evidence we have that valency is a directed function. The tetrahedron is now accepted as the most suitable model of the carbon atom to be visualized whenever carbon is thought of; moreover, it is held that the directions in which valency acts are appropriately pictured if they are regarded as proceeding from the centre of mass to the four solid angles of the tetrahedron. In such a case, two affinities proceeding from each of two carbon atoms do not meet and overlap but cross, each pair at a considerable angle through which they must be deflected to bring them into contact. Von Baeyer has suggested that this angle, ½(109° 28'), is the measure of the strain imposed upon the affinities and that the existence of this strain afiords an explanation of the readiness with which ethylene lapses into a derivative of ethane when suitable opportunity is given to combine with some other substance. Another way of looking at the matter is to suppose that the affinities do not, as it were, overlap but merely cross each other and that the angle of approach referred to is a direct measure of the degree of unsaturatedness: such a view is more in accordance with Thomsen's contention. In any case, the ethenoid condition of unsaturatedness at the junction of two carbon atoms is a centre at which altogether peculiar properties, chemical and physical, are developed—the most noteworthy being the enhanced refractive power. The ethenoid symbol C=C is therefore of peculiar significance. It is a remarkable fact that the properties of ring systems generally are in accordance with the above hypothesis-the degree of unsaturatedness diminishing as “ the angle of approach ” is diminished, the more nearly the affinities can be pictured as overlapping.

The most stable arrangement of the carbon affinities would appear to be that in benzene and compounds of the benzene type—whatever that may be. The determination of the “structure” of this hydrocarbon has given rise to a large amount of paper warfare. Two tendencies may be said to have been brought together in the course of this discussion: on the one hand, the desire to arrive at a determination of the actual structure; on the other, the desire to devise formulae which shall be faithful expressions of functional behaviour and broadly indicative of the structural relationship of the constituent elements. Thexlatter is perhaps the tendency which is now in the ascendant: we are beginning to realise, particularly in the case of carbon compounds, that formulae are primarily expressive of behaviour-being based on the observation of behaviour. Thus in the case of all parafiinoid compounds, the symbol C-C has a distinctive meaning, as indicating saturation; in the case of ethenoid compounds, the symbol C=C has an equally distinctive meaning, indicating a particular degree of unsaturatedness.

From this point of view, therefore, the benzene symbol originally proposed by Kekulé is misleading, inasmuch as it indicates that the hydrocarbon contains three ethenoid junctions; it should therefore be an eminently unsaturated compound, which is not the case. On this account the centric formula is to be preferred as an expression of the properties of the compound. The non-metallic elements other than carbon all form volatile hydrides and methides from which their fundamental valencies can be deduced without difficulty. Chlorine, oxygen, nitrogen and silicon may be regarded as typical of the four classes into which the non-metals fall. But the number of hydrogen and methyl radicles which the atom carries cannot be taken as the measure of absolute valency in the case of elements of the chlorine, oxygen and nitrogen classes. The hydrides of the elements of these classes must all be regarded as more or less unsaturated compounds, the fact that gases such as hydrogen chloride and ammonia are intensely soluble in water being clearly a proof that their molecules are greatly attracted by and have great attraction for water molecules; it is remarkable, however, that although hydrogen chloride and ammonia are easily soluble in water and also combine readily with one another, they are gases which are by no means easily condensed—in other words, the molecules in each gas have little tendency to associate among themselves. It may also be pointed out that, to account for the properties of liquid water, it is necessary to suppose that the simple molecules represented by the symbol H2O have a very considerable mutual afhnity and that water consists largely of complex molecules.[2] Taking into account 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 is drawn to certain points of importance in the hope that it may be clear that the problems cannot be discussed usefully in the formal manner which is too frequently adopted. Our knowledge of valency cannot be expressed in a few symbols or in a few formal statements.  (H. E. A.) 

  1. Stanislao Cannizzaro, A Course of Chemical Philosophy (1858). Alembic Club Reprints, No. 18. [1910.]
  2. On this account it is desirable to confine the term water to the liquid and to distinguish the simple molecule represented by the symbol H2O by a separate name—that proposed is Hydrogen. Liquid water is probably a mixture of several polyhydrones together with more or less hvdrone.