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VALENCY
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VALENCY . The See also:doctrine of valency, in See also:chemistry, may be defined as the doctrine of the combining See also:power of the atoms or elementary radicles of which See also:compound molecules consist. The conception that each elementary See also:atom has a de-finite atom-fixing power, enunciated by See also:Frankland in 1852, is the See also:foundation of the See also:system of rational or structural formulae which now plays so See also:great a See also:part in chemical See also:science. See also:Frank-See also:land dealt more particularly with the valency of the metallic elements, in which he was specially interested at the See also:time; but in See also:conjunction with his co-worker See also:Kolbe, he subsequently applied it to compounds of See also:carbon. At that time (1852-56), the application of See also:Avogadro's theorem to the determination of atomic weights was not yet recognized; it was only when See also:Cannizzaro' made this clear that it became possible to develop the doctrine of valency upon a consistent basis. See also:Kekule, whose services in this See also: Frankland held that each element has a certain maximum valency but may See also:manifest one or more subordinate valencies, the See also:affinities in See also:abeyance in cases in which only the See also:lower valency is manifest satisfying each other mutually. By a logical See also:extension of this view, elements have been divided into those of See also: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. Kekule 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 Kekule, that of fixed valency is probably the one that has met with least See also:acceptance even among chemists of his own school. At the See also:present See also:day it is, so far as I am aware, without supporters." But he adds, " Yet Kekule held it to the last." And such a fact cannot be overlooked: that Kekule 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 Frankland school—cannot be so easily disposed of. He saw clearly that structure is the determining See also:factor to be taken into See also:account in all such discussions; he also considered that it was necessary always to make use of univalent or See also: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 See also:condition that must be fulfilled if the molecular See also:weight of a compound is to be placed beyond question. He therefore objected to the use of compounds such as ammonium' chloride and See also:phosphorus pentachloride as criteria of valency, as they undergo decomposition when volatilized. This objection has been somewhat robbed of its force by Brereton See also:Baker's observation that decomposition can be prevented if the utmost 1 Stanislao Cannizzaro, A Course of Chemical See also:Philosophy (1858). See also:Alembic See also:Club Reprints, No. 18. [191o.]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, See also:CH4, the simplest hydride the element forms, the first See also:parent of the mighty See also:host of compounds numbering thousands upon thousands which are the subject-See also:matter of organic chemistry. Carbon, it is well known, is distinguished from all other elements by forming a great variety of compounds with See also:hydrogen—the See also:hydrocarbons; from these, in turn, other See also: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 See also: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 See also: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 See also:interest, as showing how difficult it is to arrive at any understanding of the manner in which valency is exercised. Apparently the compound represented by the See also:symbol See also:CH2 cannot exist, all attempts to isolate it having failed, the hydro-carbon See also:ethylene, formed by the See also:union of two such See also:groups, being obtained in its See also: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 See also: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 See also: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 See also:sulphur compound carbon monsulphide, CS, recently discovered by See also:Sir See also: In rgoo some excitement was aroused by the See also:discovery by Gomberg of a remarkable See also:hydrocarbon formed by the withdrawal of the See also:chlorine atom from chlorotriphenylmethane, C(See also: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 See also:TRIPHENYLMETHANE). The homologues of methane—the hydrocarbons of the See also:paraffin or C.H2,,+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 See also:combination with other molecules. But it is important to guard against the See also:assumption that they are actually saturated in any See also:absolute sense. Even gases such as See also:helium and See also:argon, destitute as they appear to be of all chemical activity, must be credited with the See also: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 See also:charcoal a not inconsiderable amount of See also:heat is liberated beyond that given out in the See also:mere liquefaction of the See also:gas. The See also:argument may be extended to hydrogen and the paraffins 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 See also:formula C, H2,,}2—has given rise to much discussion, the subject being one which affords an opportunity for great difference of See also:opinion. In ethylene, C2H4, each carbon atom is attached to only two hydrogen atoms, as two affinities of each atom are therefore See also: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, See also:H3C.See also: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 See also:Julius See also:Thomsen, on the basis of determinations of the heat of See also:combustion of the hydrocarbons, that the two carbon atoms in ethylene are less firmly See also:united in ethylene than are those in ethane; moreover, that in See also:acetylene, C2H2, in which there are three affinities at the disposal of each of the two carbon atoms, the union is even less See also: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 paraffin type may be cited as arguments against them. In dealing with such a problem, it is necessary to take into account the See also:evidence we have that valency is a directed function. The See also:tetrahedron is now accepted as the most suitable See also: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 See also: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 See also:cross, eachpair at a considerable See also:angle through which they must be deflected to bring them into contact. Von See also:Baeyer has suggested that this angle, 2(Io9° 28'), is the measure of the See also:strain imposed upon the affinities and that the existence of this strain affords 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 See also: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 See also:peculiar properties, chemical and See also:physical, are See also: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 See also:ring systems generally are in accordance with the above See also: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 See also:stable arrangement of the carbon affinities would appear to be that in See also: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 See also:hand, the See also: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. The latter 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 paraffinoid 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 Kekule 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 See also:form volatile hydrides and methides from which their fundamental valencies can be deduced without difficulty. Chlorine, oxygen, See also:nitrogen and See also: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 See also:ammonia are intensely soluble in See also:water being clearly a proof that their molecules are greatly attracted by and have great attraction for water molecules; it is remark-able, 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 See also: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 See also:H2O have a very considerable mutual affinity and that water consists largely of complex molecules.) Taking into account 1 On this account it is desirable to confine the See also:term water to the liquid and to distinguish the simple See also:molecule represented by the symbol H2O by a See also:separate name—that proposed is Hydrone. Liquid water is probably a mixture of several polyhydrones together with more or less hvdrone 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 affinity. 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 Kekule and See also:left by him as a See also:legacy to be decided upon. When hydrogen chloride and ammonia enter into combination to form ammonium chloride, for example, do they combine in some See also: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 See also: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 See also: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)(See also:C2H6)(CH2•See also:CO2H)Cl, are optic-ally active may be said to have set the question at See also:rest, as See also:optical activity is only to be expected in the case of a compound of See also:asymmetric structure having the four radicles separately associated with and arranged around the sulphur atom. If it he 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 See also:stage, however, now that See also:Barlow and See also: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 See also:close-packed, homogeneous assemblage of the See also:spheres of influence of the component atoms. According to this view, valency acquires See also:volume significance. For example, the hydrogen atom being represented by a See also:sphere of unit volume, that of the tetrad carbon atom is represented by one of four times this unit volume; the monad elements — chlorine, See also:bromine and See also: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 See also:change considerably. For example, taking the spheres of atomic influence of carbon IS of volume 4 and those of hydrogen, chlorine and bromine as of volume r, they find that benzene, C6H6i hexachlorobenzene, C6C16, 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 rr•o, 5.5, 22.8 and 27.8—the so-called atomic volumes deduced by See also: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: 13o•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 See also: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 i be inserted into the space already occupied by an atom of valency m, a See also:gap is produced which must be filled up by another atom of valency r if the close packing is to be re-stored 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 r and of an atom of hydrogen of volume r, the nitrogen atom retaining its fundamental valency 3. This geometric conception affords a See also:justification of Kekule's conception of fixed valency; at the same See also:tin.e it gives expression to the view he advocated that a distinction was to be See also:drawn between atomic and molecular compounds; but it also supports the contention of Kekule'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 See also: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 See also: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 Kekule's molecular compounds and as the fore-runners 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 See also:accord with the assumption of one and only one fundamental valency of the See also: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 See also:representation of the facts: we are still unable to apply dynamic considerations to the explanation of valency. From the time of See also:Faraday onwards, chemists have been willing to regard chemical affinity as See also:electrical in its origin; on this account, the atomic-See also:charge hypothesis advocated by See also: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 See also: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 See also:polarity. Attempts have been made of See also:late to apply the electronic hypothesis—these attempts, however, have involved little more than a See also: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 See also:property that may be said to be the fundamental property of matter—if we could ex-See also:plain 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 See also:sketch above given is but a See also:bare outline of the subject, one in which See also:attention is drawn to certain points of importance in the See also: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. Additional information and CommentsThere are no comments yet for this article.
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