Kekule (1851), on the other hand, regarded valency as a fixed invariable property, asserting that this property must be considered as invari able as the atom itself. He argued that varia tons in valency are more apparent than real.
To him phosphorus trichloride was a true mole cule formed by °atomic while the pentachloride was a complex of the molecules Pa, and Cl, formed by the saturation of the one with the other, there being no redistribu tion of atoms in this saturation. The formula of phosphorus pentachloride was therefore written as PCI.. Cl, and its dissociation into PC18 and at elevated temperatures was taken as an evidence of a peculiar type of loose com bination.
The views of Kekule could not be harmon ized with certain known facts even at that early period. Our present-day knowledge shows beyond dispute that even elements of appar ently invariable valencies may exhibit variation in this property under certain conditions. For instance, although the science of organic chem istry has for its foundation the quadrivalency of carbon, the element may in a few isolated instances show the valency of two. Nef and others have advanced convincing arguments in favor of the bivalency of carbon in hydro cyanic acid (structurally represented as C= N —H), in isonitriles R —N = C, and in fulminic acid C = N — 0-H. The case of oxygen, the prevailing valency of which is two, is equally interesting. Thus oxygen compounds have been described in which the doctrine of invariable valency breaks down. There is no doubt that in the oxonium derivatives - al C H 0 and \ CI' \ I (Ca.), \ (caw,/ \ oxygen could function as a quadrivalent atom. According to Gomberg triphenyl methyl (C.HOBC, contains a trivalent carbon atom, although this has been questioned, and reasons have been advanced in favor of the adoption of (C4HB)BC*--C(GH.)s as the more rational formula of triphenyl methyl. In view of these observations it is safe to assume that there are very few elements possessing constant valencies. As far as we know, hydro gen, sodium and potassium always show the valency of one. Calcium and barium appear to be always bivalent. Other isolated examples of invariable valency might be mentioned; but as a rule valency is a variable property. Al though the theory of valency was first formu lated by Frankland, Kekule and others in the early 50's of the 19th century, its germ is to be found in the systems, theories and laws propounded by earlier investigators. Our present-day conception of valency is therefore intimately related to Lavoisier s analytical methods, Dallon's atomic theory, Gay-Lassac's volume relations, Avogadro's hypothesis, and to other theories and laws that have contributed to our better understanding of the science of chemistry during the past century. A number of theories have been proposed to expkin the variable property of valency. Abegg stares that each element possesses two kinds of vilency, namely, normal and contra-valency. In metals the normal valency is electropositive, it• non metals electronegative. When one vale icy in an atom becomes saturated the remainirg val encies will be strongly influenced. Neff claims that the unused valencies in a compound saturate one another, and J. van't Hoff seeks an explanation of variability in the shape and form of the atoms. According to Friend there are three kinds of valencies: 1. Free negative valency inherent in elements which can combine with hydrogen. 2. Free positive valency, the numerical value of which is not so easily de termined. 3. Residual or latent valency. These
differ from the free valencies in that they can only be called out in pairs of equal and opposite sign. Werner distinguishes two kinds of valency, principal or primary, and auxiliary or secondary. According to his theory nitrogen, sulphur, chlorine, oxygen, platinum, gold and other elements may in some of their compounds bring into action their secondary valencies, forming complex derivatives. With this theory Werner endeavors to explain the constitution of double salts, crystalline hydrates and complex molecules.
It is generally admitted that none of these theories can account for all known facts. Each one, however, has made its contribution to the proper appreciation of observed phenomena. We are equally uncertain as to the true nature of valency itself. Of the theories that have been put forward to clear the situation the electrochemical theory and the corpuscular or electronic theory have been most favorably received. As early as 1806 attention had been directed by Sir Humphry Davy to the relation of chemical affinity and electrical force, and attempts had been made to employ this as a basis for a theory of chemical combination. Later in the century Faraday summed up his studies on electrolysis in a general law which states that the electrical charge which is con veyed by an ion of a metal in the electrolysis of a solution of a salt is directly proportional to the valency of the ion. With these earlier con clusions as a basis, the electrochemical theory was revived by Helmholtz. The theory had many adherents during the latter part of the 19th century. The corpuscular or electronic theory formulated by J. J. Thomson starts with the assumption that corpuscles or electrons are constituents of all atoms; so that the chief difference between one atom and another is not one of quality—since all atoms contain electrons—but is a difference of quantity, and of distribution or arrangement. The electrons carry minus charges of electricity, and yet they are ordinarily present in a neutral atom. This being the case, we should look for a corre sponding positive charge, somewhere in the atom, and the hypothesis has been brought for ward that the positive charges constitute the nucleus of the atom. When two electronic sys tems, e.g., that of hydrogen and chlorine, come into contact, one of the electrons gets detached from the atom and describes an orbit around the other atom. Each atom was originally neutral; but the system from which the elec tron was removed becomes a positive atom, and the system to which the electron is added be comes a negative atonr. This transfer of an electron then makes hydrogen a positive sys tem and makes chlorine a negative system. Here we have a plausible explanation of the valency of an element. An atom minus one electron, or an atom plus one electron may be considered as electrically charged, and there fore capable of attracting oppositely charged 'atoms to form electrically neutral systems. An atom less two electrons, or, with two electrons in excess, would have twice the ability to com bine. It would have the valency of two. Con sult Friend, 'Theory of Valency) ; Werner, 'Theory of Valency' ; von Meyer, Ernst,