In this new theory no assumptions were made as to the nature of the forces connecting the atoms in a molecule, and hence no op position in character between the atoms was required. Every atom was regarded as having a certain number of units of combining power, and every atomic link as involving the utilization of one of these units by each of the atoms concerned. Double and triple links, in which each of two atoms used up two or three of these units in combining with one another, were also recognized. The valency of an atom in the numerical sense was the number of such units which it possessed. On paper, the links were repre sented by lines, giving such formulae as This theory had an astounding success in its application to or ganic chemistry. It was found possible to determine from the reactions of a substance the order in which the constituent atoms were linked in the molecule, and thus to assign to the vast hordes of organic compounds "structural formulae" which accounted for their existence and to a large extent also for their properties. For an account of further extensions of this theory, which made it possible to determine the actual positions in space of the various atoms and groups in a molecule, see STEREOCHEMISTRY. In con sequence of these triumphs the older theory was disregarded, and chemists failed to realise that the views of Berzelius were quite satisfactory if their application was limited to the oppositely charged radicals in a salt.
The electrical theory again became prominent in and after 1887, with the rise of the Arrhenius theory of salt solutions which showed that when a salt is dissolved in water it is very largely broken up into oppositely charged ions (sodium chloride, for example, into positive sodium and negative chlorine ions) which have to a great extent the properties of free molecules. On the other hand atoms forming part of a complex radical do not separate in this way; thus potassium chlorate, KC103, breaks up in solution into potassium, K.+, and chlorate, C103, ions, but the latter do not dissociate further into chlorine and oxygen. This made it clear that two kinds of links can be formed between atoms in a molecule, one of which is broken on solution in water (and other similar solvents) with the formation of ions, while the other is not. To the first kind the theory of Berzelius, that the atoms are held together by their electrical charges, obviously ap plies : the work of Faraday and of Arrhenius shows that such atoms actually exist in solution with electrical charges, and further support is given by the observation that solvents of high dielectric capacity promote ionization, since such solvents would weaken the attraction between charged atoms. The final confirmation of the theory has been given in recent years through the investigation of crystal structure, mainly by W. H. and W. L. Bragg and by Rubens, who have shown that with many salts (including sodium chloride itself), the ions are present as such in the solid crystal.
It still remained to explain the second kind of linkage (non ionized), and especially two of its peculiarities: (I) the fact that in many compounds the valency of an element is the same whether it is expressed in ionized or in non-ionized links, and (2) the relation between this numerical value and the position of the element in the periodic table. This was the state of opinion towards the end of the 19th century.
Werner's Theory.—In 1893, an entirely new theory of val ency was put forward by Alfred Werner, which was subject to quite different laws, and dealt primarily with a different—and previously somewhat neglected—class of compounds. It had al ready become recognised that there were many substances, some of them quite stable, the structure of which could not be ex plained on the current theory of valency. Most of these were formed by the combination of molecules which appeared to be already saturated, and so to have used up all their combining power; and often, though not always, the component molecules could easily be separated again. Such substances were known as "molecular compounds" and were disregarded in the current the ories of molecular structure, it being supposed that molecules re tained the power of combining loosely with one another, in virtue of some force inferior to true valency. The most conspicuous ex ample* are the compounds which many salts form with "water of crystallization," and those which many substances form with ammonia (see AmmiriEs). All attempts to formulate these on the accepted theories of valency had proved unsatisfactory. Werner collected an immense number of examples of such compounds, and showed that it was possible to formulate them on a new principle, to which he gave the name of co-ordination. The essence of this principle was that the combining powers of an atom depended not on the nature but on the number of the atoms or groups to which it was attached. This number, which he called the co-ordination number, and which determined the formula of the compound and, in the new sense, the valency of the central atom, did not, as did the ordinary valencies, change from one atom to the next, but was most frequently six, sometimes four, and less often had other values. Thus platinic chloride, PtC14, forms a series of addition compounds with ammonia, and also with potassium chloride. Werner showed that in all of these the platinum has six groups— chlorine atoms or ammonia molecules—attached to it by links which are not ionized in solution, while any further atoms in the molecule are ionized, and are presumably attached by weaker links in the "outer sphere" of the platinum, outside the co-ordina tion complex. This is shown by the following formulae, in which the atoms enclosed in square brackets are those which are found not to ionize: It will be seen that in the series of transformations the platinum atom always retains a group of six atoms or molecules attached to it by non-ionized links. In its application to addition corn pounds of this kind, Werner was able to show that his theory was quite satisfactory. He subsequently extended it to the whole of chemistry; but in this he was not followed by the majority of chemists. In particular the complicated relations of the great mass of compounds of carbon—some hundreds of thousands of substances—were found to be adequately explained by the older structural theory. Indeed Werner's theory, even in its application to inorganic compounds, was not regarded very seriously by chemists until, in 1911, he was able by its means to predict the occurrence of optical activity (see STEREOCHEMISTRY) in certain structures, and to verify his predictions by experiment. It then became recognised that the theory in its original sphere—that of inorganic chemistry—had strong claims to be accepted.