Valency

In this way we can now see that the theory of Werner is only one aspect of a physical process of which the older structural theory is another. The co-ordinate link and the ordinary covalency are of the same kind, consisting of pairs of shared electrons, but the manner in which they arise is different.

The electronic theory thus accounts for all the various kinds of linkage which can arise between atoms by the redistribution of their valency electrons. The cause of this redistribution is the possibility of producing a more stable arrangement of the electrons.

Octet Theory.

So far, it has been assumed that the stable forms attained are in general those which occur in the inert gases. The physicists have shown (see ATOMIC STRUCTURE) that the electrons of an isolated atom arrange themselves in groups or shells, and the outermost group contains eight electrons in all the inert gases except helium, which has a single group of two. Hence in the majority of molecules, and in nearly all those hitherto discussed, the result of the combination is to give an outer group of eight electrons to all the atoms concerned except those of hydrogen, which have two. This is the basis of the octet theory, originally proposed by Lewis, and developed in great detail by Irving Langmuir. But if the largest valency group possible is eight, and every covalency consists of two electrons, the highest covalency possible for any atom must be four. The existence of such bodies as sulphur hexafluoride SE's, a very stable gas, and of the co-ordination compounds already mentioned, shows that a covalency of six is possible, and hence a valency group of twelve; and we know of atoms which exert a covalency of eight, and so must have a valency group of 16 electrons, such as osmium in the octafluoride OsF8. This seems to open up almost infinite possi bilities of combination; but it is found that it is not every atom which can form these large groups. The size of the valency group of an element is limited according to the position which it occupies in the periodic table; but the limitation does not depend on the (vertical) periodic group to which it belongs, as does that of the ordinary valency; it is determined by the (horizontal) period. For hydrogen it is four (covalency two; one of the two links must of course be co-ordinate) ; for the elements of the first short period (lithium to fluorine) it is eight (covalency, four) ; for those of the second short and the first long periods (sodium to chlorine, and potassium to bromine) it is 12 (covalency, six) ; whilst for the heavier elements it is 16, corresponding to a co valency of eight. This refers to the maximum size possible, which is not always reached.

The electrons surrounding an atom in a molecule can thus be divided into two parts, the valency group, including the shared , electrons, and the inner groups, the electrons of which are all unshared, and which may be called the core; and changes of valency may either be due to changes in the size or the utilization (extent of sharing) of the valency group, or to changes in the size of the core. In many elements, including all those of the

first two periods, the core cannot be changed by chemical means: it consists of a series of complete electronic groups, which are too stable to be affected by chemical forces. (These are the elements included in the Abridged Classification in the article PERIODIC LAW.) With these, a change of valency can only occur through the valency group being completed, or, if it is already complete, being more fully shared. Thus boron, with three valency electrons, can form three covalencies as in the fluoride sharing each of its electrons with a fluorine atom, and receiving one electron in return from each. Though this only gives it six valency elec trons, it cannot combine with a fourth (neutral) fluorine atom, because it has no more valency electrons to offer. But with a fluorine ion (which has a complete octet) it can form a co ordinate link, if the ion shares a pair of its electrons with the boron: We thus get the four-covalent complex which, however, necessarily has the negative charge given it by the fluorine ion, and so appears as the anion of a salt such as In this way the boron changes its valency from three to five (four covalencies and one electrovalency). In the same way the nitrogen of ammonia, though it has a complete octet, only shares six of these electrons. It can, however, increase its valency by sharing the other pair with a hydrogen ion: The complex now has the positive charge of the hydrogen ion, and so forms the ammonium cation, as in Cl. In this way a trivalent element can increase its valency to five, its covalency rising to four, while the complex acquires one unit of charge, that is, one electrovalency. Further changes of the same kind are possible with the heavier elements, the valency group of which is not limited to eight. Thus silicon, with four valency electrons, can form the tetrafluoride SiF4, with a complete and fully shared octet. It is, however, able to hold a valency group of twelve, and can obtain this by forming two co-ordinate links with two fluorine ions: acquiring at the same time two negative charges. We thus get the anion of the silicofluorides (such as K2[SiF6]) in which the valency of the silicon has risen from four to eight (six covalencies and two electrovalencies). The same thing can happen without the production of an electrovalency, if an atom shares a lone pair of its electrons with, say, an oxygen atom, which needs two to make up its octet. For example, chlorine completes its octet by combining with a hydroxyl (H-0-) group in hypochlorous acid H-O-Cl. In this molecule the chlorine has three lone pairs, so that it can form co-ordinate links with as many as three oxygen atoms, giving the series If, as is commonly done, we regard this link to the oxygen as equivalent to two valencies (in the older structural formulae it was necessarily written as a double link), the chlorine here ap pears with the valencies of 1, 3, 5 and 7.