Valency

It will be observed that all these changes of valency in which the core of the atom is unaffected, occur by two units at a time; and this is the main cause of the belief held in the early days of the theory of valency that the various valencies of an atom were either all even or all odd.

Transitional Elements.

The elements commonly called transitional, those which are enclosed in frames in the Bohr classi fication (see ATOMIC STRUCTURE) and are relegated to table 2 in the article PERIODIC LAW, have in the outermost (valency) group of the isolated atom one or two electrons, and in the next group, which forms the outermost group of the core, not the stable numbers 2, 8 or 18, but some number intermediate between 8 and 18. A group of this size does not hold its electrons very firmly, and one or two of them can be detached by chemical action. Thus iron (26), in which the successive groups of the free atom are 2, 8, 14, 2, can lose its two outermost electrons in the usual way to form the bivalent cation Fe++=2, 8, 14. But by more ener getic treatment a third electron can be detached from this ferrous ion leaving the trivalent ferric ion Fe"'=2, 8, 13. In this par ticular element the process in nearly all the compounds stops here, but with others it goes further : thus manganese [(25) = 2, 8, 13, 2] can have valencies of 2, 3, 4, 5, 6 and 7. In the last of these states the core is reduced to 18=2, 8, 8, the stable arrangement of argon. Thus the transitional elements show valencies vary

ing by single units, often over a wide range, the limit being that in which the core is reduced to the size of the preceding inert gas, or in other words, the highest valency being that correspond ing to the periodic group to which the element belongs.

Copper and gold, though not strictly transitional elements, have many of the properties of such elements. They have the outer electronic groups 18, I. The single electron of the outer most group is readily detached to give the univalent cuprous and aurous ions. But the 18 group, though it is normally very stable, is less so when the positive charge on the nucleus is barely suffi cient to neutralize it, or in other words when its electrons are moving in a very weak electric field. Hence, in copper one, and in gold two, of these 18 electrons can be detached from the group, giving the cupric and auric compounds, such as and with the respective groups 17 and 16. Silver, which has a similar structure, might be expected to behave in the same way; but for some reason which is not understood the 18 group in silver is more stable and cannot be broken down ; hence silver is almost invariably univalent. In the elements following these three in the table, from zinc, cadmium and mercury onwards, the in creased charge on the nucleus holds the 18 group so firmly that its electrons cannot be removed. (N. V. S.)