VALENCY, in chemistry, is the power which atoms possess of combining with a definite number of other atoms, and is us ually expressed numerically by the number of hydrogen atoms which an atom can combine with or replace.
When towards the middle of the 19th century the relative weights of atoms and molecules became accurately known, espe cially through the reintroduction by Cannizzaro in 1858 of Avo gadro's principle (181I) of the relation between the molecular weight and the density of a gas, it was clear that the number of atoms with which a given atom could combine varied with the nature of that atom : that, for example, a chlorine atom could combine with one hydrogen atom, an oxygen atom with two, a nitrogen atom with three, and a carbon atom with four, as in the compounds HC1, }LC. These characteristic combining powers were shown to extend to more complicated compounds: two of the four hydrogen atoms in methane, CH4, could be re placed by one oxygen atom, or all four by two, or three by one nitrogen atom, as in H2CO, HCN. Thus it appeared that every atom could combine with or replace a definite number of other atoms of a specified kind, and hydrogen, as it was the light est atom, and was never found to combine with more than one atom of another element, was selected as the standard. The term, valency, introduced in 1868, is now adopted to express both the power of combination in general, and its numerical value.
As the examination of the valency of elements was extended, there was much dispute as to whether or not it was constant in value for any one element. It soon became evident that the valency of many elements was variable : for a time it was main tained that the variations were always by two units, so that ele ments could be divided into those of odd and those of even val ency; but before long it was recognised that while this was true of many elements, there were others of which the valency must be admitted to vary by single units at a time, unless very improb able assumptions were made as to the structure of some of their compounds.
The discovery of the periodic classification of the elements through the work of Newlands, Mendeleyev and Lothar Meyer (1864-1869) brought to light a new regularity in the valencies of the elements (see PERIODIC LAW). Mendeleyev pointed out that all the elements of a periodic group had the same valency, and that this changed by one unit as we pass from one group to the next. In particular, the highest valency of an element, as shown in its highest oxide, is equal to the ordinal number of the group to which it belongs. On the other hand, the valency in the hy drides seems to rise with that in the oxides from one to four, and then to fall by steps of one unit to unity again. This is illus trated by the following table, giving the two "short periods" of the periodic classification.
Thus the mysterious periodicity of properties revealed by the table was most clearly exemplified in the valency.
Earlier Theories.—All discussions of valency in the 19th cen tury were handicapped by the absence of any satisfactory theory of its cause. Berzelius, in the earlier part of the century, had assumed that atoms were electrically charged, and were held to gether in the molecule by the attraction of opposite charges. This view was supported by the phenomena of electrolysis, and espe cially by the work of Davy and Faraday. Its validity as a general theory of valency was, however, destroyed about 1840 by the discovery that under some conditions "electropositive" could be replaced by "electronegative" atoms without any fundamental modification of the properties of the molecule; thus part of the hydrogen in acetic acid can be replaced by chlorine, and the prod uct resembles the original acid in many ways, and obviously must have a similar structure. The attempts of Berzelius to stretch his theory to explain such phenomena led to its falling into discredit, and it was replaced by what ultimately became the theory of structural chemistry.