VALENCY is that property of elements by virtue of which they unite to form compounds. The atom of hydrogen has never been known to combine with more than one atom of another element In terms of this property hydrogen always shows the valency of one. Its valency is therefore taken as the unit of comparison. A number of elements form compounds with hydrogen. Chlorine forms HC1, oxygen forms 1120, nitrogen forms. NFU and carbon forms C114.. Therefore, with hydrogen as the unit, the valency of these elements is one two, three and four respectively. Or, as A. VV. Hofmann expressed it In 1865: eh takes the whole atom power of chlorine, 35.5, to engage one atom of hydrogen; whereas the atom-power of oxygen, 16, suffices to engage two hydrogen atoms; and the atom-powers of nitrogen and carbon suffice, respectively, to engage three and four hydrogen atoms.' There are a number of elements With which hydrogen does not combine. The va lencies of these elements can only be ascertained by indirect methods. In such instances infor mation is gained by a study of the compounds of elements which can combine with hydrogen. For example, one atom of chlorine forms a stable compound with an atom of hydrogen. The same element will also form the following compounds: KCI, ZnCI, MoCI,, Wa.. Therefore, the valencies of potassium, zinc, aluminium, tin, molybdenum and tungsten are one, two, three, four, five and six respec tively. The highest valency that an element has been known to exhibit is eight. The ele ments of the argon series (helium, neon, argon, krypton, xenon, niton) are remark able in that they form no well-defined com pounds with other elements. They are described as elements with no chemical affinity. In other words, their valency is zero. With this prop erty as a basis, elements may be classified as monads, diads, triads, tetrads, pentads, hexads, heptads and octads, the terms having reference to the valencies that are displayed in the tion of compounds. Elements are also desig nated as univalent, bivalent, tervalent, quadriv alent, etc. The theory of valency has also been extended to include radicles which remain unchanged during chemical transformations. The radicles CH., CN, CHBCO, are univalent, SO. is bivalent, PO4 is tervalent, PBC', is quadriv alent, etc. As early as 1861 Crum Brown had
suggested that the molecule of hydrogen is a complex of two atoms held together by a common bond, H — H. According to this sys 0 tem carbon dioxide is C ammonia is H H 0 N—H, and acetic acid is The \H bonds in each case represent the valencies of the elements forming the compounds. This system has also been employed by Kekule, Frankland and others, and is sometimes used at the present day to emphasize the valency of each element in a given product. The system has been found especially fruitful in the domain of organic chemistry. An element does not always exhibit the same valency. It may possess a characteristic active valency which remains constant through a large series of chemical changes, but this valency is often a dependent variable, being influenced by physical as well as chemical conditions. The tervalent nitrogen in NH, assumes other valencies toward oxyger•; sulphur may act as a diad, a tetrad, and a hexad; tin is a diad and a tetrad. In the compounds VC13, VC13, VCL, and VOCL van adium functions with the valencies of two, three, four and five. The variable property of valency was recognized by some of the founders of the theory. In 1852 Franldand published a memoir setting forth his views on valency in these words: aThe compounds of nitrogen, phosphorus, antimony and arsenic especially, exhibit the tendency of these elements to form compounds containing three or five atoms of other elements i and it is in these proportions that their affinities are best satisfied: thus in the ternal group we have NO3, NH3, NI., NS,, PO. PCI., SbO,, SbCI., As09, ABM, AsCL, etc.• and in the five-atom group NO. NH40, NI44I, PO., PH.I, etc. Without offering any hypothesis regarding the cause of this sys tematic grouping of atoms, it is sufficiently evi dent from the examples just given, that such a tendency or law prevails, and that, no matter what the character of the uniting atoms may be, the combining power of the attracting element is always satisfied by the same number of these atoms." It is to be noted that Frankland em ployed the unit 8 for the atomic weight of oxygen, and the unit 16 for the atomic weight of sulphur.