VALENCY OF ELEMENTS.] C H E M I S T R Y 473 chemists are in the habit of employing various kinds of rational formulae, called constitutional formulas, graphic for mulae, &c., which not only express the molecular composition of the compounds to which they apply, but also embody cer tain assumptions as to the manner in which the constituent atoms are arranged, and convey more or less information with regard to the nature of the compound itself, viz., the class to which it belongs, the manner in which it is formed, and the behaviour it will exhibit under various circum stances. Before explaining these formulae it will be neces sary, however, to consider the differences in combining power exhibited by the various elements. It is found that the number of atoms of a given element, of chlorine, for example, which unite with an atom of each of the other elements is very variable. Thus, hydrogen unites with but a single atom of chlorine, zinc with two, boron with three, silicon with four, and phosphorus with five. Those elements which are equivalent in combining or displacing power to a single atom of hydrogen are said to be univaleiit or monad elements ; whilst those which are equivalent to two atoms of hydrogen are termed bivalent or dyad elements ; and those equivalent to three, four, five, or six atoms of hydrogen triad, tetrad, pentad, or hexad elements. But not only is the combining power or valency (atomicity) of the elements different, it is also observed that one element may combine with another in several pro portions, or that its valency may vary ; for example, phos phorus forms two chlorides represented by the formulae PC1 3 and PC1 5 , and nitrogen the series of oxides repre sented by the formulae N 2 , NO , N 2 3 , N 2 4 , N 2 5 . In explanation of these facts it is supposed that each element has a certain number of " units of affinity," which may be entirely, or only in part, engaged when it enters into combination with other elements ; and in those cases in which the entire number of units of affinity are not engaged by other elements, it is supposed that those which are thus disengaged neutralize each other, as it were. For example, in pentachloride of phosphorus the five units of affinity possessed by the phosphorus atom are satisfied by the five monad atoms of chlorine, but in the trichloride two are disengaged, and, it may be supposed, satisfy each other. Compounds in which all the units of affinity of the con tained elements are engaged are said to be saturated, whilst those in which the affinities of the contained elements are not all engaged by other elements are said to be unsatu- rated. According to this view, it is necessary to assume that, in all unsaturated compounds, two, or some even number of affinities are disengaged; and also that all elements which combine with an even number of monad atoms cannot combine with an odd number, and vice versa, in other words, that the number of units of affinity active in the case of any given element must be always either an even or an odd number, and that it cannot be at one time an even and at another an odd number. There are, how ever, a few remarkable exceptions to this " law." Thus, it must be supposed that in nitric oxide, NO, an odd number of affinities are disengaged, since a single atom of dyad oxygen is united with a single atom of nitrogen, which in all its compounds with other elements acts either as a triad or pentad. When nitric peroxide, N 2 O 4 , is converted into gas, it decomposes, and at about 180 C. its vapour entirely consists of molecules of the composition N0 2 ; while at temperatures between this and C. it consists of a mixture in different proportions of the two kinds of molecules, N 2 4 andNO 2 . The oxide NO 2 must be regarded as another instance of a compound in which an odd number of affinities of one of the contained elements are disengaged, since it contains two atoms of dyad oxygen united with a single atom of triad or pentad nitrogen. Again, when hexa- chloride of tungsten is converted into vapour it is decom posed into chlorine and a pentachloride, having a normal vapour density, but as in the majority of its compounds tungsten acts as a hexad, we apparently must regard its pentachloride as a compound in which an odd number of free affinities are disengaged. Hitherto no explanation has been given of these exceptions to what appears to be a law of almost universal application, viz., that the sum of the units of affinity of all the atoms in a compound is an even number. The number of units of affinity active in the case of any particular element is largely dependent, however, upon the nature of the element or elements with which it is asso ciated. Thus, an atom of iodine only combines with one of hydrogen, but may unite with three of chlorine, which never combines with more than a single atom of hydrogen ; an atom of phosphorus unites with only three atoms of hydrogen, but with_five of chlorine, or with four of hydrogen and one of iodine ; and the chlorides corresponding to the higher oxides of lead, nickel, manganese, and arsenic, PbO 2 , Ni 2 3 , MnO 2 , and As 2 5 do not exist as stable compounds, but only the lower chlorides, PbCl 2 , NiCl 2 , MnCl 2 , and AsCl 3 , are known. It is difficult, therefore, to classify the elements according to their valencies ; indeed, an absolute classification is scarcely possible. In the following table a number of the elements are arranged mostly according to their apparent maximum valencies : Monads. Dyads. Triads. Hydrogen. Oxygen. Boron. Fluorine. Barium. Gold. Chlorine. Strontium. Bromine. Calcium. Iodine. Magnesium. Zinc. Potassium. Cadmium. Sodium. Copper. Lithium. Mercury. Silver. Tetrads. Pentads. Hexads. Carbon. Nitrogen. Sulphur. Silicon. Phosphorus. Selenium. Tin. Arsenic. Tellurium. Lead. Aluminium. Antimony. Tungsten. Indium. Bismuth. Molybdenum. Thallium. Iron. Chromium. Manganese. Cobalt. Nickel. Platinum. Palladium. The valency of an element is usually expressed by dashes or Roman numerals placed on the right of its symbol, thus : H , O", B ", C T , P T , Mo TI ; but in constructing graphic formulae the symbols of the elements are written with as many lines attached to each symbol as the element which it represents has units of affinity. Graphic formulae are employed to express the manner in which it is assumed that the constituent atoms of com pounds are associated together ; for example, the trioxide of sulphur is usually regarded as a compound of an atom of hexad sulphur with three atoms of dyad oxygen, and this hypothesis is illustrated by the graphic formula O When this oxide is brought into contact with water it com bines with it forming sulphuric acid, H 2 S0 4 , and it ia
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