VALENCY. 797 VALENCY. Na + H CI = Xa 01 + H K + H CI = K CI + H Eviiiently, an atom of sodium or potassium is equiiahiit to an atom of hyilrogon, and as the latter is univalent, the former, too, must be uni- valent. Similarly, an atom of calcium takes the place of two atoms of hydrogen : Ca + |^[!} =CaCl,,+2n and therefore the element calcium, or rather an atom of this element, is considered di-valent. Thus a few simple compounds and a few simple reactions led to a knowledge of the valencies peculiar to all of the elements. The conception of valency has proved especiallj' useful in the domain of organic chemistrj-, i.e. the chemistry of the compounds of carbon. Our modern "struc- tural theory' is based entirely on the assumption that an atom of carbon is invariably quadriva- lent : and the usefulness of the structural theory can hardly be overestimated. For it exhibits in a clear and simple manner the relations between similar as well as different compounds, and, above all, it permits of determining the exact number of dirt'erent compounds that may have the same composition and the same molecular weight, and thus permits of foretelling the existence of compounds before they have been actually ob- tained. Graphically each unit of combining capacity of an atom is represented by a dash added to its symbol. The valencies of different elements are thus denoted as follows; H— CI— — O— —X— — ('— etc. I I When two atoms combine, at least one valency of each is employed, and in compounds like the following, the atoms are said to be linked to- gether by single 'bonds,' each 'bond' evidently representing two 'valencies' or 'affinities' (i.e. one unit combining capacity of each of the com- bining atoms) : H I H— 01 H— O— H H— N— H H— C— H H H Hydrochloric acid Water Ammonia Marsli-gas The graphic representation of valency suggests an important question, viz.: Are the valencies of an atom forces acting only in certain directions, or do they act, like gravity, in all directions? A further question naturally suggests itself in the ease of atoms having more than unit valency, viz. : Are the several affinities equal to one an- other in power ? To answer these questions is a matter not of idle speculation, but of necessity in the case — again — of the compounds of carbon. The study of these compounds has led chemists to make the following assumptions: (1) the four valencies of carbon are in all respects equal ; (21 they act in four different directions, which are perfectly symmetrical with respect to the carbon atom. The carbon atom is, namely, imagined to be placed at the centre of a regular tetrahedron, and four equal forces are assumed to act in the directions of the four vertices of the tetrahedron. A further assumption that thrusts itself upon the organic chemist is that in every compound capable of independent existence all the valencies of the constituent atoms are satis- lied by condiination, and that no valency is 'free.' ithout these assumptions organic chemistry can make no progress. The.se assumptions made, there is hardly a general fact that remains un- accounted for. The assumptions, though hypo- thetical in character, are therefore incorporated as principles of science, and thus in connection with the compounds of carbon chemistry answers in a sense the question stated at the beginning of this article, viz.: In what manner does affinity act in holding together the atoms of compounds? In the case of other elements than carbon, the application of the idea of valency has been much less useful and much less successful. In fact, the obstacles in the way of consistently applying the idea to the several elements are so great that the idea would ])robably have been abandoned long ago, were it not for its great usefulness in the case of carbon. The chief obstacles are as follows: Firstly, the valencies of most elements are found to be variable and hence unreliable as a basis for predicting the constitution of un- known substances. Tluis. while in ammonia (XHj) the atom of nitrogen is tri-valent (be- cause combined with three univalent atoms of hydrogen), in nitric oxide (XO| it is divalent (because combined with one divalent atom of oxygen), and in ammonium chloride (Nll.ll) it is penta-valent (because combined with live uni- valent atoms, viz. four hydrogens and one chlorine). In other compounds nitrogen seems to have still other valencies. Turning to iron, we find it divalent in ferrous cliloride ( FeClj) and tri-valent in ferric chloride (FeClj). Chlorine is univalent when combined with hydro- gen, and quinqui-valent when combined "with oxygen. Sulphur is di-valent when combined with hydrogen, and hexa-valent when combined with oxygen. Phosphorus is tri-valent when combined with hydrogen, and quinqui-valent when combined with o.xygen. Oxygen is divalent in nearl.y all of its compounds; yet in <li-methyl ether hj'drochloride o.xygen must be assumed to be quadrivalent. Even in the case of carbon an exception is known: in ordinary carbonic oxide (CO) the carbon atom is apparently di-va- lent (because combined with one divalent atom of oxygen) — unless we assume that the oxygen atom in this compound is quadrivalent, and hence that the compound is an exception to the rule according to which oxygen is divalent. Further, it has been stated above that the atoms of hydrogen, chlorine, iodine, and sodium were primarily assumed to be univalent. One might therefore expect that in all comliinations of any two or three elements one atom of one would combine with one, and only one atom of the other. Yet the compound called trichloride of i<idine has the formula ICI,. Is iodine tri-valent in this compound? Another compound, a hydride of sodium, has the formula HNa,. Is hydrogen in this compound divalent? And is. therefore, the valency even of hydrogen variable? Again when we find the molecule of hydrogen g:is to be made >ip of two liydrogen atoms, we conclude that the affinity of each of these atoms is satis- fied by that of the other atom. But the molecules of certain univalent elements (the vapors of sodium, potassium, iodine, at high temperatures, etc.) are known to be made up each of a single
