VALENCY ( Lat. ralcntia, strength, from valerc, to be strong, able; connected with 01r. fiaith, power, MO. galeti, to be able, Goth. icaldan, 011G. Ger. !mite'''. AS. ircaldan, Eng. /vivid), VALENCE, or kw:mull-Y. In chem istry. the combining capacity of an atom, With reference to the number of other atoms with which it can be directly combined. The concep of valency is an offspring of the atomic and molecular theories. These theories led to a knowl edge of the numbers of different atoms making up the molecules of compounds; but as in many cases very different compounds were found to have the same composition and the same molec ular weight, it became clear that differences in the chemical and physical properties of com pounds must often be caused by differences in the manner of combination of their :Hams. Those• differences of combination had to be investigated, and thus arose the question, In what manner does affinity act in holding together the atoms of compounds? In considering various com pounds with a view to obtaining some light on this question, chemists were gradually led to a series of assumptions, the incorporation of which in atomic chemistry has proved exceedingly fruit ful.
First of all it was observed that, in compounds of hydrogen with some one other element, one atom of hydrogen can hold in combination only one atom of the other element ; this was shown by such compounds as hydrochloric acid (11C1), hydrobromic acid (HBr), hydriodic acid (HI), etc., and the fact was expressed by saying that hydrogen is uniralcut or monad. The val encies of certain other elements were then found by considering their compounds with hydrogen, on the principle that by every unit of their combining capacities the atoms of those elements can hold one atom of hydrogen; for hydrogen itself is univalent, i.e. has unit com bining capacity. From the compounds already mentioned it may be seen that the atoms of chlorine, bromine, and iodine are, like hydrogen, univalent; for each atom of these elements can be combined with only one atom of hydrogen.
Similarly, compounds like water (H,0), sul phureted hydrogen ammonia (NIL), and marsh-gas (CH,) show that oxygen and sulphur are divalent, that nitrogen is trivalent, and that carbon is quadrivalent. The valencies peculiar to some of the elements being thus established by an inspection of their compounds with hydrogen, the valencies of the other ele ments can be found by studying their com pounds not necessarily with hydrogen. but with any element of known valency. Thus compounds like sodium chloride (Na postassium chloride (KCl), etc.. show that time atoms of sodium and potassium are univalent; for they can hold in combination only one atom of another univalent element. The same thing is shown by the fact that sodium or potassium takes the place of one atom of hydrogen in acids; for example, in hydro ehlorie acid: Na H CI = Na CI -I- II K H Cl = K Cl + H Evidently, an atom of sodium or potassium is equivalent to an atom of hydrogen, and as the latter is univalent, the former, too, must he uni valent. Similarly, an atom of calcium takes the place of two atoms of hydrogen: Ca + 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 valeneies peculiar to all of the elements. The conception of valency has proved especially useful in the domain of organic chemistry, 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 exhibit': 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 different 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.