Page:Outlines of Physical Chemistry - 1899.djvu/48

 28 OUTLINES OF PHYSICAL CHEMISTRY

composed NH 4 C1 molecules are mixed with the dissociation products. The conclusion to be drawn from this is that PF 5, PC1 5 , NH 4 C1, &c. represent single molecules capable of existence in the gaseous state. Nitrogen and phosphorus are therefore pentavalent. In an analogous manner the existence of trimethyl sulphonium iodide, (CH 3 ) 3 S1, proves that sulphur is tetravalent. In short, the exceptions have become so evident and so numerous that the theory of constant valency has become untenable.

We are thus forced into the dilemma — whether to place only limited confidence in this beautiful theory of constant valency, or to modify the conception and admit a certain variability of valency.

But if we choose this latter alternative are we not going to be led to a deplorable confusion and compromise the study of the chemical constitution of substances ? Are we not going to allow the theorist in his study to dispose of atomicity just as he pleases, and assign the most impro- bable structural formulae to compounds ?

Fortunately, no! for MendeUeff and Lothar Meyer have shown by their classifications of the elements that variable valency obeys certain rules and is a periodic function of the atomic weights.

��Mendel6epp's Classification of the Elements

In the table on page 29 the elements are arranged in the order of their atomic weights. After each seventh element a new horizontal series commences, and by this division of the elements we obtain seven vertical columns which constitute the true natural families.

Each family is divided into two sub-groups, and the most perfect analogy exists between the members of these sub-groups. For example, the second column contains the sub-groups Be, Mg, Zn, Cd, Hg, and Ca, Sr, Ba. The fifth column is subdivided into N, P, As, Sb, Bi on the

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