Page:Outlines of Physical Chemistry - 1899.djvu/47

 and 8 respectively.

I may cite here as examples :

Sulphur group. — Tetra-substituted sulphonium iodides and hydroxides : (CH 3 ) 3 SI, (CH 3 ) 3 S.OH.

Nitrogen group. — The ammonium salts. Ammonium behaves exactly like a monovalent metallic radical, re- sembling sodium and potassium, and can isomorphicaUy replace the alkaline metals. There may even exist an ammonium amalgam (?).

Phosphonium iodide, H 4 PI, the organic ammonium and phosphonium salts such as (CH 3 ) 4 NI, (C 2 H 5 ) 4 N.OH.

Phosphorus pentachloride, PC1 5.

In many compounds, therefore, we must assume sulphur to be tetravalent, and nitrogen pentavalent.

But all these substances belong precisely to that class of compounds which are easily dissociated by heat. Their vapour density is abnormal, and corresponds to a mixture of products resulting from the decomposition.

Thus KehuU was able to say : ' We can only deduce the valency of the elements from the examination of the gaseous compounds. Now ammonium salts do not exist in the gaseous state any more than phosphorus penta- chloride does. In the gaseous state I only know NH 3 and PC1 3 ; nitrogen and phosphorus are thus trivalent. As for the ammonium salts (in the solid state) they are addition compounds : NH 3 .HC1, CH 3 .C0 2 H.NH 3, &c. And in the same way solid phosphorus chloride is a combination of PC1 3 with CV

This artifice served the purpose, and for a long time the theory of constant valency was accepted.

But phosphorus pentafluoride exists in the gaseous state as PF 6. Phosphorus pentachloride has a normal vapour density under the conditions already mentioned (seepage 16). The density of the vapour from ammonium chloride does molecules, which shows that a certain number of unde-

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