Note 14 — Chapter XXI · CHROMIUM, MOLYBDENUM, TUNGSTEN, URANIUM, AND MANGANESE

The atomic weight of uranium was formerly taken as half the present one, U = 120, and the oxides U2O3, suboxide UO, and green oxide U3O4, were of the same types as the oxides of iron. With a certain resemblance to the elements of the iron group, uranium presents many points of distinction which do not permit its being grouped with them. Thus uranium forms a very stable oxide, U2O3(U = 120), but does not give the corresponding chloride U2Cl6 (Roscoe, however, in 1874 obtained UCl5, like MoCl5 and WCl5), and under those circumstances (the ignition of oxide of uranium mixed with charcoal, in a stream of chlorine), when the formation of this compound might be expected, it gives (U = 120) the chloride UCl2, which is characterised by its volatility; this is not a property, to such an extent, of any of the bichlorides, RCl2, of the iron group.

The alteration or doubling of the atomic weight of uranium—i.e. the recognition of U = 240—was made for the first time in the first (Russian) edition of this work (1871), and in my memoir of the same year in Liebig's Annalen, on the ground that with an atomic weight 120, uranium could not be placed in the periodic system. I think it will not be superfluous to add the following remarks on this subject: (1) In the other groups (K—Rb—Cs, Ca—Sr—Ba, Cl—Br—I) the acid character of the oxides decreases and their basic character increases with the rise of atomic weight, and therefore we should expect to find the same in the group Cr—Mo—W—U, and if CrO₃, MoO₃, WO₃ be the anhydrides of acids then we indeed find a decrease in their acid character, and therefore uranium trioxide, UO₃, should be a very feeble anhydride, but its basic properties should also be very feeble. Uranic oxide does indeed show these properties, as was pointed out above (Note 10). (2) Chromium and its analogues, besides the oxides RO₃, also form lower grades of oxidation RO₂, R₂O₃, and the same is seen in uranium; it forms UO₃, UO₂, U₂O₃ and their compounds. (3) Molybdenum and tungsten, in being reduced from RO₃, easily and frequently give an intermediate oxide of a blue colour, and uranium shows the same property; giving the so-called green oxide which, according to present views, must be regarded as U₃O₈ = UO₂2UO₃, analogous to Mo₃O₈. (4) The higher chlorides, RCl₆, possible for the elements of this group, are either unstable (WCl₆) or do not exist at all (Cr); but there is one single lower volatile compound, which is decomposed by water, and liable to further reduction into a non-volatile chlorine product and the metal. The same is observed in uranium, which forms an easily volatile chloride, UCl₄, decomposed by water. (5) The high sp. gr. of uranium (18·6) is explained by its analogy to tungsten (sp. gr. 19·1). (6) For uranium, as for chromium and tungsten, yellow tints predominate in the form RO₃, whilst the lower forms are green and blue. (7) Zimmermann (1881) determined the vapour densities of uranous bromide, UBr₄, and chloride, UCl₄ (19·4 and 13·2), and they were found to correspond to the formulæ given above—that is, they confirmed the higher atomic weight U = 240. Roscoe, a great authority on the metals of this group, was the first to accept the proposed atomic weight of uranium, U = 240, which since Zimmermann's work has been generally recognised.