Atomic Weight of Uranium

The earlier investigations into the atomic weight of uranium are of little more than historic interest, owing in the first place to the erroneous idea that the oxide was the element, and also to the fact that the analytical results obtained were often widely discordant, probably due to the impure nature of the materials studied. The valency of the element was assumed to be 3 and the atomic weight appeared to be about 120. After the recognition of the true nature of the oxide and the isolation of the metal, Peligot made a series of analyses of the oxalate and acetate of uranium, and from his results the atomic weight 120 (O = 16) was again obtained.

In 1872 Mendeleeff pointed out that there was no place in the Periodic Table for a trivalent element of atomic weight 120, and drew attention to the similarity of uranium to chromium, molybdenum, and tungsten; he therefore suggested that the atomic weight should be doubled, so that uranium could be placed below these elements in the table. He also formulated the oxides, by analogy with those of the other elements in the group, as follows: uranous oxide UO₂, urano-uranic oxide U₃O₈ or UO₂.2UO₃, uranic oxide UO₃; thus suggesting that the element was both tetra- and hexa-valent. The correctness of these suggestions was confirmed by the determination of the specific heat of the metal and the application of Dulong and Petit's Law, and also by vapour-density determinations of uranous chloride and bromide, and later of uranic fluoride.

In 1902 Richards and Merigold analysed uranous bromide, and determined the bromine as silver bromide. The uranous bromide was carefully purified by sublimation in an atmosphere of either bromine or nitrogen, oxygen being rigorously excluded. The operations, however, were carried out in glass vessels, and a correction had to be made for the presence of a small amount of sodium bromide in the uranous bromide. A further source of error, namely, that at the high temperature of sublimation the uranous bromide reacts with the glass to form small quantities of the oxybromide UO₂Br₂, was not taken into account, so that the values obtained must be considered too high. The vessel, containing a known weight of the uranous bromide, was opened under water, dilute hydrogen peroxide added, and the bromine precipitated in one series of experiments by excess of silver nitrate, and in another series by the addition of the exact weight of silver dissolved in nitric acid. The results were as follows:

UBr₄:4AgBr :: 74.2908:100.0000, whence U = 238.4.
UBr₄:4AgBr :: 74.2960:100.0000, whence U = 238.4.
UBr₄:4Ag :: 129.3280:100.0000, whence U = 238.4.

In 1914 Honigschmid repeated the experiments of Richards and Merigold, using apparatus of silica. Uranous bromide was carefully purified by sublimation either in bromine vapour or in nitrogen, and the product was then melted and transferred to a silica weighing-tube in absence of air. The ratio UBr₄:4AgBr was estimated gravimetrically, and the UBr₄:4Ag by titration. The results were as follows (Ag = 107.880, Br = 79.916):

1. After sublimation in bromine,
   UBr₄:4AgBr :: 74.24803:100.00000, whence U = 238.09.
   UBr₄:4Ag :: 129.2547:100.0000, whence U = 238.11.
2. After sublimation in nitrogen,
   UBr₄:4AgBr :: 74.26192:100.00000, whence U = 238.19.
   UBr₄:4Ag :: 129.2722:100.0000, whence U = 238.18.

The lower values obtained in the former experiments were due to adsorption of traces of bromine during sublimation, and the values from the second series of experiments are more reliable. For these researches, the uranium salt was derived from minerals obtained from the Joachimsthal deposits, the age of which is estimated to be 250 million years; in order to see if the atomic weight of uranium, which is considered to be a mixture of two isotopes (see below), varied with the age of the mineral, Honigschmid and Horovitz made further experiments, the methods being precisely as above, but the uranium was obtained from a very pure crystalline form of uraninite from Morogoro in East Africa, the age of which is about 800 million years. The results were as follows:

1. After sublimation in bromine,
   UBr₄:4AgBr :: 74.24378:100.00000, whence U = 238.05.
   UBr₄:4Ag :: 129.2450:100.0000, whence U = 238.06.
2. After sublimation in nitrogen,
   UBr₄:4AgBr :: 74.25872:100.00000, whence U = 238.16.
   UBr₄:4Ag :: 129.2702:100.0000, whence U = 238.17.

These results are identical with those of the earlier experiments, and as before the higher results are more correct.

This work is the most reliable done up to the present (January 1926), and the true atomic weight appears to be slightly below 238.2. The International Committee for 1925 accept the value

U = 238.17.

Uranium is thus the terminal member of the periodic system, possessing a greater atomic mass than any other known element.