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Atomistry » Thorium » Physical Properties » Atomic Weight, History | ||
Atomistry » Thorium » Physical Properties » Atomic Weight, History » |
Atomic Weights of Thorium, History
It was assumed by Berzelius, who first investigated thorium and its compounds, that thoria is analogous to magnesia, and should therefore be represented by the formula ThO, the metal having an atomic weight of approximately 116. The formula ThO2 was first adopted by Delafontaine in 1863, on account of the isomorphism of thoria with tinstone and rutile, and of certain complex salts of thorium with similar salts of zirconium. That the atomic weight of thorium is about 232 is proved by the following facts:
Numerous attempts have been made to determine accurately the atomic weight of thorium. Berzelius converted the oxide into the sulphate and then precipitated the latter with barium, obtaining the mean value of 238.9, calculated according to modern atomic weight values; Chydenius analysed the simple and complex sulphates, and also the acetate, formate, and oxalate. The mean atomic weight calculated from his values is 236.83. Delafontaine estimated the thoria in the hvdrated sulphate Th(SO4)2.9H2O, and obtained the value 232.34. The foregoing results are now merely of historical interest. Cleve analysed the anhydrous sulphate as well as the oxalate; Nilson estimated the thoria in Th(SO4)2.9H2O and also in the anhydrous sulphate; Krliss and Nilson likewise determined the ratio ThO2:2SO3. Estimations have also been made by Hermann, Brauner, Urbain, W. Biltz, and by Meyer and Gumperz. The figure adopted by the International Atomic Weights Committee from 1900 to 1908 was 232.5. In 1909 the value was changed to 232.42, and at the present time (1917) the figure accepted is 232.4. This value is in need of revision, for it is almost impossible to be certain that thoria (which figures in all the above ratios) was weighed free from moisture and adsorbed gases. |
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