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 ThO_{2} 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:
 The radioactivity of thorium shows it to be an element of high atomic weight.
 The specific heat of thorium is 0.02787. Assuming a mean atomic heat of 6.4, the atomic weight by Dulong and Petit's Law is about 230.
 The transparency of thorium to Xrays is in accordance with a value for the atomic weight equal to four times the equivalent weight.
 Not only is thoria isomorphous with cassiterite and rutile, but the complex nitrates of thorium and cerium of the type MgR^{iv}(NO_{3})_{6}.8H_{2}O, and thorium and uranous sulphates R^{iv}(SO_{4})_{2}.9H_{2}O, are also respectively isomorphous. Hence by analysing these compounds and applying Mitscherlich's Law of Isomorphism an atomic weight of 232 is arrived at.
 An appropriate position is found for thorium in the periodic system if its atomic weight is 232.
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(SO_{4})_{2}.9H_{2}O, 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(SO_{4})_{2}.9H_{2}O and also in the anhydrous sulphate; Krliss and Nilson likewise determined the ratio ThO_{2}:2SO_{3}. 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. 


