Atomic Weight of Bismuth

Until comparatively recent years the values obtained for the atomic weight of bismuth by different investigators varied considerably. This was, in general, due to two causes: firstly, the difficulty of obtaining pure bismuth compounds, and secondly, the tendency for inorganic salts of bismuth to form basic complexes through hydrolysis. The earliest values were obtained by Lagerhjelm, who converted metallic bismuth into oxide, sulphide and sulphate respectively and determined the ratios 2Bi:Bi₂O₃, 2Bi:Bi₂S₃, and 2Bi:Bi₂(SO₄)₃. Subsequently many investigators repeated these determinations and obtained values varying from 208 to 210. Dumas converted the metal into chloride and determined the ratio 3Ag:BiCl₃. Marignac prepared bismuth trioxide from bismuth nitrate. The nitrate was first purified and then converted into basic nitrate, the latter being converted into the oxide by heating. In one series he reduced the oxide to metal by heating in a current of hydrogen and determined the ratio 2Bi:Bi₂O₃; in another he converted the oxide into the sulphate and determined the ratio Bi₂O₃:Bi₂(SO₄)₃. Classen, in 1890, realising the difficulty of obtaining pure bismuth, purified the metal electrolytically; from the ratio 2Bi:Bi₂O₃ he obtained the value 208.92, which approximates closely to that now generally accepted. Later, Gutbier and his collaborators adopted a variety of methods and obtained values all of which were in the neighbourhood of 208. More recently, Honigschmid and Birckenbach prepared specimens of the chloride and bromide of bismuth with great care, taking special precautions to exclude all moisture, and from their analyses of these salts obtained a mean value of 209. Classen and his collaborators concluded that organic compounds of bismuth would be more suitable for atomic weight determinations as they were less liable to form basic complexes. They prepared specimens of bismuth triphenyl, and from the ratio 2BiPh₃:Bi₂O₃ derived the mean value 209.00.

In 1921 the value adopted by the International Committee was 208.00; at present (1936) the accepted value is 209.00. This is in agreement with the work of Aston, who determined the mass number of bismuth to be 209.

Bismuth is a simple element, no inactive isotopes having been discovered. Radioactive isotopes, such as thorium C, are known, however.