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Atomic Weight of Ruthenium

Approximate Atomic Weight of Ruthenium

That the atomic weight of ruthenium is approximately 100 and not a multiple or sub-multiple of this amount is evident from various considerations, namely:
  1. Its specific heat is 0.0611. Assuming a mean atomic heat of 6.4, the atomic weight of ruthenium, according to Dulong and Petit's Law, is approximately 105.
  2. The most appropriate position for ruthenium in the Periodic Table is as the first member of the second series of horizontal triads. Hence its atomic weight should lie between that of molybdenum (96) and rhodium (102.9), but nearer in point of magnitude to the last named since an element should be in Group VII between it and molybdenum, but has yet to be discovered.
  3. The alkali chlor-ruthenates are isomorphous with the chlor-platinates, and therefore, by the application of Mitscherlich's Law, they must be assumed to contain, like the platinum derivatives, one atom of ruthenium, their generic formula being M2RuCl6. Similarly, ruthenium dioxide is isomorphous with cassiterite, SnO2, and with rutile, TiO2, from which it may be concluded that its formula is RuO2. Analyses of these compounds indicate that the atomic weight of ruthenium is 101.7.

Exact Atomic Weight of Ruthenium

For many years the accepted value for the atomic weight of ruthenium was 103, a figure derived from the work of Claus. This chemist decomposed potassium chlor-ruthenite, K2RuCl5, by heating it in a stream of hydrogen, noted the loss in weight, and separated and determined the amounts of ruthenium and potassium chloride in the residue. From his values for the percentages of ruthenium, potassium chloride, and chlorine expelled, the corresponding atomic weights deduced for ruthenium are 103.2, 107.4, and 97.1 respectively. These results are obviously of no present value.

In 1889, Joly determined the percentage of ruthenium in three pure ruthenium compounds, viz. (i) ruthenium dioxide, RuO2, (ii) ruthenium nitrosochloride, Ru(NO)Cl3.H2O, and (iii) ammonium nitrosochlor-ruthenate, (NH4)2Ru(NO)Cl5. In each case reduction to the metal was brought about by heating the compound in a stream of hydrogen. The mean results were as follow:

4 exptsRu:RuO2::76.060:100.000Ru = 101.668
2 exptsRu:Ru(NO)Cl3.H2O::39.720:100.000Ru = 101.734
2 exptsRu:(NH4)2Ru(NO)Cl5::29.455:100.000Ru = 101.613


In 1912, Vogt analysed ruthenium dioxide by reducing it to the metal in a stream of hydrogen, obtaining as a mean of six experiments, the following result:

Ru:RuO2::76.053:100.000 whence Ru = 101.63

The International Committee for 1918 on Atomic Weights accepts the value Ru = 101.7. Further determinations of this constant are clearly desirable.

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