Atomic Weight of Rhodium

That the atomic weight of rhodium is approximately 103 and not a multiple or fraction of this amount is evident from a variety of considerations. These may by summarised as follows:

1. According to Dulong and Petit's Law the atomic weight of rhodium is of the order of 110, its specific heat being 0.058, and a mean atomic heat of 6.4 being assumed.
2. According to the Periodic Classification, rhodium, which in its properties forms a mean between cobalt and iridium, and between ruthenium and palladium, should have an atomic weight intermediate between the values for these extreme elements, namely, from 102 to 105.
3. Rhodium, like cobalt, iron, and aluminium, yields a well-defined series of alums. Application of Mitscherlich's Law of Isomorphism indicates that these have the general formula:
   
   M₂SO₄.Rh₂(SO₄)₈.24H₂O.

Analyses of these salts prove that the atomic weight of rhodium is approximately 103.

The earliest determinations of the atomic weight of rhodium, published by Berzelius in 1828, are now only of historical interest, his results, obtained from analyses of the sodium and potassium chlor-rhodites, Na₃RhCl₆ and K₂RhCl₅, varying from Rh = 103.1 to Rh = 106.1.

The first reasonably accurate determinations were those of Jorgensen in 1883, who reduced chloropentammine-rhodium dichloride,

[Rh(NH₃)₅Cl]Cl₂,

and the corresponding bromine compound, [Rh(NH₃)₅Br]Br₂, to the metal by heating them in a stream of hydrogen. Subsequent experimenters, namely, Seubert and Kobbe, Huttlinger, Dittmar, and Renz, have used the same two compounds. The results are as follow:

Rh(NH₃)₅Cl₃:Rh::100.000:x

give x within 34.953-34.984, whence Rh within 102.91-103.06.

Rh(NH₃)₅Br₃:Rh::100.000:x

give x within 24.053-24.065, whence Rh within 102.90-102.97.

Jorgensen made one analysis of the bromine derivative for bromine, whilst Seubert and Kobbe made one estimation of chlorine in the other compound, the results being as follow:

Rh(NH₃)₅Br₃:3AgBr::100.000:131.62 whence Rh = 103.14

Rh(NH₃)₅Cl₃:3AgCl::100.000:145.94 whence Rh = 103.13

The value accepted by the International Committee on Atomic Weights for 1918 is:

Rh = 102.9,

which, judging from the concordance between the preceding results, appears to be a close approximation to the correct value.