Atomic Weight of Rubidium, history

Like the other alkali-metals, rubidium is univalent, forming compounds of the type RbX, so that its hydrogen equivalent is the same as its atomic weight. Its properties and those of its compounds indicate it to be a member of the first group of the periodic system, with an atomic weight of the order Rb=85. This view finds confirmation in the isomorphism of its salts with those of potassium and caesium; in the close approximation in the molecular heats of the carbonate and chloride to those of the corresponding salts of sodium and potassium; and in the depression of the freezing-points of such solvents as bismuth chloride and mercuric chloride produced by rubidium chloride.

The various attempts to determine the atomic weight of rubidium are given in the appended table. The values computed for the atomic weight as given in the last column are not in the original memoirs, but have been calculated from the ratios in the adjoining column, employing for the purpose the modern atomic weights of silver, chlorine, and bromine, Ag=107.880, Cl=35.457, and Br=79.916.

The most reliable results are those of Archibald shown in heavy-face type, the mean value for the atomic weight of rubidium being 85.448.

The current table of the International Committee on Atomic Weights gives the value

Rb = 85.45.