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Atomic Weight of Caesium, history

The chemical properties of caesium indicate its close relationship to the other alkali-metals. It is univalent, forming compounds of the type CsX, its atomic weight and hydrogen equivalent being the same. Its atomic weight is of the order Cs=133: a value confirmed by the specific-heat method; by the isomorphism of the caesium compounds with those of potassium, ammonium, and rubidium; by the correspondence of the properties of the metal and its compounds with the periodic system; by the formation of a univalent cation; and by the depression of the freezing-point of bismuth chloride and mercuric chloride produced by caesium chloride.

Early Determinations of Caesium Weight

The earliest determination of the atomic weight of caesium with any approach to accuracy was that of Johnson and Allen. The mean of four experiments gave the value

AgCl:CsCl = 100:117.499,

whence the atomic weight of caesium is

Cs = 132.963.

In 1863 Bunsen obtained the ratio

AgCl:CsCl = 100:117.467

as the mean of three experiments, the corresponding value for the atomic weight being

Cs = 132.917.

Godeffroy employed material of doubtful purity, his ratio being AgCl:CsCl = 100:117.164,


Cs = 132.484.

Modern Determinations of Caesium Weight

With a view to calculating the atomic weight of caesium, Richards and Archibald in 1903 determined with great accuracy the values of five different and independent ratios.The current table of the International Committee on Atomic Weights gives

Cs = 132.81.

Several investigators have experimented with a view to proving the existence of a metal analogous to caesium, but with a higher atomic weight. Careful examination of caesium salts has given no indication of the presence of this "ekacaesium."

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