Atomic Weights of Thallium History

The atomic weight of thallium is equal to its combining weight in thallous salts, or three times its combining weight in thallic salts. This follows from the isomorphism of (i.) numerous thallous, potassium, rubidium, and caesium salts; (ii.) various double thallic halides with the corresponding indium salts. It is, moreover, in harmony with Dulong and Petit's Law.

The atomic weight of thallium has been determined with great care by Crookes, who effected the synthesis of thallous nitrate. As the mean of ten extremely concordant experiments, he obtained the following result (O=16.000; N = 14.008): -

Tl:TlNO₃::100:130.3910 ⇒ Tl = 204.034.

The determinations of Lepierre may be looked upon as yielding corroborative evidence in favour of Crookes's value, the results ranging from 204.021 to 204.300 for the atomic weight, and being derived from the analyses of thallous sulphate, thallous nitrate, and thallic oxide. The value at present accepted for the atomic weight of thallium is Tl = 204.0.