Terbium atomic weight

Following the adventurous story of terbium it is actually impossible to decide by now who was the true discoverer, Mosander, Delafontaine or Smith The element names, as indicated above were applied inconsistently, and we cannot know whether they referred to the same substance. Did Mosander find the same substance and called it erbium that finally became terbium with Delafontaine, or was Bunsen correct and consequently Mosander s fraction was a mixture only No data were reported that would allow us to state now, at this late date, what substances were identical, no characteristic spectral lines, no exact atomic weight values are at our disposal as yet. 

Roscoe discovered tungsten pentachloride and pentabromide, uranium pentachloride,2 and niobium trichloride (which he found gave NbOClg and CO when heated in COg). He determined the vapour densities of PbClg and TlCl, showed that Delafontaine s supposed new rare-earth metal philippium is a mixture of yttrium and terbium, and redetermined the atomic weight of carbon (C = 12 002). Roscoe published several papers and a book on spectrum analysis. 

Lenssen was another early pioneer who was prepared to make predictions on the basis of his system. For example, he predicted the atomic weights of erbium and terbium, neither of which had yet been isolated. This is mentioned just to emphasize again that Mendeleev did not invent the idea of making predictions using classification systems for the elements, as seems to be popularly believed.The fact remains, however, that Lenssen s predictions were found to be incorrect. 

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