Didymium atomic weight

It thus appears that seven little studied elements remained outside of the table This septuplet of homeless elements consisted of indium (In), thorium (Th) and the five rare earths—erbium (Er), yttrium (Yt), cerium (Ce), lanthanum (La), and didymium (Di). Question marks and wrong atomic weights reigned in the last rows of Mendeleev s system, and a new place had to be found for the homeless septuplet We will call this problematic accommodation issue the rare-earth crisis. 

Previously, the differences in atomic weight values between lanthanum, cerium and didymium had been too small to differentiate between these three elements. This undermined Mendeleev s use of the atomic weight as the characteristic property of basis substances, and led Mendeleev to believing in physically real atoms and an internal matter constituting these atoms. 

Brauner s later work on the atomic weights of rare-earth elements over a period of years was, in general, very accurate. He also used volumetric methods. In 1882 he concluded that didymium is complex, but since Cleve published on this at the same time he gave up the work in deference to him. Auer von Welsbach first separated didymium into praseodymium and neodymium, but Brauner found that he had inverted the atomic weights. 

There seemed to be no doubt about the existence of lanthanum and didymium. Around 1850, Marignac in Switzerland determined their atomic weights and carefully investigated the chemistry of the two elements. 

Naturally, Brauner did not obtain pentavalent didymium . We know now that lanthanides cannot reach this oxidation state. However, trying to determine the atomic mass of didymium more correctly, Brauner decided to obtain the element in as pure a form as possible. He discovered that didymium separated from samarium could be divided into three fractions somewhat differing in molecular weights. Brauner performed this experiment in 1883 but he had to stop further research for some reasons. It was a great pity since he was so close to ending the story of the old didymium . 

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