Erbium atomic weight

Atomic number 69 Standard state Tm (c) Erbium Atomic weight 169.4... 

ISOTOPES There are 39 isotopes of erbium, six of which are stable Er-162, Er-164, Er-166, Er-167, Er-168, and Er-170. These six isotopes make up the total atomic weight (mass) of erbium, and all the other isotopes are artificially made and short-lived. Their half-lives... 

Cleve s fame rests chiefly, however, on his discoveries among the rare earths. After obtaining some erbia from which all the ytterbia and scandia had been removed, and after noticing that the atomic weight of the erbium was not constant, he succeeded in resolving the earth into three constituents erbia, holmia, and thulia (21). The absorption bands of holmium had already been noticed by the Swiss chemists M. Delafontaine... 

For example, the isotopic composition and the atomic weight of neodyminm," dysprosium and erbium have been determined using synthetic mixtnres prepared gravimetrically from highly enriched isotopes of neodymium in the form of oxides of weU defined pnrity by TIMS. No natnral isotopic variation was found in terrestrial neodymium, dysprosium or erbium samples. These isotopic compositions of Dy and Er measnred by TIMS are accepted as the best measurements from a single terrestrial source as noted in the table of isotopic composition of elements, 2001. °... 

The values in square brackets are those that were effectively used by Mendeleev in 1870. Notice that the atomic weight of erbium was tripled, while the atomic weight of thorium was doubled, just as in the case of uranium ... 

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. 

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. 

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. 

There are many elements over which Mendeleev dehberated for considerable periods of time and published several accounts. These include indium, erbium, and lanthanum, all of which involved subtle arguments having to do with atomic weight corrections, some of which are examined below. 

---