Rare earth elements placement

The position of yttrium in rare earth chemistry has always been interesting and this is also the case with respect to complex formation. The electrostatic model suggests placement of yttrium between holmium and thulium. It has been shown that it is not the case [14]. When one considers the stability constant data of group 1 ligands, yttrium is similar to the heavy rare earths. When the second group of ligands is considered, yttrium exhibits a behaviour similar to the lighter rare earth elements. 

An even more asymmetrical aspect concerns the precise placement of the rare earth element block, which in many tables is still shown to begin one element to the right of the start of the d block. This is despite the fact that several articles have pointed out that the f block truly belongs between the s and d blocks (Jensen, 1986). 

Mendeleev himself referred to the rare earths as podvodnyi kamen (or underwater stone), also translated as stumbling block This evocative term already hints to what extent the placement of the rare-earth elements had troubled... 

Mendeleev. After his discovery of the periodic system in February 1869, Mendeleev tirelessly laboured at the clarification, illustration, refinement, and optimisation of his periodic classification of the elements. Not surprisingly, the accommodation of the rare-earth elements formed an important part of Mendeleev s research program, which lasted approximately 2 years— from 1869 till 1871. But at the end of 1871, Mendeleev threw in the towel, distancing himself from the various discussions concerning the placement of the rare earths which were held at the end of the nineteenth century. 

The accommodation of cerium went smoothly as Mendeleev had correctly determined its atomic weight and oxide formulae. According to the dualism of cerium, this rare-earth element exhibited two oxidation states (+III and +IV), making its placement in the fourth group very natural. As Mendeleev reasoned in his article On the Placement of Cerium in the Periodic System of Elements ... 

This homologous placement of the rare-earth elements shows to what extend Mendeleev s viewpoints had changed by the end of the first half of 1870. What catches the eye is that Dmitrii Ivanovich had switched from placing the rare-earth elements as a group in the periodic system to an individual placement of each element separately (Fig. 11.6). That is, by breaking up the natural group of rare earths, Mendeleev ended up with a set of distinct elements which he set out to accommodate on an individual basis in dijferent groups of the system. 

While this individual accommodation of the rare-earth elements represented an interesting step forward, it did not remove all problems. Mendeleev had always used a web of analogies in determining the positions of the chemical elements, but this methodology could no longer be apphed in the case of the rare earths. As Nathan Brooks observed, the rare-earth elements exposed a serious weakness in Mendeleev s approach to solving the placement of elements in his periodic system. Not surprisingly, Mendeleev remained doubtful as to the new positions of the rare-earth elements. 

Moseley s work was also used to settle the question of the placement of the rare earth elements, a task that had eluded Mendeleev and other early pioneers of the periodic table. Mendeleev had stated that the placement of the rare earths was one of the most difficult problems of all those confronting the periodic law. The rare earths were notoriously difficult to separate chemically. Since they appeared to differ only shghtly in atomic weight and properties, no one had been able to find a satisfactory way of fitting them into the periodic table. According to William Crookes,... 

This tome has been referred to, during its preparation, as the cosmic volume. This was intended to suggest the key role the rare earths play in science from the terrestrial to the celestial. For example, the periodic table of the elements is certainly one of the grandest of human intellectual generalizations and the placement of the rare earths in that scheme completed it. Cosmic is not too grand a term. 

But the placement of difficult elements such as beryllium, the accommodation of the newly discovered noble gases, and the ongoing struggle to position the rare earths all contributed to an atmosphere of productive debate that surrounded the periodic system. These factors may well have contributed just as much as the... 

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