[174] Annalen, 1904, 331, 334.
The Rare Earth Elements, and the Periodic Classification
At the time of the introduction of the periodic classification the rare earth elements were generally believed to be divalent. This belief, which has persisted until quite recently,[175] was based chiefly on the electropositive character of the metals, and their general chemical resemblance to the elements of the alkaline earths; the isomorphism of the tungstates of calcium and the cerium elements, and of the molybdates of lead and the cerium elements, also supports this view. The physical evidence in favour of Mendelejeff’s view, however, is quite overwhelming; the specific heats of the metals, the equivalent conductivities of the chlorides, and molecular weight determinations by means of vapour densities and the boiling point method, prove beyond doubt that the elements are in fact trivalent.
[175] See Wyrouboff, Bull. Soc. franc. Min. 1896, 19, 219; Wyrouboff and Verneuil, Compt. rend. 1897, 124, 1230 and 1300; ibid., 1899, 128, 1573; etc.
In deciding in favour of the trivalent nature of the rare earth metals, Mendelejeff was influenced chiefly by the fact that there was no room in the table for divalent elements with the equivalent weights then assigned to the cerium and yttrium elements. At that time, only the six oxides obtained by Mosander were known; of these the accepted equivalents and atomic weights were as follows:
| Element. | Equivalent. | Atomic Weight. |
|---|---|---|
| Lanthanum | 46 | 92 |
| Cerium | 46 | 92 |
| Didymium | 48 | 96 |
| Yttrium | 31 | 62 |
| Erbium | 56 | 112 |
the values for terbium being uncertain. If cerium be considered trivalent in the cerous salts, its atomic weight becomes 138, that of barium being 136. Mendelejeff placed cerium in Group IV, series 8, in the position which it still occupies; he pointed out that the accepted equivalent must be too low, and suggested that the atomic weight should be at least 140, almost exactly the value accepted to-day.
This choice left the positions in Group III, series 8, horizontally before cerium, and in Group IV, series 10, vertically below it (see [figure]), to be filled by the two elements, lanthanum and didymium. No chemical evidence being available to decide the choice, he provisionally assigned didymium to the first (Group III, series 8), and lanthanum to the second (Group IV, series 10) position, at the same time expressing the opinion that didymium was probably a mixture of closely related elements. Yttrium then fell into place in Group III, series 6, above didymium, and erbium in Group III, series 10, below it. To the vacant space above yttrium in Group III, series 4, he assigned the hypothetical element Eka-boron, with atomic weight 44; this space is now occupied by scandium, which corresponds almost exactly in properties to the metal described by the Russian chemist. A part of the table illustrating these positions is shown in [Fig. 4].
The determination of the specific heats of the metals by Hillebrand and Norton in 1875, whilst confirming the trivalency of the elements, rendered it necessary to alter the position of lanthanum, which was placed in Group III, series 8, instead of didymium, which was thus left without a place. This first indication that all the rare earth elements could not be fitted into the table without difficulties was soon followed by the discovery of several other members of the group, for which places could not easily be found.
| Group | 0 | I | II | III | IV | V | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A | B | A | B | A | B | A | B | A | |||||
| Series | 1 | H | |||||||||||
| „ | 2 | Li | Be | B | C | ||||||||
| „ | 3 | Na | Mg | Al | Si | ||||||||
| „ | 4 | K | Ca | Eka- boron | Ti | ||||||||
| „ | 5 | Cu | Zn | ||||||||||
| „ | 6 | Sr | Yt | Zr | |||||||||
| „ | 7 | Ag | Cd | Sn | |||||||||
| „ | 8 | Ba | Di? | Ce | |||||||||
| „ | 9 | ||||||||||||
| „ | 10 | Er | La? | ||||||||||
| „ | 11 | Au | Hg | Pb | |||||||||
| „ | 12 | Th | |||||||||||