This list does not include all the names which have been put forward to designate what have been claimed from time to time as new elements; whilst the individuality of some of those included is not yet fully established, and the homogeneity of others has been called in question. The uncertainty is more pronounced among the yttrium elements than among the cerium elements; owing to the opportunities for investigation furnished by the commercial treatment of monazite, the chemistry of the cerium group may be regarded as complete.
In the following table the elements are arranged in order of increasing atomic weight, and it can be seen at once how closely the division into groups follows this order:
| Element | Atomic Wt. | Colour of Salts | |||
|---|---|---|---|---|---|
| - | Scandium, Sc | 44·1 | Colourless | ||
| Yttrium, Yt | 89·0 | Colourless | |||
| Cerium Group. | - | Lanthanum, La | 139·0 | Colourless | |
| Cerium, Ce | 140·25 | Cerous, colourless; ceric, orange to red | |||
| Praseodymium, Pr | 140·6 | Green | |||
| Neodymium, Nd | 144·3 | Red to reddish-violet | |||
| Samarium, Sa | 150·4 | Topaz yellow | |||
| Terbium Group. | - | Europium, Eu | 152·0 | Faint rose | |
| Gadolinium, Gd | 157·3 | Colourless | |||
| Terbium, Tb | 159·2 | Colourless | |||
| Erbium Group. | - | Dysprosium, Dy | 162·5 | Bright green | |
| Holmium, Ho | 163·5 | Yellow to orange | |||
| Erbium, Er | 167·7 | Deep rose | |||
| Thulium, Tm | 168·5 | Bluish-green | |||
| Ytterbium Group. | - | Ytterbium, Yb | 172·0 | Colourless | |
| Lutecium, Lu | 174·0 | Colourless | |||
In their chemical relations, the rare earth elements may be placed between the metals of the alkaline earths, and the trivalent metals iron, aluminium, and chromium. With the exceptions of cerium in the ceric salts, and of samarium and europium in the recently discovered dichlorides, they are uniformly trivalent, but the oxides are very strong bases, and the salts very slightly hydrolysed in dilute solutions; generally, therefore, they resemble the calcium family rather than the aluminium group. Among the common salts, the oxalates, phosphates, chromates, iodates, fluorides, carbonates, tartrates, and borates are almost insoluble; the sulphates are only sparingly soluble at ordinary temperatures. Among the double salts, the alkali double sulphates are of great importance from their employment for separations; the tendency to the formation of complex salts is greater among the yttrium than among the cerium elements, increasing with the atomic weight, and with the decrease in basic strength of the oxides.
The great similarity in chemical behaviour of the rare earth elements is apparent not only in the similarity in composition, solubility and chemical properties of the salts—which is so great that the general account of the compounds which follows applies almost in its entirety to each member of the group—but also in the crystallographic relations between corresponding compounds. Many of the salt hydrates form isomorphous series; the sulphate octohydrates, for example, appear to be isomorphous throughout the whole group, and probably the relation would be found to apply even more completely than is generally accepted, if the necessary data were forthcoming. Of great interest and practical importance is the isomorphism between the nitrates and double nitrates of the cerium elements and bismuth, which has been utilised with such valuable results in the processes of fractional crystallisation.
The Metals.
—The earlier attempts to reduce compounds of the rare earth elements to the metallic condition, by means of metallic sodium or potassium, did not yield pure products; nor did the use of aluminium or magnesium lead to results of practical importance. The metals were first obtained in a coherent physical condition by Hillebrand and Norton,[147] by electrolysis of the fused chlorides. These investigators obtained cerium, lanthanum, and the so-called didymium, and measured their specific heats; their results confirmed the atomic weights assigned to the elements by Mendelejeff, except in the case of lanthanum. Their method has since been elaborated by Muthmann, Hofer and Weiss,[148] who have prepared large quantities of the cerium elements in the pure state. More recently, Hirsch has prepared metallic cerium in large quantities,[149] and has studied its properties.
[147] Pogg. Ann. 1875, 155, 631; 156, 466.
[148] Annalen, 1902, 320, 231; see also Muthmann and Weiss, ibid. 1904, 331, 1.
[149] Met. Chem. Eng. 1911, 9, 543.