[157] Melikoff and Pissarjewski, Zeitsch. anorg. Chem. 1899, 21, 70; Melikoff and Klimento, Chem. Zentr. 1902, 1, 172.
Oxides.
—In their most stable state of oxidation, the rare earth elements are generally trivalent. In the case of cerium, the dioxide, CeO₂, is more stable than the sesquioxide Ce₂O₃, but the ceric salts are unstable, and are very readily reduced to cerous compounds, corresponding to the oxide Ce₂O₃. Higher oxides are known with certainty among the other elements only in the cases of praseodymium and terbium, but these do not give rise to salts.
The oxides R₂O₃ are fairly strong bases, being comparable in strength to the alkaline earths, and far more strongly basic than alumina and oxides of other trivalent elements; thus they liberate ammonia from ammonium compounds, whilst the salts they form with strong acids are not easily hydrolysed. Their relative strengths as bases are expressed in the following series, in which the elements are placed in order of diminishing electropositive character:[158]
La, Ce´´, Pr, Nd, Yt, Eu, Gd, Sa, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Ceiv.
[158] The position of yttrium in this series is not known with certainty; it is probably as positive as neodymium. It is usually stated (see Meyer and Hauser, pp. 32-33) that the terbia oxides are intermediate in basic strength between the ceria and yttria earths, though the arrangement into two series, consisting of the cerium and yttrium groups respectively, is generally adopted; the electropositive character of the elements in each series then weakens as the atomic weight rises, scandium being of course exceptional.
It will be seen that, with the exception of scandium and yttrium, the metals of the cerium and yttrium groups become less electropositive as the atomic weight increases.
This arrangement is obtained by ascertaining the order in which the various hydroxides are precipitated from a solution by gradual addition of a dilute solution of a strong base. The weakest base is precipitated first, and the strongest last; those intermediate in strength are thrown down in ascending order of strength. Similar results may be obtained by the fractional decomposition of the nitrates by heat; in this case the nitrate of the weakest base is decomposed at the lowest temperature. This order is also confirmed, as far as the data are available, by measurements of the equivalent conductivities of solutions of the salts (see, for example, [p. 122]).
Quite recently, a very different order has been obtained from a consideration of the dissociation tensions, and of the heats of dissociation of the anhydrous sulphates.[159] In the following table the elements are arranged in the order of the increase of the dissociation tension (T) measured at 900°, which is the same as the order of decrease of the heats of dissociation (Q):
| Element | At. Wt. | T. (Mm. Hg.) | Q. | ||
|---|---|---|---|---|---|
| La | 139 | ·0 | 2 | 59·8 | |
| Yt | 89 | ·0 | 3 | 58·9 | |
| Lu | 174 | ·0 | 3 | ·5 | 58·5 |
| Yb | 172 | ·0 | 4 | 58·2 | |
| Er | 167 | ·7 | 5 | 57·6 | |
| Pr | 140 | ·6 | 5 | ·5 | 57·4 |
| Nd | 144 | ·3 | 6 | 57·2 | |
| Gd | 157 | ·3 | 7 | 56·9 | |
| Sa | 150 | ·4 | 8 | 56·5 | |
| Sc | 44 | ·1 | 11 | 54·5 | |
| Ce | 140 | ·25 | 52·4 | ||