The Rare Earths: Their Occurrence, Chemistry, and Technology

by Stanley Isaac Levy

[178] Monats. 1881, 3, 1; Zeitsch. Elektrochem. 1908, 14, 525.

In support of this arrangement, he quotes the fact that some of the elements appear to be able to form higher oxides in the presence of other oxides, which act as oxygen carriers (see [pp. 174], [177-8]), though these higher oxides are certainly not salt-forming. He also deduces, from the rates of hydrolysis of the sulphates, that the elements fall into two parallel series, according to the strengths of the hydroxides as bases, on which ground he justifies the distribution throughout series 8 and 9. There can be no doubt, however, that this disposition is far less in accordance with the behaviour and properties of the rare earth elements than is the first arrangement, which places them in a transition zone between barium and tantalum; it is impossible, for example, to reconcile the properties of praseodymium with those of columbium and tantalum, or to find the slightest analogy between neodymium and molybdenum or tungsten, as the second arrangement requires.

The analogy of the rare earth group to the elements of Group VIII has been pointed out by many authors.[179] On the ground that the rare earth elements cannot be spread over the table in series 8-10, Steele[180] favours the early classification of Thomsen, according to which the elements are divided into three groups. The first, corresponding to Groups I and II of Mendelejeff’s table, consists of two sub-groups, each containing seven elements[181]; the second, corresponding to the first two long series of the periodic table, has two sub-groups, each of seventeen elements, of which the first and last seven are analogous—these elements fall into the same groups in the periodic table—whilst the middle three are interperiodic. These interperiodic elements are those which Mendelejeff places in Group VIII. The third division consists of one (or two) group(s) of thirty-one elements; here again, the first and last seven are analogous, whilst the interperiodic elements, which are seventeen in number, include the rare earth metals.

[179] Compare Biltz, Ber. 1902, 35, 562.

[180] Chem. News, 1901, 84, 345.

[181] The inert gases are not included.

Steele’s idea has been extended by Werner,[182] who has drawn up a table to illustrate it. In this classification, the elements are arranged in order of atomic weight, but arbitrary gaps are left in such a way that similar elements may fall into the same vertical columns, as in the periodic table. The arrangement has the advantage that the interperiodic elements, consisting of the rare earth elements and the elements placed in Group VIII of the periodic table, here do fall in the middle of their respective periods, but it has several drawbacks, and does not represent the transition of properties from element to element so well as the helical representation of the periodic table, which brings out most clearly the true relations between the elements, and the anomalous position of the rare earth metals.

[182] Ber. 1905, 38, 914.

Mention must be made at this point of the theory of ‘Meta-elements’ put forward in 1888 by Sir William Crookes.[183] From his work on the cathode luminescence of some of the oxides (see [next chapter]), that author was led to the conclusion that several of the then-accepted rare earth elements, notably samarium and yttrium, were in reality heterogeneous, consisting of large numbers of very closely related bodies, differing so very slightly in properties that only the most refined methods could perceive the variations; for these he proposed the name Meta-elements. Though it has been proved that the differences observed by Crookes in the luminescence spectra were really due to the presence of very small quantities of impurities, his paper is of great interest, in that it contains a theory of evolution of the elements, and postulates the possibility of their decay. Modern developments in radioactivity have not only lent a curious force to these speculations, but even support his contention that a chemical element, in the ordinary sense of the word, is not necessarily homogeneous.[184] In the field of the rare earths, also, the homogeneity of elements is even now continually being called into question (see [Thulium], p. 204). In any case, we have in the rare earth elements a series of bodies in which the change of properties from one member to another—and the consequent possibility of easy separation—is so very slight, and so far without parallel in the whole field of chemistry, that we are at least justified in asking whether some extension of our ordinary conception of an element is not required.

[183] Trans. Chem. Soc. 1888, 53, 487.