Chapter 73 Chapter 73
5. Setting things in order and interpretation
In effect, f r o m o b s e r v i n g a n d e x p e r i e n c i n g the c o n s t a n t m a t e r i a l t r a n s f o r m a t i o n s g o i n g on in the w o r l d , the a s s u m p t i o n a p p e a r e d o b v i o u s t h a t the different m a t e r i a l s consist of i d e n t i c a l c o m p o n e n t s a n d t h a t s o m e c h a n g e s t a k i n g p l a c e in t h e m are t h e causes of the t r a n s f o r m a t i o n s . T h i s was the c o n c e p t of all p h i l o s o p h i c a l schools in a n t i q u i t y , w h e t h e r a s s u m i n g one single p r i m e v a l c o m p o n e n t , c a l l i n g it water, fire o r a t o m , o r a s s u m i n g a few such c o m p o n e n t s like e.g. A r i s t o t l e did. T h e c o n c e p t of the c h e m i c a l e l e m e n t a p p e a r e d with Boyle, however, he d i d n o t d i s m i s s t h e i d e a t h a t these e l e m e n t s m a y c o n s i s t of c o m m o n c o m p o n e n t s , of the A r i s t o t e l i a n f o u r if you like, b u t t h e y a r e of n o interest for t h e chemist, since c h e m i c a l e l e m e n t s c a n n o t be further d e c o m p o s e d by the m e t h o d s of a n a l y t i c a l c h e m i s t r y . Boyle m a d e n o s t a t e m e n t as to h o w m a n y e l e m e n t s exist, n o r d i d L a v o i s i e r , w h o gave the exact definition of the c h e m i c a l element. It was D a l t o n ' s a t o m i c t h e o r y w h i c h e x c l u d e d the p o s s i b i l i t y of the existence of the p r i m e v a l element(s), of a n y sort of further divisibility. T h i s t h e o r y says t h a t every c h e m i c a l e l e m e n t c o n s i s t s of its p a r t i c u l a r , i n d e p e n d e n t a n d indivisible a t o m s differing in m a t e r i a l q u a l i t y f r o m t h e a t o m s of all o t h e r elements. T h i s is the r e a s o n why the p r o p e r t i e s of i n d i v i d u a l c h e m i c a l e l e m e n t s differ. T h i s last s t a t e m e n t was - as a m a t t e r o f fact - a c c e p t e d g e n e r a l l y since Boyle w i t h o u t a n y special r e a s o n i n g , since it was this differentness on the basis of which new c h e m i c a l e l e m e n t s c o u l d be d i s c o v e r e d . A s u b s t a n c e c o u l d be d e c l a r e d a n e w e l e m e n t on the basis of its p r o p e r t i e s differing f r o m all o t h e r elements. T h e p r o p e r t i e s in q u e s t i o n were u s u a l l y perceivable, such as c o l o u r , state, melting point,
DISCOVERY AND SEPARATION OF THE RARE EARTHS 69 chemical reactions, etc. Dalton, however, introduced a new, m o r e abstract character- istic: atomic weight. This should have been a more exact characteristic than all earlier ones, had not its determination encountered so m a n y uncertainties. Atomic weight was determined by analysis, by finding the percentual composition of the compounds of the element in question. This part of the j o b was fairly simple, because it could be performed in the laboratory, with increasing accuracy as analytical methods of the age developed. However, to determine the atomic weight, it is necessary to know the atomic composition of the molecule, and this could only be assumed on the basis of analogies, no experimental method existed for determin- ing it. Berzelius's genius is truly admirable, using the analytical methods at his disposal he was able to determine the atomic weights of 53 elements between 1807 and 1820 at a fairly acceptable level. We know his laboratory from the description of his pupil W6hler who worked with him in 1823-1824.
'The laboratory consisted of two c o m m o n rooms with the simplest possible equipment. There was no furnace, no exhaust hood, no running water, no gas pipe.
In one of the r o o m s there were two c o m m o n long pine-wood tables, one for Berzelius, one for me. On the walls there were some shelves with chemicals, in the middle of the r o o m a glass-blowing table and a mercury tank. In the c o m e r was a sink made of fayence and a wash tub under it, where Anna, Berzelius's cook, washed up our vessels each morning. In the other r o o m we had the balances and a big c u p b o a r d with instruments. In the kitchen close by where Anna cooked there was a small furnace rarely used and a sand bath continually heated. There was also a small w o r k s h o p with a lathe. Berzelius was usually cheerful, he talked much during work, he liked to tell jokes and liked to listen to jokes, except when he had one of his rather frequent headaches. Then he retired for days and did not come into the l a b o r a t o r y . . . ' (W6hler 1875).
Berzelius's achievement under such conditions is outstanding. His results for atomic weights, at least the integers, are still correct, except where the number of atoms in the molecule that he assumed was incorrect, in such cases he arrived to half or double, two thirds or one and a half times the correct value. T w o discoveries, made still in Berzelius's life time, helped to a certain extent, namely Mitscherlich's isomorphism law and the statement of Dulong and Petit that for metals the product of specific heat and atomic weight is a constant value (atomic heat law). Taking these statements into account Berzelius corrected his atomic weight values from time to time. In his textbook published in 1836, the atomic weights of two rare earth elements are listed: 46.01 for cerium and 32.31 for yttrium (Berzelius 1836). Since Berzelius considered the double hydrogen a t o m as unit, the above values shall be taken twice: 92.02 and 64.62. Berzelius obtained these values by assuming that the composition of the oxides is - analogously to earth alkali metal oxides - C e O and YO, respectively, i.e., to use an expression of a later period, he assumed that Ce and Y are bivalent. The researchers who in the following decades worked on atomic weights of rare earth elements also took them to be bivalent, as well as the rare earth elements discovered subsequently. A m o n g those mentioned in this chapter, particularly Marignac and Brauner determined atomic weights with accuracies astonishingly good in their age.
During the century-long reign of Dalton's atomic theory some concepts involving primeval element hypotheses, c o m m o n components of atoms always propped up.
The growing group of rare earth elements was particularly puzzling for chemists, since their chemical properties were extremely similar, atomic weight determinations gave largely differing results for nominally one and the same element, by reason of the intermingling of the elements, by reason of working in most cases with mixtures that were separated into individual elements only later. In fact, it was the difference that appeared in the atomic weights of different samples that stirred the researchers to discover new rare earth elements. Can these elements be considered chemical elements at all - this is how many chemists put the question.
The human mind wishes to systematize things and phenomena. So it also wished to systematize chemical elements. A certain system was defined by chemical proper- ties, by the chemical reactions to the compounds, which was the basis on which the qualitative wet analysis system for detecting elements developed. It was started perhaps with Bergman in the 18th century and improved in the early 19th century by Pfaff and Rose to achieve the form which up to the 20th century, to the age of instrumental analysis, was the standard method of analysis, described in Fresenius's book Anleitung zur qualitativen chemischen Analyse which first appeared in 1841 and later had many new editions and was translated into many languages (Szabadv~ry 1966, p. 121). The elements discovered later usually fitted into one of Fresenius's analytical groups and consequently were fitted into the next editions of the book.
Rare earth elements, however, did not fit into this system. The concept of atomic weight opened up a new possibility for systematization: grouping the elements by increasing atomic weight appeared a numerical and hence more exact principle than their properties. The idea was not conceived in Lothar Meyer's and Mendeleev's mind without antecedents, systematization by atomic weight was taken up almost immediately after the concept of atomic weight was born. Its history from D6bereiner through Gladstone, Odling, Beguyer de Chancourtois to Newland, to mention only the most important predecessors, has been told many times (Szabadv~ry 1961). However, the scientists mentioned were unable to come to important conclusions through their attempts, by reason of the basic concept, since the atomic weights were uncertain. Not only numerically, due to experimental errors, but also on principle. Dalton took hydrogen as unity and related all other elements to it, Berzelius calculated either with the 2H unit or else with O = 1 0 0 which he considered more reliable. Others took what later was called equivalent weight for atomic weight, and even in this there was a difference in many cases between the analytical and the electrochemical value. Elemental gases, with their two-atomic molecules were another source of confusion. Hence very many different value types were current, and this chaos began to make the sense of the concept of atomic weight itself questionable. No wonder that the different systems of elements based on such questionable values were not really reassuring. The French Academy of Science declared, regarding one such table, that it was pure figure mysticism.
When Newland reported in the Royal Society that he observed a periodically recurring similarity in properties when elements are being put into the order of their atomic weights, somebody remarked that he should try to list the elements in alphabetic order, maybe he could detect some regularity in that case too.
DISCOVERY AND SEPARATION OF THE RARE EARTHS 71 It is well-known that it was Cannizzaro who finally, by deliberate thinking, cleared up things and defined equivalent weight, atomic weight and molecular weight unequivocally in his small book Sunto di un corso difilosofia chimica (1858) and pro- posed to accept these definitions at the congress of chemists held in Karlsruhe in 1860. Two young scientists also participated at this congress: Lothar Meyer from Bres- lau and Dmitry Mendeleev from St. Petersburgh. At the congress, as usual, there was much talk and debate over Cannizzaro's proposals. Some found them worthy, some did not, some accepted them, some did not. N o n e the less finally the whole chemical world began to think in these categories. Meyer and Mendeleev were immediately under the influence of Cannizzaro's concept, as we know from their personal reminiscences. Mendeleev, for instance, wrote the following: 'I received a copy of the book and read in it on the long railway journey home. At home, I re-read it and was fascinated by the clarity with which the author interpreted the major points discussed. All of a sudden the scales fell from my eyes, my doubts disappeared and security took their place. If some years later I was able to contribute to some extent to order in these matters, it was due not insignificantly to Cannizzaro's book' (Danzer 1974).
Both Meyer and Mendeleev independently of one another started to group elements by their atomic weight which by now was consistently interpreted and by their periodically recurring properties. It is unnecessary for me to write about the table, which is by now taught in secondary school. Obviously the periodic table was not born in the form we know it today, both authors continuously changed and modified it. On this subject very much could be written, but we are here interested solely in the relationship of rare earth elements and the periodic table. Rare earth elements, in this field too, caused considerable confusion and problems.
Lothar Meyer solved the question very simply. He just took no notice of rare earth elements, they are not listed in his element table. He was not fully convinced of their being elements. In Mendeleev's first table which appeared in 1869 and was very different from what we know now as a periodic table, cerium, lanthanum and didymium were listed with the atomic weights of 92, 94 and 95, respectively, corresponding to bivalence at a unit O -- 16, and also erbium and yttrium with atomic weights of 56 and 60, respectively, with interrogation marks. In this table no grouping of elements according to valence appears as yet (Mendeleev 1869). The table which appeared in 1871 is already in the usual form. In this year Mendeleev performed an important change - arbitrarily - with rare earth elements: he assumed that they were trivalent, and correspondingly recalculated their atomic weights, that is, instead of YO and LaO he calculated with
Y203
andLa203.
He then listed yttrium with an atomic weight of 88, lanthanum with 137 and cerium with 138. He supported his assumption with specific heat measurements of cerium (Mendeleev 1871). The assumption proved true, Cleve confirmed it in the following years analytically, Hillebrand (in 1876) with specific-heat determinations for lanthanum and didymium (Hillebrand 1876). It was Hillebrand who in the previous year, using Bunsen's electrolytic method, obtained metallic cerium, lanthanum and didymium in satisfactory purity for the measurements (Hillebrand and Norton 1875). This was in fact the first actual proof for the reality of Mendeleev's periodical system, confirmed later in a striking manner by the discovery of some predicted elements such asFig. 11. Dmitry Ivanovich Mendeleev and Bohuslav Brauner.
gallium, scandium and germanium. F r o m then on the table served as a sort of map where to look for as yet unknown elements. It proved useful in numerous cases, however, for rare earth elements it failed. As their number increased, there was simply no place to put them in the periodic table. They gave headaches to the author of the table and to others. Among these, Brauner was an outstanding figure, a personal close friend to Mendeleev and one of the first zealous backers of his table.
His activity was concentrated above all on atomic wieght determinations, including more exact determinations of rare earth element values only calculated by Mendeleev. We have seen earlier that the anomalies in atomic weight determi- nations led Brauner to the assumption that didymium was not a homogeneous element, confirmed soon afterward by Auer. The results of the atomic weight determinations carried on for decades were: Ce, 140.22; La, 148.92; Pr, 140.9; Nd, 144.3; Sm, 150.4; i.e., the values still valid at present. He took great pains to place them in the periodical table. He attempted to complete Mendeleev's periods with two further periods, then gave up this idea. He also proposed some sort of interperiodical arrangement (Brauner 1902), similarly to Steele before him (Steele 1901). Finally his proposal that all rare earth elements with the exception of yttrium and scandium should be placed into a single square, the square of lanthanum, remained (Brauner 1899). Independently of him Retgers also made the same proposal (Retgers 1895). Brauner believed in the existence of some primeval element, all individual elements being forms differentiated and condensed in different degrees.
D I S C O V E R Y A N D S E P A R A T I O N O F T H E R A R E E A R T H S 73
l i e 4
A 4@
H 1,O08(
0 16 F Ig B I 1 - . 2'/.I
t V ~I,1~ - . W,
¢Cr 611,1 -- N
¢ l h ~,~,0 -- - - Fe M -- I01,~
t~,, G&4 . . ! 1 2 Q [ tGa 70 - , I t 4 h
l g o
1 ~ ¸ ~ !
Fig. 12. T h e P e r i o d i c S y s t e m o f J u l i u s T h o m s e n .
In rare earth elements the extent of differentiation is very small, this is the reason that they are so similar. Mendeleev who stiffly adhered to Dalton's concept did not agree with Brauner's theory, none the less the location of rare earth elements according to Brauner's proposal is still the most widespread solution of the problem, in most periodic tables lanthanum with the atomic n u m b e r 57 is followed by hafnium with the atomic number 72, and the rare earth elements are listed separately somewhere at the b o t t o m of the table.
At the time, of course, no atomic numbers existed as yet, they were introduced by Van den Brock in 1912, assuming that the nuclear charge of the individual elements and hence the n u m b e r of electrons revolving around it is identical with the ordinal number occupied by the given element in the Mendeleev table (Van den Brock 1913). At the time, shortly after the discovery of radioactivity, atomic structure investigations were still at their very beginnings, statements now already based on experimental facts that the atoms are not indivisible, but consist - as so m a n y earlier nature philosophic concepts maintained - of c o m m o n components. It was thus
impossible to know how many rare earth elements are still to be expected into that c o m m o n square. N o n e the less, several scientists attempted to predict from the proportions of atomic weight increases, and fairly well as a matter of fact, how many rare earth elements exist all in all, how many are still unknown. Thomsen, for instance, constructed a very peculiar periodic system in 1895, very similar in its arrangement to the later periodical system based on Bohr's electron configuration table. He assigned 16 Places between lanthanum and tantalum to rare earth elements. Ten were known at the time (not taking into account yttrium and scandium, which held their proper places from the start and caused no problem).
Europium, dysprosium, holmium and lutetium were as yet unknown, at least to Thomsen. This leaves two unknown rare earth elements at the time, a surprisingly exact prediction! (Thomsen 1895).
Moseley, a researcher who died at an early age in World War I at the siege of the Dardanelles, discovered, in 1913, a mathematically expressible relationship between the frequency of X-rays emitted by the element serving as anticathode and its atomic number. This method yielded the possibility to determine atomic numbers of chemical elements experimentally. One could in this manner unequivocally state whether the substance in question is truly an element and if so, one could find its place in the periodical table. The first to make use of this possibility was Urbain: in 1914 he submitted all rare earth elements discovered in the latter times to the Moseley check. The tests confirmed that they were true elements. Thus, the range of rare earth elements would reach from lanthanum with the atomic number 57 to the atomic number 72, that is, actually 16 places. Among them the element with the number 61 was as yet unknown and there was no unequivocal opinion regarding the element with the number 72. All other elements fitted well into the system.
We have mentioned at the end of the previous chapter that Urbain announced the discovery of a further rare earth element in 1911, he called it celtium (Urbain 1911).
This was the only one among the samples whose elemental nature was not confirmed by Moseley. It is difficult to convince a scientist of his error. Urbain did not believe Moseley in the celtium case, he considered it the last rare earth element with the atomic number 72. Further elucidation of the question was post poned by the War, and Moseley fell in its early stage. After the war, however, Urbain and Dauviilier studied the would-be celtium by X-ray spectrometry until they found some very pale lines that might putatively be considered proof of the existence of celtium and of its atomic number 72 (Urbain and Dauvillier 1922). However, the whose assumption proved to be incorrect, though Urbain still continued to defend c e r i u m for a long time.
In the meantime, Bohr developed his electron shell theory applying the quantum theory. Bohr thereby interpreted the Mendeleev table theoretically: new periods in Mendeleev's system begin at the elements where the filling-up of a new electron shell begins and last until that electron shell is completed, explaining the periodicity of chemical properties, since chemical properties depend above all on the actual external electron shell.
Rare earth elements were a problem for Bohr too, which he could only solve by them making an exception. After lanthanum the filling of the external shell stops and
DISCOVERY AND SEPARATION OF THE RARE EARTHS 75
Fig. 13. George de Hevesy.
continues on a shell closer to the interior by two. This will then also explain the great chemical similarity of rare earth elements, their external electron shell being identical. The reason for this anomaly is explained by various quantum chemical and energetical theories, their discussion, however, is not the business of the science historian, on the one hand, because he is no specialist in quantum chemistry, and, on the other hand, because he does not fully believe in them. In nature order prevails, so why this apparent anomaly? Maybe rare earth elements will one day shake our present knowledge and conceptions. N o n e the less, rare earth elements were direct proof of the correctness of Bohr's theory, since according to his electron configuration table, filling up of the N shell is complete at lutetium, atomic number 71, and atomic number 72 corresponds to the continuation of filling up the electron shell interrupted after lanthanum. Consequently the element 72 will not have the properties of rare earth elements, but rather those of the elements in the next column, one should therefore look for it in zirconium ores. At the time George de Hevesy - who made friends earlier with Bohr in Rutherford's institute in Manchester - worked in the Bohr institute. Before World War I, de Hevesy also worked with Moseley. Together with Dirk Coster, also working in Bohr's institute, they began to investigate Norwegian zirconium ores and detected a new element in it, first only by X-ray spectrometry in 1922 (Coster and de Hevesy 1923), which they named hafnium, from the Latin name of Copenhagen. Later de Hevesy succeeded to separate it chemically from zirconium based on the differing solubility of the fluorides and to obtain the pure element via reducing the fluoride by metallic sodium (de Hevesy 1925). Hafnium turned out to be unequivocally the element 72 and had no properties similar to rare earth elements. Thus Bohr was fully confirmed.