User:Double sharp/Rare earths history

(10.1016/B978-0-444-53590-0.00001-7)

This seems a good thing to summarise for the article: not only is it describing an important thing in the history of the PT, it links up as a 2ARY review of a huge swath of 1ARY sources how the initial difficulties DIM had with the rare earths are still being felt today.

Rare earths were problematic for DIM; he kept them separate in his 1869 system, but it broke the sequence of increasing atomic weights
By the next year he realised that he had wrong atomic weights due to wrong valencies, but then he ran into a problem: normally similar elements in a family had differing atomic weights (like F-Cl-Br-I), but here like for transition elements in group VIII he had very similar atomic weights
So in 1870 he thought rare earths and transitions were a different kind of element altogether!
But later he thought the periodic law should be absolute and without exceptions, so he corrected the valencies and tried to slot them into the rest of the PT with other elements: La in group III, Ce in group IV, Di in group V (he had no proof of this valence but assumed it would be found), giving up analogy to transition elements
He then tried studying the rare earths himself; by the next year he gave up without results
Czech chemist Brauner continued his work, trying to accommodate them even after Di was split, putting Pr in group V, Nd in group VI, but all attempts to show these valences by experiment failed.
So by 1902 he gave up and advanced the "asteroidal" hypothesis: all the rare earths belonged in group IV like Ce, just like how between Mars and Jupiter there is not a planet but an asteroid belt. Many others came up with something similar around this time, but the chemistry world was split between "all in group III", "all in group IV", or "spanning group III and IV"!
Also it was not clear where the series ended (was the element before Ta a rare earth or not?) and how many rare earths there were until Moseley (1914) and Bohr (1913-22)
But Bohr quantum theory rather favoured the older idea that the rare earths were indeed a different kind of element in the sense that they didn't fit in any group. So some long 32 tables came around. Mostly were ignored in textbooks though which either continued distributing rare earths among the groups like Mendeleev or giving "asteroidal" tables
Reason was that although Bohr favoured the idea that the actinoids are a second rare earth series, most chemists were unconvinced because valencies, and there was some confusion about where exactly 6d or 5f begins filling, or even if it actually matters (vide Seaborg and Jorgensen)
Matter was only settled when Seaborg made elements past 92, and they clearly were not transition metal homologues
So after WWII it became more popular to cut the rare earths out of all groups. Jensen well describes why it went to Ce-Lu / Th-Lr mostly
But only a decade later questions started arising because electron configuration of Yb was corrected, so Landau & Lifshitz already called for Lu-Lr (at least, this is how Hamilton and other physicists later interpreted it)
So we now had a situation where most people agreed on an accommodation where the rare earths were not in any group - but there was no agreement on Ce-Lu / Th-Lr vs La-Yb / Ac-No (most physicists who investigated the problem favoured the latter, but textbooks mostly did not listen as before)
Mostly chemists did not pay attention to the physicists, but with Jensen 1982 they did sit up and take notice - so IUPAC 1988 reports admitted that electron configurations demanded Sc-Y-Lu-Lr (citing Jensen and the physicists), and that La-Yb / Ac-No should be the ones taken out if one transitioned from 32 to 18 column. But because Sc-Y-La-Ac was common, IUPAC decided to show a compromise Sc-Y-*-**
They didn't do it consistently either, because in the next Red Book of 1990, 18 column was shown as Sc-Y-*-** but 32 column was Sc-Y-Lu-Lr
This Sc-Y-*-** goes back to the old asteroidal pre-quantum idea, and the debate continued anyway through the 2000s and 2010s
The evidence for Sc-Y-Lu given by physicists and chemists from 1960s onwards is mostly accepted by those researching specifically as specialised sources on the PT, and by the IUPAC report has made significant inroads into textbooks. Basically: it is the form that most chemists who think about the matter agree on. Sources on accommodation and historical development on the PT mostly support this arrangement too (Scerri, The Periodic Table, Its Story and Its Significance; Scerri, The Periodic Table: A Very Short Introduction; Thyssen and Binnemans, Accommodation of Rare Earths Into the Periodic Table)
But still there is disagreement, whence the 2015 starting of the IUPAC project

To summarise for the section (obviously, not all bullet points will go in, otherwise WP:SLICEDBREAD, LOL)

clotty prose, just coughing out from the head first. ALl cites Thyssen, we know where the rest is

The rare earths have always caused difficulty for the periodic table. In Dmitri Mendeleev's arrangement of 1869, they were cast to one side, but this interrupted the sequence of increasing atomic weights. After doing some work on the problem in the next year, he realised that he had the wrong atomic weights (because the rare earths were wrongly thought to have a valence of +2, when it was actually +3). Based on these new atomic weights, his 1871 table then attempted to slot the rare earth elements into the system as he would any other: on the grounds of maximum oxidation states, lanthanum was placed in group III, cerium in group IV, didymium in group V. But he could not get further, and neither he nor the Czech chemist Bohuslav Brauner (who continued Mendeleev's work on the rare earths even after Mendeleev had given up) could prove pentavalence for didymium, or for its constituents even after it was later split into praseodymium and neodymium. Therefore, Brauner in 1902, like many others around the same time, decided that the rare earths should be better accommodated according to an "asteroidal hypothesis": just like between Mars and Jupiter there is not one planet but an asteroid belt, so there should be many elements fitting in one space on the periodic table. There was however no agreement on whether the rare earths all belonged in group IV, in group III, or in a mix of all of them.

The question of how many rare earths, and their limits, was itself also not settled until the work of Moseley and Bohr. Bohr's quantum theory resolved these questions but also justified the idea that the rare earths in fact are not well accommodated in the existing groups (like Mendeleev) or "asteroidally" (like Brauner), but in fact form a new group characterised by the filling of f orbitals. Similarly, Bohr expected that the known actinides were actually the beginning of another f series. Textbooks, however, were slow to update to this new view because these elements behaved more like transition metals; only after Seaborg synthesised later actinides was this accepted.

Therefore, with the rise of electronic periodic tables, a classification of the rare earths as coming between groups III and IV was generally accepted. Early spectroscopic studies assigned the configuration of barium as [Xe]6s², and that of lanthanum as [Xe]5d¹6s². The latter seemed to open the d block, and appeared analogous to scandium [Ar]3d¹4s² and yttrium [Kr]4d¹5s². Most rare earths were then assigned the configuration [Xe]4fⁿ5d¹6s², including ytterbium [Xe]4f¹³5d¹6s² and lutetium [Xe]4f¹⁴5d¹6s². Lutetium then appeared to have a 4f differentiating electron and therefore as the ending of the f block; and thus group 3 became generally classified in textbooks as scandium, yttrium, lanthanum, and actinium.

However, only a few years later (CHECK DATE FROM SCERRI), the rare earths' electron configurations were reassigned by new studies. Ytterbium was found to be [Xe]4f¹⁴6s², and thus lutetium was differentiated from ytterbium by a 5d and not a 4f electron. Most of the rare earths and actinides were found to lack the extra d electron hanging up in the old configuration (only La, Ce, Gd, Lu; Ac, Pa, U, Np, Cm have it; Th exceptionally has two d electrons hanging up; the rest have none, with Lr as a unique anomaly). This made lutetium an equally plausible candidate to lanthanum as the element that begins the third row of the d block. Similarly to all other d block groups, going from yttrium (in period 5) to lutetium (in period 6) adds a filled 4f shell to the core; this does not occur passing from yttrium to lanthanum. This completion of the f block at ytterbium was interpreted by some physicists as evidence that group 3 should actually run scandium, yttrium, lutetium, and lawrencium.

Significant evidence accumulated over the next decades for this revised group 3 with lutetium, mostly at first from physicists (though not exclusively)[CHISTYAKOV]: the evidence involved physical and chemical properties of the metals concerned, as well as consistency of trends in the periodic table. The issue was mostly drawn to chemists' attention by an article by William B. Jensen writing in 1982. In a report of 1988, IUPAC admitted that electron configurations supported group 3 with lutetium, but because the lanthanum option remained the most common in textbooks and posters, it ended up showing a compromise which it still does today. This compromise shows all lanthanides are actinides under yttrium in group 3, as a throwback to Brauner's "asteroidal hypothesis".

Despite the existence of this compromise, the debate remains present in textbooks and journals today. Most textbooks continue to show the form with lanthanum under yttrium, but in recent decades more and more have been moving to either the compromise or the form with lutetium under yttrium. The lutetium form tends to be supported by the majority of periodic table specialists and researchers, but not unanimously: some consider chemical trends to support lanthanum under yttrium (cite Restrepo and Vernon), whereas others differ on how the electron configurations should be interpreted. In 2015, renewed attention was drawn to the subject by investigation of the highly unstable lawrencium, which science news articles linked to the dispute: those with views on the group 3 question were split on which side it supported. This spurred IUPAC to begin a project that year aimed at resolving the matter, as the matter was felt to be of considerable importance to physicists, chemists, and students and instructors who are often puzzled by the inconsistency between different periodic tables on this point. As yet, no results are publicly available.

I THINK THIS IS STILL TOO LONG BUT AT LEAST IT IS SOMETHING