User:Double sharp/Group 3 section

Dispute on composition
Published periodic tables show variation regarding the heavier members of group 3, which begins with scandium and yttrium. They are most commonly lanthanum and actinium (group 3 as Sc-Y-La-Ac), but many physicists and chemists have argued that they should be lutetium and lawrencium (Sc-Y-Lu-Lr). The spaces below yttrium are sometimes left blank (Sc-Y-*-**): this is found in the so-called "IUPAC periodic table" that appears on the IUPAC website, though IUPAC has clarified that it "has not approved any specific form of the periodic table, and an IUPAC-approved form does not exist". Chemical, physical, and electronic concerns have been used to discuss the problem, but the chemist and philosopher of science Eric Scerri considers such arguments inconclusive, pointing to cases where different properties favour different compositions of group 3.

Early investigators measuring the electron configurations of the rare earth elements concluded that they mostly had the configuration fx−1d1s2 with few exceptions. This suggested that lanthanum ([Xe]5d16s2) was the first 5d element and so that group 3 should be Sc-Y-La-Ac; fourteen f-block elements then follow, with lutetium as the last 4f element completing the 4f subshell. This placement was generally adopted in the 1940s as electron configurations became the basis for placing elements on the periodic table, and has been supported more recently by American chemist Laurence Lavelle on the grounds that lanthanum and actinium ([Rn]6d17s2) indeed both lack f-electrons and therefore should not be considered as f-block elements.

However, later measurements proved that in fact most of the lanthanides and the actinides have the configuration fxs2, with only a few (La, Ce, Gd, Lu; Ac, Pa, U, Np, Cm) having the old configuration. In particular, the 4f subshell actually finishes filling at ytterbium ([Xe]4f146s2) and so lutetium ([Xe]4f145d16s2) appears as an equally valid candidate to lanthanum ([Xe]5d16s2) for being the first 5d element. According to American chemist and chemical historian William B. Jensen, this supports treating lanthanum and actinium ([Rn]6d17s2) as exceptions to the Madelung rule, where a d-electron replaces the idealised f-electron, exactly like what happens for thorium ([Rn]6d27s2) that is unanimously regarded as an f-block element despite also lacking an f-electron. Consequently, after the corrected configurations became known, many authors began to advocate that lutetium and lawrencium should replace lanthanum and actinium in group 3. Most authors writing on the group 3 problem support the Sc-Y-Lu-Lr form, but many textbooks continued to show Sc-Y-La-Ac.

The Sc-Y-*-** form is a compromise between Sc-Y-La-Ac and Sc-Y-Lu-Lr formats, and a reversion to the 1902 "asteroid hypothesis" of Czech chemist Bohuslav Brauner, where all the rare earths would take one place in the periodic table just as between Mars and Jupiter there is an asteroid belt rather than a planet. While it clarifies the similarities between the lanthanides, it mixes the d- and f-blocks together, creates a 15-element-wide f-block when an f-subshell can only contain 14 electrons, and leaves it ambiguous if the group only contains scandium and yttrium, or if it also extends to include all thirty lanthanides and actinides. Jensen considers this form to be "chemical nonsense" based on an "antiquated interpretation" of the lanthanides and actinides, noting that many of them can use their f-electrons as valence electrons and exhibit maximal oxidation states greater than +3, contradicting their assignment to group 3.

In 2015, IUPAC began a project to decide if group 3 should be Sc-Y-La-Ac or Sc-Y-Lu-Lr: it was chaired by Scerri and included (among others) Jensen and Lavelle. It considered the question to be "of considerable importance" for chemists, physicists, and students, noting that the variation in published periodic tables on this point typically puzzled students and instructors. The IUPAC project released a provisional report in 2021. It concluded that the group 3 dispute cannot be decided by any objective means and that that made it more important for IUPAC to decide it as a matter of convention. According to the report, assigning electron configurations to atoms, and similarly assigning elements to blocks, represents an approximation: it makes the point that thorium is universally classified as an f-block element even though it lacks f-electrons. The report points out that if lanthanum and actinium are included in group 3, then the d-block must be split "into two highly uneven portions"; whereas if lutetium and lawrencium are included in group 3, no such split is required. It also points out that if the spaces below yttrium are left blank, then 15 elements occur in the f-block rows, even though by quantum mechanics an f-subshell can accommodate at most 14 electrons. Therefore, the report recommended considering scandium, yttrium, lutetium, and lawrencium as the group 3 elements. The reasons given were to display all elements in order of increasing atomic number, avoid the d-block split, and to have the blocks follow the widths quantum mechanics demands of them (2, 6, 10, and 14): no other possible version achieves all three. The report noted that some practitioners of a specialised branch of relativistic quantum chemistry concerned with the properties of superheavy elements uphold grouping together 15 rather than 14 f-block elements, but it considered that to be "interest-dependence" and stated that such findings "should not be imposed on the majority of users of the periodic table".