Block-splitting: s-block vs d-block

[Rene]

Colleagues

Eric’s The Periodic Table: Its Story and Significance (2020) includes a 32-column table in which the d-block is split into one and nine columns (p. 399, figure 13.14). I’ve attached an example of such a table.

The accompanying text suggests this appears to be the least likely arrangement, since such uneven block-splitting “does not occur in any other block" (p. 401). The s-block is acknowledged as an exception, since its first two members (H and He) are generally separated, though this is described as an even 50%–50% split. For convenience, I’ve appended the relevant text.

I wonder if this reasoning might deserve a second look. Viewed as whole, the s-block division is actually 1 vs 13 — numerically even more unbalanced than the d-block’s 1 vs 9. This suggests that asymmetry alone cannot be the deciding factor. Perhaps the more relevant question is whether a given split has sufficient chemical, historical, or pragmatic justification.

Certainly the composition of group 3 as Sc-Y-La-Ac, which gives rise to a split d-block in the rarely seen 32-column form, has a long history, and its pragmatic justification is grounded in the delayed appearance of the first f-electron. Chemically, Jensen gave Sc-Y-Lu-Lr a red hot go in his 1982 JChemEd article, but Eric later referred to this work as being too selective in terms of the evidence put forward (Scerri & Parsons 2018, p. 143). In this case group 3 as Sc-Y-Lu-Lr avoids a split d-block in the 32 column form.

Scerri ER & Parsons W 2018, What elements belong in Group 3 of the periodic table?, in Scerri E & Restrepo G (eds), Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table, pp. 140–151, Oxford University Press, New York

One might say that, pragmatically, fewer splits are preferable—yet the perceived importance of the delayed appearance of the first f-electron seems to outweigh neatness.

In a fashion, He over Ne, and La under Y offset one another: He over Ne ignores electron configuration and results in a split block while La under Y respects electron configuration and likewise results in a split block.

I raise this not as a definitive answer, but as an invitation to discussion.

thank you, René

Extract from Eric’s The Periodic Table: Its Story and Significance (2020), p. 401:

"Although some books include a periodic table arranged like figure 13.14 it does not seem to be a popular design and for rather obvious reasons. Presenting the periodic table in this fashion requires that the d-block of the periodic table be split up into two highly unequal portions containing one block that is only one element wide and another block that is nine elements wide. Given that this behavior does not occur in any other block in the periodic table, it would appear to be the least likely of the three possible tables to reflect the actual arrangement of the elements in nature. As suggested earlier though, we should beware of arguments based on symmetry and regularity. In addition, there is already one block, or at least the first members of that block, that are generally separated, namely, hydrogen and helium in the s-block. Nevertheless, this splitting of two members of the s-block occurs in an even or 50%-50% fashion, whereas the splitting of the d-block as seen in figure 13.14 is clearly very uneven."

[johnmarks9]

René,

Neither s- nor d- is split in the following PT:

It is the arrangement used by Ramsay in 1915 and resurrected by Scerri in 2006.

Regards,

John

[ERIC SCERRI]

On Sep 22, 2025, at 7:12 AM, johnmarks9 <johnm...@hotmail.com> wrote:

René,

Neither s- nor d- is split in the following PT:

<RS correspondence.png>

It is the arrangement used by Ramsay in 1915 and resurrected by Scerri in 2006.

Regards,

John

On Monday, September 22, 2025 at 2:16:04 PM UTC+2 Rene wrote:

Colleagues

Eric’s The Periodic Table: Its Story and Significance (2020) includes a 32-column table in which the d-block is split into one and nine columns (p. 399, figure 13.14). I’ve attached an example of such a table.

The accompanying text suggests this appears to be the least likely arrangement, since such uneven block-splitting “does not occur in any other block" (p. 401). The s-block is acknowledged as an exception, since its first two members (H and He) are generally separated, though this is described as an even 50%–50% split. For convenience, I’ve appended the relevant text.

I wonder if this reasoning might deserve a second look. Viewed as whole, the s-block division is actually 1 vs 13 — numerically even more unbalanced than the d-block’s 1 vs 9. This suggests that asymmetry alone cannot be the deciding factor. Perhaps the more relevant question is whether a given split has sufficient chemical, historical, or pragmatic justification.

Thank you for raising these issues Rene.

Can you explain what you mean when you write that the s-block is actually split 1- 13 ?

Surely not the fact that elements in group 13 have outer s orbital electrons in addition to full d orbitals?  Do you really believe that taking this peciliar feature of electronic configurations is so important?  

If so, then why not claim that some elements in the f-block are also really part of the s-block?  But I havnt thought this through fully yet.

I won’t comment on your separate point regarding group 3, only to say that, in my view, you are again attaching too much importance to the details of electronic configurations.

This is a little surprising from somebody who constantly champions the macroscopic chemical properties of elements.

It also ignores the long standing discussion regarding the dual sense of the term “element”.  See the collection of articles that I co-edited with Elena Ghidaudi, “What is a chemical element?”, OUP, 2020 and several journal articles on this issue.

regards

Eric

P.S.  Thanks for sending me your canasta package John Marks.  I just received a message to say that it has cleared customs.

Certainly the composition of group 3 as Sc-Y-La-Ac, which gives rise to a split d-block in the rarely seen 32-column form, has a long history, and its pragmatic justification is grounded in the delayed appearance of the first f-electron. Chemically, Jensen gave Sc-Y-Lu-Lr a red hot go in his 1982 JChemEd article, but Eric later referred to this work as being too selective in terms of the evidence put forward (Scerri & Parsons 2018, p. 143). In this case group 3 as Sc-Y-Lu-Lr avoids a split d-block in the 32 column form.

Scerri ER & Parsons W 2018, What elements belong in Group 3 of the periodic table?, in Scerri E & Restrepo G (eds), Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table, pp. 140–151, Oxford University Press, New York

One might say that, pragmatically, fewer splits are preferable—yet the perceived importance of the delayed appearance of the first f-electron seems to outweigh neatness.

In a fashion, He over Ne, and La under Y offset one another: He over Ne ignores electron configuration and results in a split block while La under Y respects electron configuration and likewise results in a split block.

I raise this not as a definitive answer, but as an invitation to discussion.

thank you, René

Extract from Eric’s The Periodic Table: Its Story and Significance (2020), p. 401:

"Although some books include a periodic table arranged like figure 13.14 it does not seem to be a popular design and for rather obvious reasons. Presenting the periodic table in this fashion requires that the d-block of the periodic table be split up into two highly unequal portions containing one block that is only one element wide and another block that is nine elements wide. Given that this behavior does not occur in any other block in the periodic table, it would appear to be the least likely of the three possible tables to reflect the actual arrangement of the elements in nature. As suggested earlier though, we should beware of arguments based on symmetry and regularity. In addition, there is already one block, or at least the first members of that block, that are generally separated, namely, hydrogen and helium in the s-block. Nevertheless, this splitting of two members of the s-block occurs in an even or 50%-50% fashion, whereas the splitting of the d-block as seen in figure 13.14 is clearly very uneven."

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[Rene]

On 23 Sep 2025, at 00:12, johnmarks9 <johnm...@hotmail.com> wrote:

René,

Neither s- nor d- is split in the following PT:

It is the arrangement used by Ramsay in 1915 and resurrected by Scerri in 2006.

Regards,

John

Thanks for your contribution John.

For the s-block, there’s a clear split: H–He are isolated at upper left, with Li–Ra off to the right (about six intervening columns in this scheme).

For the d-block, it’s fragmented into separated slices: e.g., the Mn–Ni, Tc–Pd, and Re–Pt quartets sit on the left, while the remaining d-elements appear elsewhere—so the block is visibly split.

For the p-block, it’s bifurcated: the halogens and noble gases run down the far left, whereas the rest of the p-block sits on the far right. Each of these two p-sectors is further interrupted by inserted d and f series (e.g., three 4-element d segments and two f rows alongside the left p-column; additional d/f insertions next to the right p-columns), effectively breaking the p-block into multiple sub-parts.

In this Ramsay–Sommerfeld layout, the s-, d-, and p-blocks are thus all split—some of them more than once—so block integrity isn't a guiding constraint here.

Eric’s 2006 periodic table https://www.meta-synthesis.com/webbook/35_pt/pt_database.php?PT_id=20 has a split p-block.

The popularity of the conventional periodic table suggests that the presence of a split block is not, in itself, considered problematic. Conversely, the absence of split blocks in the left-step table doesn't appear to have helped its wider acceptance.

René

[johnmarks9]

I think we´re at cross-purposes, René, with differing understandings of "split". 

If you want the blocks preserved, that can only be at the cost of representing them separately and destroying the table format:

The split of the d-block in this PT 

is inevitable if the f-block is to be included in the body of the table. The 1s block is directly followed by the 2s block which then continues vertically. 

Unless you use something like Newlands Revisited:

where they are all vertical. But that also inevitably splits all blocks (in your sense) by the hiving off of the H-He "echo" - which is F-Ne, etc., in the p-bloc, Cu-Zn, etc., in the d-block and Tu-Yb, etc., in the f-block.

The complication of splitting arises from the aufspaltung occurring after the 3s:

So your "splitting" can´t really be avoided if you want to retain a periodic table format.

Regards,

John

[Rene]

On 23 Sep 2025, at 04:00, ERIC SCERRI <sce...@g.ucla.edu> wrote:

Thank you for raising these issues Rene.

Thanks for your generous and thoughtful reply Eric. I appreciate the spirit in which you engaged with my note. As I recall, the subject of block-splitting is not one that list members have discussed much.

Can you explain what you mean when you write that the s-block is actually split 1- 13 ?

Surely not the fact that elements in group 13 have outer s orbital electrons in addition to full d orbitals?  Do you really believe that taking this peciliar feature of electronic configurations is so important?  

If so, then why not claim that some elements in the f-block are also really part of the s-block?  But I havnt thought this through fully yet.

I wasn’t referring to group 13 or to any electronic configuration detail. I meant simply that in the conventional table helium is separated from the other 13 s-elements (Li–Fr and Be–Ra). That yields a numerical division of one element versus thirteen. My point was not that this should carry great theoretical weight in itself, only that if such an uneven split is tolerated in the s-block, then asymmetry alone seems insufficient as a reason to dismiss the d-block split.

I won’t comment on your separate point regarding group 3, only to say that, in my view, you are again attaching too much importance to the details of electronic configurations.

Well, the importance given to the details of electronic configurations is a general theme in the literature, including the delayed appearance of the first f-electron. This importance was such that B-Al, originally placed over Sc, came to be progressively repositioned from the 1930’s onwards over Ga—even though the periodic trends going down B-Al-Sc-Y-La are smoother. It was a case of electron configuration consistency winning out over the periodic law. Conversely, configuration details were pragmatically overruled in the case of He over Ne.

Your comment prompted me to reconsider the start of the f-block, a topic which I intend to elaborate in a separate post.

This is a little surprising from somebody who constantly champions the macroscopic chemical properties of elements.

Yes, I've highlighted such properties on an ad hoc basis so as to shed extra light on the placement of some problem elements.

It also ignores the long standing discussion regarding the dual sense of the term “element”.  See the collection of articles that I co-edited with Elena Ghidaudi, “What is a chemical element?”, OUP, 2020 and several journal articles on this issue.

I acknowledge the long-standing discussion regarding the dual sense of the term “element”.

That said, since the periodic table is fundamentally a manifestation of the periodic law, I suggest that the dual sense becomes less central in this context. As Mendeleev (1899; 1901, in Jensen 2005, p. 200) noted, it was atomic weight that served as the staging ground for the discovery of the law, and a law expresses a relationship between variables—in this case, atomic weight as the independent variable and chemical/physical properties as the dependent one. Here the properties in view are those of the most stable form of the element under ambient conditions.

As Jensen (1986, p. 498) put it:

“What all of this means is simply that the periodic table is a true natural classification based on the simultaneous consideration of as many property–atomic number maps as possible. Since none of these maps exhibits perfect periodicity, the result is the best ‘averaged’ representation and one which is consequently imperfect with regard to any single property considered in isolation, be it maximum oxidation state or electronic configuration. In short, the table should combine the best features of both the traditional chemical table and the more recent electronic configuration tables.”

I submit that the “best features” Jensen refers to are exemplified in the conventional placements of H over Li, He over Ne, and B-Al over Ga.

regards

Eric

sincerely, René

• Jensen WB 1986, Classification, symmetry and the periodic table, Comput. Math. with Appl. 12B, 12, 487–510 (508)
• —— (ed.) 2005, Mendeleev on the Periodic Law: Selected writings, 1869–1905, Dover Publications, Mineola, New York

On 22 Sep 2025, at 22:15, Rene <re...@iinet.net.au> wrote:

Colleagues

Eric’s The Periodic Table: Its Story and Significance (2020) includes a 32-column table in which the d-block is split into one and nine columns (p. 399, figure 13.14). I’ve attached an example of such a table.

The accompanying text suggests this appears to be the least likely arrangement, since such uneven block-splitting “does not occur in any other block" (p. 401). The s-block is acknowledged as an exception, since its first two members (H and He) are generally separated, though this is described as an even 50%–50% split. For convenience, I’ve appended the relevant text.

I wonder if this reasoning might deserve a second look. Viewed as whole, the s-block division is actually 1 vs 13 — numerically even more unbalanced than the d-block’s 1 vs 9. This suggests that asymmetry alone cannot be the deciding factor. Perhaps the more relevant question is whether a given split has sufficient chemical, historical, or pragmatic justification.

Certainly the composition of group 3 as Sc-Y-La-Ac, which gives rise to a split d-block in the rarely seen 32-column form, has a long history, and its pragmatic justification is grounded in the delayed appearance of the first f-electron. Chemically, Jensen gave Sc-Y-Lu-Lr a red hot go in his 1982 JChemEd article, but Eric later referred to this work as being too selective in terms of the evidence put forward (Scerri & Parsons 2018, p. 143). In this case group 3 as Sc-Y-Lu-Lr avoids a split d-block in the 32 column form.

Scerri ER & Parsons W 2018, What elements belong in Group 3 of the periodic table?, in Scerri E & Restrepo G (eds), Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table, pp. 140–151, Oxford University Press, New York

One might say that, pragmatically, fewer splits are preferable—yet the perceived importance of the delayed appearance of the first f-electron seems to outweigh neatness.

In a fashion, He over Ne, and La under Y offset one another: He over Ne ignores electron configuration and results in a split block while La under Y respects electron configuration and likewise results in a split block.

I raise this not as a definitive answer, but as an invitation to discussion.

thank you, René

Extract from Eric’s The Periodic Table: Its Story and Significance (2020), p. 401:

"Although some books include a periodic table arranged like figure 13.14 it does not seem to be a popular design and for rather obvious reasons. Presenting the periodic table in this fashion requires that the d-block of the periodic table be split up into two highly unequal portions containing one block that is only one element wide and another block that is nine elements wide. Given that this behavior does not occur in any other block in the periodic table, it would appear to be the least likely of the three possible tables to reflect the actual arrangement of the elements in nature. As suggested earlier though, we should beware of arguments based on symmetry and regularity. In addition, there is already one block, or at least the first members of that block, that are generally separated, namely, hydrogen and helium in the s-block. Nevertheless, this splitting of two members of the s-block occurs in an even or 50%-50% fashion, whereas the splitting of the d-block as seen in figure 13.14 is clearly very uneven."

<PastedGraphic-7.png>

[ERIC SCERRI]

On Sep 25, 2025, at 6:04 PM, Rene <re...@iinet.net.au> wrote:

On 23 Sep 2025, at 04:00, ERIC SCERRI <sce...@g.ucla.edu> wrote:

Thank you for raising these issues Rene.

Thanks for your generous and thoughtful reply Eric. I appreciate the spirit in which you engaged with my note. As I recall, the subject of block-splitting is not one that list members have discussed much.

Can you explain what you mean when you write that the s-block is actually split 1- 13 ?

Surely not the fact that elements in group 13 have outer s orbital electrons in addition to full d orbitals?  Do you really believe that taking this peciliar feature of electronic configurations is so important?  

If so, then why not claim that some elements in the f-block are also really part of the s-block?  But I havnt thought this through fully yet.

I wasn’t referring to group 13 or to any electronic configuration detail. I meant simply that in the conventional table helium is separated from the other 13 s-elements (Li–Fr and Be–Ra). That yields a numerical division of one element versus thirteen. My point was not that this should carry great theoretical weight in itself, only that if such an uneven split is tolerated in the s-block, then asymmetry alone seems insufficient as a reason to dismiss the d-block split.

Thanks for the clarification.  The two kinds of split you are discussing are not comparable.  In the case you discuss one single element is traditionally removed from the s-block because of its chemical properties as you have so often argued for in rejecting the left-step table over the years.

The d-block split really is a split or separation whereby Sc, Y, La and Ac are separated right down the middle as it were from a set of other d-block elements (Ti, Zr, Hf, Rf).

The two cases are entirely different.

To be comparable we would need to be speaking of separating H, Li, Na, K etc from Be, Mg, Ca, etc.

ERIC SCERRI

I won’t comment on your separate point regarding group 3, only to say that, in my view, you are again attaching too much importance to the details of electronic configurations.

Well, the importance given to the details of electronic configurations is a general theme in the literature, including the delayed appearance of the first f-electron. This importance was such that B-Al, originally placed over Sc, came to be progressively repositioned from the 1930’s onwards over Ga—even though the periodic trends going down B-Al-Sc-Y-La are smoother. It was a case of electron configuration consistency winning out over the periodic law. Conversely, configuration details were pragmatically overruled in the case of He over Ne.

Please cite any article detailing the history of this development.  Was it really a case of putting electronic structure over chemical behavior as you suggest?

Your comment prompted me to reconsider the start of the f-block, a topic which I intend to elaborate in a separate post.

This is a little surprising from somebody who constantly champions the macroscopic chemical properties of elements.

Yes, I've highlighted such properties on an ad hoc basis so as to shed extra light on the placement of some problem elements.

It also ignores the long standing discussion regarding the dual sense of the term “element”.  See the collection of articles that I co-edited with Elena Ghidaudi, “What is a chemical element?”, OUP, 2020 and several journal articles on this issue.

I acknowledge the long-standing discussion regarding the dual sense of the term “element”.

That said, since the periodic table is fundamentally a manifestation of the periodic law, I suggest that the dual sense becomes less central in this context.

What do you understand by the dual nature of elements Rene? 

I’m afraid I dont see how what follows below has any bearing on this distinction.

As Mendeleev (1899; 1901, in Jensen 2005, p. 200) noted, it was atomic weight that served as the staging ground for the discovery of the law, and a law expresses a relationship between variables—in this case, atomic weight as the independent variable and chemical/physical properties as the dependent one. Here the properties in view are those of the most stable form of the element under ambient conditions.

As Jensen (1986, p. 498) put it:

“What all of this means is simply that the periodic table is a true natural classification based on the simultaneous consideration of as many property–atomic number maps as possible. Since none of these maps exhibits perfect periodicity, the result is the best ‘averaged’ representation and one which is consequently imperfect with regard to any single property considered in isolation, be it maximum oxidation state or electronic configuration. In short, the table should combine the best features of both the traditional chemical table and the more recent electronic configuration tables.”

I submit that the “best features” Jensen refers to are exemplified in the conventional placements of H over Li, He over Ne, and B-Al over Ga.

regards

Eric

sincerely, René

• Jensen WB 1986, Classification, symmetry and the periodic table, Comput. Math. with Appl. 12B, 12, 487–510 (508)
• —— (ed.) 2005, Mendeleev on the Periodic Law: Selected writings, 1869–1905, Dover Publications, Mineola, New York

On 22 Sep 2025, at 22:15, Rene <re...@iinet.net.au> wrote:

Colleagues

Eric’s The Periodic Table: Its Story and Significance (2020) includes a 32-column table in which the d-block is split into one and nine columns (p. 399, figure 13.14). I’ve attached an example of such a table.

The accompanying text suggests this appears to be the least likely arrangement, since such uneven block-splitting “does not occur in any other block" (p. 401). The s-block is acknowledged as an exception, since its first two members (H and He) are generally separated, though this is described as an even 50%–50% split. For convenience, I’ve appended the relevant text.

I wonder if this reasoning might deserve a second look. Viewed as whole, the s-block division is actually 1 vs 13 — numerically even more unbalanced than the d-block’s 1 vs 9. This suggests that asymmetry alone cannot be the deciding factor. Perhaps the more relevant question is whether a given split has sufficient chemical, historical, or pragmatic justification.

Certainly the composition of group 3 as Sc-Y-La-Ac, which gives rise to a split d-block in the rarely seen 32-column form, has a long history, and its pragmatic justification is grounded in the delayed appearance of the first f-electron. Chemically, Jensen gave Sc-Y-Lu-Lr a red hot go in his 1982 JChemEd article, but Eric later referred to this work as being too selective in terms of the evidence put forward (Scerri & Parsons 2018, p. 143). In this case group 3 as Sc-Y-Lu-Lr avoids a split d-block in the 32 column form.

Scerri ER & Parsons W 2018, What elements belong in Group 3 of the periodic table?, in Scerri E & Restrepo G (eds), Mendeleev to Oganesson: A multidisciplinary perspective on the periodic table, pp. 140–151, Oxford University Press, New York

One might say that, pragmatically, fewer splits are preferable—yet the perceived importance of the delayed appearance of the first f-electron seems to outweigh neatness.

In a fashion, He over Ne, and La under Y offset one another: He over Ne ignores electron configuration and results in a split block while La under Y respects electron configuration and likewise results in a split block.

I raise this not as a definitive answer, but as an invitation to discussion.

thank you, René

Extract from Eric’s The Periodic Table: Its Story and Significance (2020), p. 401:

"Although some books include a periodic table arranged like figure 13.14 it does not seem to be a popular design and for rather obvious reasons. Presenting the periodic table in this fashion requires that the d-block of the periodic table be split up into two highly unequal portions containing one block that is only one element wide and another block that is nine elements wide. Given that this behavior does not occur in any other block in the periodic table, it would appear to be the least likely of the three possible tables to reflect the actual arrangement of the elements in nature. As suggested earlier though, we should beware of arguments based on symmetry and regularity. In addition, there is already one block, or at least the first members of that block, that are generally separated, namely, hydrogen and helium in the s-block. Nevertheless, this splitting of two members of the s-block occurs in an even or 50%-50% fashion, whereas the splitting of the d-block as seen in figure 13.14 is clearly very uneven."

<PastedGraphic-7.png>

[Rene]

On 26 Sep 2025, at 12:29, ERIC SCERRI <sce...@g.ucla.edu> wrote:

On Sep 25, 2025, at 6:04 PM, Rene <re...@iinet.net.au> wrote:

...in the conventional table helium is separated from the other 13 s-elements (Li–Fr and Be–Ra). That yields a numerical division of one element versus thirteen. My point was not that this should carry great theoretical weight in itself, only that if such an uneven split is tolerated in the s-block, then asymmetry alone seems insufficient as a reason to dismiss the d-block split.

Thanks for the clarification.  The two kinds of split you are discussing are not comparable.  In the case you discuss one single element is traditionally removed from the s-block because of its chemical properties as you have so often argued for in rejecting the left-step table over the years.

The d-block split really is a split or separation whereby Sc, Y, La and Ac are separated right down the middle as it were from a set of other d-block elements (Ti, Zr, Hf, Rf).

The two cases are entirely different.

To be comparable we would need to be speaking of separating H, Li, Na, K etc from Be, Mg, Ca, etc.

ERIC SCERRI

Thank you Eric.

I can see the distinction between the kinds of split, though at the level of principle I argue that both cases show the integrity of a block can be compromised when other considerations are judged more important. The real question seems to me not whether the splits are identical in form, or how the extent of one compares to the other, but how far we are prepared to privilege block integrity vs tolerate block irregularity for didactic, historical, pragmatic, electronic, or chemical reasons.

I believe this is consistent with your caveat that "we should beware of arguments based on symmetry and regularity". 

I won’t comment on your separate point regarding group 3, only to say that, in my view, you are again attaching too much importance to the details of electronic configurations.

Well, the importance given to the details of electronic configurations is a general theme in the literature, including the delayed appearance of the first f-electron. This importance was such that B-Al, originally placed over Sc, came to be progressively repositioned from the 1930’s onwards over Ga—even though the periodic trends going down B-Al-Sc-Y-La are smoother. It was a case of electron configuration consistency winning out over the periodic law. Conversely, configuration details were pragmatically overruled in the case of He over Ne.

Please cite any article detailing the history of this development.  Was it really a case of putting electronic structure over chemical behavior as you suggest?

There’s no such article I’m aware of however I suggest the smoking gun is found in Parkes (1943, p. 677):

“Relationships of the elements of Group III. The exact sub-classification to be adopted in this group has in the past occasioned some difficulty, but it is now generally agreed that B and Al are best associated with Ga, In and Tl. This is supported by the conclusions at present accepted for the electronic configurations of these elements…”

Parkes GD (ed.) 1943, Mellor’s modern inorganic chemistry, impression of 1939 edition, Longmans, Green and Co., London

This regularisation appears to have occurred from the 1930s to (effectively) the 1960s, with the acceptance of modern electronic configuration theory. Pauling seems to have been the longest to hold out.

Tracing the rest of the story obliges me to cite more references:

• A colleague (Eugen as I recall) mentioned to me a while ago:

“When I studied chemistry in the 1950s…the groups of elements were designed according to empirical chemistry, with only a little impact from quantum theory. Group 3 was B-Al-Sc-Y-La…".

• The 1967 edition of Parkes, which followed the 1961 edition of Mellor’s says:

"As in the case of Group II, there has been considerable discussion as to whether boron and aluminium are to be considered as primarily related to scandium, yttrium, etc., or to gallium, indium and thallium; but the view is now generally held that the association of boron and aluminium with gallium, indium and thallium is, on the whole, the best arrangement." (p. 735)

• Moving forward, Pauling (1988, pp. 635, 697) refers to the congeners of B as being Al-Sc-Y-La. He discusses Ga-In-Tl separately in his chapter on Cu, Zn, Ga and their congeners. He writes, "The metals Zn, Cd, and Hg (group IIb) are also much different from the alkaline earth metals (group II), as are Ga and its congeners (group IIIb) from the elements of group III.”

Pauling L 1988, General chemistry, Dover Publications, New York. This edition is said to be an unabridged, slightly altered and corrected republication of the 3rd 1970 edition.

• Greenwood & Earnshaw (2002, pp. 222–226) note that although the Group 13 elements (B–Al–Ga–In–Tl) all share an ns2np1 outer configuration, their underlying cores differ markedly: B and Al have noble-gas cores, Ga and In add a filled d10 shell, and Tl adds both 4f14 and d10. These differences produce irregularities in the expected trends, such as the unexpectedly high ionization energy of Ga (from the d-block contraction) and the reversal of the In–Tl trend (from the lanthanide contraction). By contrast, the Group 3 elements Sc, Y, and La all possess noble-gas-type cores, leading to smooth, regular property trends (e.g. a steady decrease in ionisation energy from B and Al down through Sc, Y, and La).

Greenwood NN & Earnshaw A 2002, Chemistry of the elements, 2nd ed., Butterworth-Heinemann, Oxford

In response to your closing question: Yes, this was a case of putting electronic structure over chemical behavior, thereby demonstrating the importance placed on electronic configuration.

It...ignores the long standing discussion regarding the dual sense of the term “element”.  See the collection of articles that I co-edited with Elena Ghidaudi, “What is a chemical element?”, OUP, 2020 and several journal articles on this issue.

I acknowledge the long-standing discussion regarding the dual sense of the term “element”.

That said, since the periodic table is fundamentally a manifestation of the periodic law, I suggest that the dual sense becomes less central in this context. 

What do you understand by the dual nature of elements Rene? 

I’m afraid I dont see how what follows below has any bearing on this distinction.

Thanks for the question. As I understand it, the “dual sense” refers to distinguishing elements as simple substances (e.g. graphite, diamond, C60) from elements as basic substances (the “carbon-ness” that persists in all compounds of C).

My view is that, in the context of the periodic table, this distinction becomes less central. The periodic law expresses relationships between two variables: Z, and chemical/physical properties, which are typically drawn from the most stable form under ambient conditions (e.g. graphite for carbon). On that basis, I see the periodic table as functioning without requiring the dual sense of an element explicitly, since it already operates with one form taken as representative. Of course, in compounds the question of the most stable form of an element becomes moot.

"What follows below" was my attempt to show that the dual nature of the elements didn't need to be invoked in mapping the periodic table.