What is the true depiction of the periodic table?

Normally with the periodic table the lanthanide series is separated out because it's long and would make the table wide. I looked for an expanded version and found this:

I found it kind of strange that scandium and yttrium are above the lanthanide and actinide series on the far left.

Does it make sense for those two elements to be there? I always assumed each column had certain characteristics like same number of valence electrons, that there were certain characteristics that held true for the whole table, but in this format I don't know how to understand the patterns or why the table ends up taking this shape.

La and Ac have $$d^1$$ electrons in their valence shells, rather than $$f^1$$ electrons.

The long table you found looks like that for a several reasons.

The trends going down Sc-Y-La are like those seen in groups 1 and 2. The trend going down Sc-Y-Lu is like that of groups 4 to about 10. Since lanthanide chemistry is basically that of trivalent alkali or alkaline earth metals, this tips the balance in favour of group 3 as Sc-Y-La.

Also helping is that group 4 is the first in which the really characteristic properties of transition metals (variable oxidation states; colour; paramagnetism) are seen. Ditto, same thing happens with group 12, which is why they're shown as post-transition metals.

And there's the lanthanide contraction, which starts at Ce and finishes at Lu. If you show Lu as being a transition metal it mushes the end of the Ln contraction into the d-block.

And there's the periodic law, which says that the chemical elements, if arranged according to their atomic numbers, show an approximate repetition of properties after certain regular but varying intervals. Here, La is the first element after Y that shows the approximate repetition in properties.

This research supports the periodic law outcome:

• Glawe H, Sanna A, Gross EKU & Marques MAL 2016, “The optimal one dimensional periodic table: a modified Pettifor chemical scale from data mining”, New Journal of Physics, vol. 18, 093011, https://iopscience.iop.org/article/10.1088/1367-2630/18/9/093011/pdf
• Restrepo G 2017, "Building classes of similar chemical elements from binary compounds and their stoichiometries", in MA Benvenuto (ed.), Elements old and new: Discoveries, developments, challenges, and environmental implications, American Chemical Society, Washington DC, pp. 95-110

Both sources point to La being more distinct from the Ln than is the case for Lu.

Finally, if you count the differentiating electron discrepancies in each block of the periodic table, noting the above table has a split-d block, you'll find that an Sc-Y-La table has 12 such discrepancies whereas an Sc-Y-Lu table has 13. The current agreed-within-IUPAC table has 14 differentiating electron discrepancies!

As many people have a hard time accepting the idea of a split-d block, I personally feel that a better solution would be a Sc-Y-Lu table with group 3 being shown as Sc-Y-Lu-Lr and La-Ac. That would have the extra benefit of resolving long-standing discussions about the composition of group 3, which have been going on since about 1982, when Jensen published his paper in JChemEd, arguing for Sc-Y-Lu:

Nothing much has to change. Just add a 3 above La-Ac in the f-block, in addition to the one over Sc-Y-Lu-Lr in the d-block.

A chemistry book chapter on this group 3 would make fascinating reading (a good thing given, to date, that group 3 is supposed to be the least studied group).

Hope that helps.

I'm sure this question will stir up a lot of debate but I agree with your analysis of yttrium and scandium. Both elements have a $d^1$ electron in their valence shell just like lutetium and lawrencium and belong in the d-block. Whereas lanthanum and actinium have $f^1$ electrons in the valence shell. Chemically they all act very similarly but electrically I would group scandium and yttrium on the right.

Of note is the representation of zinc, cadmium and mercury as post transition metals. this is another overlooked but debated categorization.

• Whereas lanthanum and actinium have f1 electrons in the valence shell. Is that really for certain? The RSC lists them as d1, rsc.org/periodic-table. #// On the 'debate' about Zn, Cd, Hg: I think there is much agreement that they behave a lot more like 'main group' elements, than transition metals. There are more proposals, which include group 11, but the term itself is not recognised (afaik) by IUPAC, probably because it cannot be rigorously defined. – Martin - マーチン Apr 25 '19 at 13:00
• La and Ac do indeed have d1 electrons. – Pretty fly for a chem nerd Jul 17 '19 at 5:32