Are the elements lanthanum and actinium considered to be in the d block or the f block of the periodic table?

Many periodic tables place lanthanum and actinium in the f-block of elements, for example, [this periodic table](http://periodic.lanl.gov/index.shtml) from Los Alamos National Laboratory.


However, [this table](http://www.rsc.org/periodic-table) from the Royal Society of Chemsitry places lanthanum and actinium in the d-block.


Which is the most appropriate placement of these two elements on the periodic table? What is the most recent scientific consensus regarding this matter?




Lanthanum and Actinium are generally both accepted as f-block elements. You will notice in the Royal Society's periodic table, La and Ac are both color coded to match the other f-block elements. They do this to show the beginning of both the Lanthanide and Actinide series, while maintaining a reasonable symmetry for their periodic table (that is, without "holes" in their table, similar to the first link you provided).


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Lanthanum and actinium are usually regarded as d-block elements (Myers, Oldham & Tocci 2004, p. 130) and generally counted as lanthanides and actinides (the rest of which occupy the f-block). Some periodic tables, like the one from LANL emphasise chemical similarities, so lanthanum and actinium are shown at the foot of the table, along with the f-block elements. That's fine, but leaves a hole under yttrium so it isn't clear which two elements go there. Other tables, like the one from the Royal Society of Chemistry, presumably rely on electron configurations to work our which elements fit under yttrium, and use colour shading to emphasise similarities in chemical behaviour.


Myers RT, Oldham KB & Tocci S 2004, *Holt Chemistry,* Holt, Rinehart and Winston, Orlando


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As per IUPAC, lanthanum and actinium are f-block elements.


source:
<https://iupac.org/what-we-do/periodic-table-of-elements/>


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From the [IUPAC red book](https://iupac.org/cms/wp-content/uploads/2016/07/Red_Book_2005.pdf):



> 
> The groups of elements in the periodic table (see inside front cover) are numbered from 1 to 18. The elements (except hydrogen) of groups 1, 2 and 13–18 are designated as main group elements and, except in group 18, the first two elements of each main group are termed typical elements. Optionally, the letters s, p, d and f may be used to distinguish different blocks of elements. For example, the elements of groups 3–12 are the d-block elements. These elements are also commonly referred to as the transition elements, though the elements of group 12 are not always included; the f-block elements are sometimes referred to as the inner transition elements. If appropriate for a particular purpose, the various groups
> may be named from the first element in each, for example elements of the boron group (B, Al, Ga, In, Tl), elements of the titanium group (Ti, Zr, Hf, Rf), etc.
> The following collective names for like elements are IUPAC-approved: alkali metals
> (Li, Na, K, Rb, Cs, Fr), alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra), pnictogens$^8$ (N, P, As, Sb, Bi), chalcogens (O, S, Se, Te, Po), halogens (F, Cl, Br, I, At), noble gases (He, Ne, Ar, Kr, Xe, Rn), lanthanoids (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), rare earth metals (Sc, Y and the lanthanoids) and actinoids (Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr).
> 
> 
> 


So to decide which block La and Ac belong to, you would have to know which group they are in (group 3 or no group). In the periodic table shown in the same document, there is only an asterisk in period 6 and 7 group 3. So according to the red book, La and Ac (and Lu and Lr) are in the f-block.


If a periodic table shows elements in group 3, this would imply they are in the d-block. This is true even if they are color-coded in the same way as the lanthanoids and actinoids.


The IUPAC red book also makes a statement about the terms lanthanide and actinide:



> 
> Although lanthanoid means ‘like lanthanum’ and so should not include lanthanum,
> lanthanum has become included by common usage. Similarly, actinoid. The ending ‘ide’ normally indicates a negative ion, and therefore lanthanoid and actinoid are preferred to lanthanide and actinide.
> 
> 
> 


The placement of La and Ac in the periodic table is [debatable](https://www.chemistryworld.com/news/new-rationale-for-15-element-wide-f-block/3009047.article), and maybe one day IUPAC will change its mind.


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If IUPAC says these elements are in the $f$ block, I'll roll with it.


The real lesson here is that the boundaries between "blocks" of the Periodic Table, like the boundary between "strong" and "weak" acids or bases or even between what is a stable compound and what isn't, is not sharp. Some other examples of a rough, spotty, changeable real world:


* Most simple magnesium compounds, even the best known hydride and boride compounds, are primarily ionic, but when they have covalent character the magnesium often bonds tetrahedrally as if using $3p$ as well as $3s$ valence orbitals. See for instance the coordinated structure given [here](https://en.wikipedia.org/wiki/Grignard_reagent) for methylmagnesium chloride in THF. Beryllium shows this effect even more prominently in its wider variety of covalent compounds.
* Calcium could be called a $d$-block element when it bonds with its $3d$ orbitals in [this calcium(I) compound](https://pubs.acs.org/doi/10.1021/ja808524y)[1](https:// doi.org/10.1021/ja105534w) (and yes it is +1, showing multiple oxidation states like a transition element).
* Cerium, the second element is the lantanide series, does some straddling of its own between $d$ and $f$ blocks as its valence in the metal is changeable between 3 (with a core-like $5f$ electron) and 4 (with thiscelectron engaged in the bonding). See Johanssen et al. [2](https://doi.org/10.1038/srep06398) and the [WP article](https://en.wikipedia.org/wiki/Cerium) citing this reference.
* Among the actinides there is also thorium, which appears to involve only $7s$ and $6d$ valence electrons in the gas phase but brings in $5f$ orbitals in the metal (see [this answer](https://chemistry.stackexchange.com/a/136833/17175) and the references therein).


**References**


1.
Krieck, Sven; Görls, Helmar; Westerhausen, Matthias (2010). "Mechanistic Elucidation of the Formation of the Inverse Ca(I) Sandwich Complex [(thf)3Ca(μ-C6H3-1,3,5-Ph3)Ca(thf)3] and Stability of Aryl-Substituted Phenylcalcium Complexes". *Journal of the American Chemical Society*. **132** (35): 12492–12501. <https://doi.org/10.1021/ja105534w>. PMID 20718434.


2.
Johansson, Börje; Luo, Wei; Li, Sa; Ahuja, Rajeev (17 September 2014). "Cerium; Crystal Structure and Position in The Periodic Table". *Scientific Reports*. **4**: 6398. Bibcode:2014NatSR...4E6398J. <https://doi.org/10.1038/srep06398>. PMC 4165975. PMID 25227991.


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