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The rare-earth elements are, according to Wikipedia (which cites IUPAC):

the fifteen lanthanides, as well as scandium and yttrium.

Could we expand this to include actinides? I would suggest . I don't know where Sc and Y would then go, though.

The alternative is to have separate tags. Is that better? I think having and is a bit weird.

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I would actually argue against combining the two, at least from a technical perspective. The chemistries of the lanthanides and the actinides are very different.

Lanthanides mostly form only trivalent ions (cerium, europium, terbium, and I think a couple others being exceptions) with nearly identical chemical properties. The big differences arise in the magnetic and excited-state behaviors downstream of the varying $\ce{4f^n}$ populations, all of which are buried below the $n=5$ valence electrons.

Actinides have a diverse chemistry, with, e.g., lots of different coordination preferences, oxidation states, and spectra of chemical reactivity. Here, AFAIK the $\ce{5f}$ electrons do participate in the actual chemistry, making it crazy, wacky, and unlike anything else above it in the Periodic Table.

So: While I'd be okay with scrubbing in favor of /, I would see value in keeping / as a separate thingummy. $\ce Y$ and $\ce{Sc}$ will just have to fend for themselves as Group III ....

Alternatively, if there really aren't enough questions to warrant separate $\ce{4f}$ and $\ce{5f}$ tags, then from an administrative perspective it may well make the most sense to just combine them, as Martin suggests. However, a quick search turns up 34 questions mentioning uranium, a subset of which would certainly be relevant to an as-yet-nonexistent ....

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  • $\begingroup$ -1 because afaik Gd doesn't form any ions apart from +3. More seriously - I don't know, I would be okay with either option actually. You are right, lanthanide and actinide chemistry is slightly different. $\endgroup$
    – orthocresol Mod
    Jan 30 '18 at 20:41
  • $\begingroup$ @orthocresol Might've been Tb, then, forming 4+ sometimes. <shrug> :-P $\endgroup$
    – hBy2Py
    Jan 30 '18 at 20:46
  • $\begingroup$ Yup, Tb goes up to +4. Sm and Yb are the other two that can stay at +2, iirc. $\endgroup$
    – orthocresol Mod
    Jan 30 '18 at 20:47
  • $\begingroup$ I support this, and it might be the better long term solution. $\endgroup$ Jan 31 '18 at 10:12
  • $\begingroup$ @orthocresol Slightly different?? Their coordination behaviors are completely dissimilar, if nothing else. $\endgroup$
    – hBy2Py
    Jan 31 '18 at 12:07
  • $\begingroup$ I’m learning to speak like a Brit. $\endgroup$
    – orthocresol Mod
    Jan 31 '18 at 12:43
  • $\begingroup$ @orthocresol Ohhh. Righto, I'll crank up the gain, then. $\endgroup$
    – hBy2Py
    Jan 31 '18 at 12:49
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I agree with the sentiment and think that we can extend the scope and rename.

That being said I would propose in accordance with the Red Book (IR-3.5, p. 51f):[1]

The following collective names for like elements are IUPAC-approved: [...] lanthanoids (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) [...] and actinoids (Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr).
[...]
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. (Emphasis mine.)

I the same rule, earlier, we are proposed what to do with scandium and yttrium:

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.

This is contrary to the current Gold Book recommendation for transition elements:[2]

An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.

However, I would in this case just roll with the Red Book and include Sc and Y in , whose tag-wiki currently already uses the Red Book definition.


  1. International Union of Pure and Applied Chemistry. Nomenclature of Inorganic Chemistry: IUPAC recommendations 2005. Prepared for publication by N.G. Connelly, T. Damhus, R.M. Hartshorn and A.T. Hutton. RSC Publishing: London, 2005. See also: Google books, IUPAC.org, full text from iupac.org (PDF, 4.1 MB), ISBN-10: 0854044388
  2. transition elements (DOI: 10.1351/goldbook.T06456) in: IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications: Oxford, 1997. XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook. Last update: 2014-02-24; version: 2.3.3.
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  • $\begingroup$ I've seen that line in the Red Book before. The -oid ending is so infrequently used, which is why I didn't bother mentioning it... But yeah, this is a good idea. From a functional pov oid or ide makes no difference in ease of finding tags (we can even make lanthanide/actinide synonyms in case people type super fast), and it's more consistent with IUPAC, so why not. $\endgroup$
    – orthocresol Mod
    Jan 30 '18 at 10:27
  • $\begingroup$ See my counterargument, Mart. Probably not anything you don't know in it, but I suspect there may be enough questions on the site to support two tags out of all this, and the two classes of elements are really not at all the same. $\endgroup$
    – hBy2Py
    Jan 30 '18 at 20:23

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