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.


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 ....

  • $\begingroup$ -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 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 Jan 30 '18 at 20:47
  • $\begingroup$ I support this, and it might be the better long term solution. $\endgroup$ – Martin - マーチン 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 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

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.
  • $\begingroup$ 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 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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