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All lanthanides except for lanthanum have suffix "-ium". Why lanthanum is so special, and why this particular word form has been considered canonical? Is using of "lanthanium" these days allowed, or it's forbidden?

It looks like in 19th century both forms were utilized, but since 1900..1910 "lanthanium" vanished from the grid:

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In 20th century only few authors ever used "lanthanium", e.g. some recent publications also include "lanthanium", but it is rather an exception than a rule. In the other hand, I haven't discovered any IUPAC rules explicitly declaring "lanthanium" as a word to be avoided.

There are also related topics on English.SE (Suffix ‘-ium’ vs. ‘-um’) and Chemistry.SE (Why do the names of most chemical elements end with -um or -ium?), but neither provides a distinct answer.

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    Related: english.stackexchange.com/questions/280490/…
    – user66974
    Sep 21 '17 at 12:03
  • But you've answered your own question. It's not used. It's not forbidden in that you won't get thrown in jail for using it (not in Britain anyway), but I doubt whether you would be able to use it in a scientific journal because scientific communication is enhanced by consistency and precision in the use of terms, and journals have house styles. (But of course I will defend to the death the right to write sulphur.)
    – David
    Sep 21 '17 at 12:33
  • @David There is still some inconsistency to these days, so if anyone could provide a historical feedback and a reference on its usage, it would be great.
    – andselisk
    Sep 21 '17 at 12:38
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I doubt that it is possible to say why lanthanum won out as the standard name. However, lanthanium is not necessarily a mistake. It was one of several names used for the element, which was discovered in 1839 – well before the establishment of IUPAC in 1919 and naming of the lanthanides in 1925, and even before the development of the periodic table in 1869.

The discovery of lanthanum was first announced in French in the Comptes rendus de l'Académie des Sciences. The name given by Mosander, from the Ancient Greek λανθάνειν (lanthanein), was reported as Lantane. It is still known as lanthane in French today. When the discovery was reported in Philosophical Letters later that year the name was translated as Latanium.

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There's no such thing as "lanthanium" - if you see it, it's simply a mistake, nothing more.

Lanthanum isn't the only element to end in "-um" rather than "-ium": so do Molybdenum, Platinum and Tantalum.

Aluminium is still spelled "Aluminum" in some places (eg the USA): its' discoverer Sir Humphry Davy originally called it "Alumium", then changed it to "Aluminum", then changed it again to "Aluminium". The second name stuck, in some regions.

And, of course, many elements don't end in "-ium" at all, and these tend to be the ones discovered first: Oxygen, Gold, Hydrogen, Sulphur etc.

The "-ium" suffix is nothing more than a convention: at some point, it was decided that newly named elements should end in -ium, and people have followed that tradition. They didn't go back and rename all the existing ones to be "goldium", "oxium", "hydrium", etc - this would just be silly.

I can't explain those ngram results - as far as I know the element has never been renamed, which would be one possible explanation. Maybe there was a typo in some popular reference book which got copied into other books before everyone noticed. Or, it might be a mistake in Google Ngrams, which cannot be infallible (it would be unique among software if it was).

If you're interested in the cultural history of the elements, I'd highly recommend a book called Periodic Tales, by Hugh Aldersly-Williams.

EDIT: as GEdgar has pointed out in the comments, it's only metals that are given -ium names by convention. As it happens, most elements are classed as metals, but not all, and each class has its own suffix, such as "-ine" for halogens, and "-on" for noble gasses.

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    I don't compare La with the entire periodic table, only with lanthanides instead, that's what I started my question with. Also, if you check out actinides, there is no such problem -- all of them have "-ium" suffix. And I would really appreciate a literature reference regarding usage of "lanthanium" vs "lanthanum".
    – andselisk
    Sep 21 '17 at 12:41
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    "at some point, it was decided that newly named elements should end in -ium" ... no the ending -ium us used only for for metals. So of course Oxygen, Hydrogen, Sulphur do not have it. Interestingly, Helium does, because it was first thought to be a metal.
    – GEdgar
    Sep 21 '17 at 12:43
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    Also, it's Davy, not Davey. And to my knowledge elements' names are not capitalized.
    – andselisk
    Sep 21 '17 at 12:44
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    @andselisk, thanks for the Davy correction, will edit. I'll also change Davy's role from inventor to discoverer, oops. Sep 21 '17 at 12:47
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    Right. For example element 117 Tennesine, should be a halogen, even if there has never been enough of it in one place to observe its properies. And 118 Oganesson should be a noble gas. So they were not given -ium names.
    – GEdgar
    Sep 21 '17 at 12:50

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