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July 14

Beginning of the Cambrian Period: which is it, 538.8, 539, or 541 years ago?

The article on the Cambrian Period, https://en.wikipedia.org/wiki/Cambrian, says that it lasted "from the end of the preceding Ediacaran Period 541 million years ago (mya)"

This disagrees with the Ediacaran article, https://en.wikipedia.org/wiki/Ediacaran which says it lasted "to the beginning of the Cambrian Period 538.8 Mya."

Also note that the end of the Ediacaran Period "marks the end of the Proterozoic Eon" However the Proterozoic Eon, https://en.wikipedia.org/wiki/Proterozoic, "is a geological eon spanning the time interval from 2500 to 538.8 million years ago."

Note that the Cambrian Period is the earliest period of the Paleozoic Era, which is the "longest of the Phanerozoic eras, lasting from 538.8 to 251.902 million years ago." Sadly, within the Proterozoic Era article, it is in disagreement with itself when it says that "The Cambrian spanned from 539–485 million years ago and is the first period of the Paleozoic Era of the Phanerozoic. "

OK, so which is it, 538.8, 539, or 541 years ago? PaulChouinard (talk) 03:19, 14 July 2023 (UTC)[reply]

I especially like the "251.902 million years ago" one. Must be a floating pimp error made when correcting from Charlemagnean time to Metric time. Abductive (reasoning) 04:53, 14 July 2023 (UTC)[reply]

Gotta watch out for them floating pimps. —Tamfang (talk) 15:18, 14 July 2023 (UTC)[reply]

These geological periods tend to be defined by particular features in the rocks that can be reliably identified in many places. Actually dating these features is another matter. So all sources may agree that the Cambrian started at one particular layer of rock, but may not agree on the exact time. Consider the end of the Cretaceous. For decades everybody has agreed that it happened at a particular iridium and soot rich layer, but estimates for the age of this layer have varied over more than a million years. PiusImpavidus (talk) 08:21, 14 July 2023 (UTC)[reply]

There's probably some false precision in dating methods shown above, making all three numbers functionally equivalent. The range of difference between your three numbers is 0.4%, which is likely smaller than the precision with which we can actually date rocks that old. --Jayron 32 12:22, 14 July 2023 (UTC)[reply]
Did you read the obvious section Cambrian#Dating the Cambrian? This explicitly notes that "A more precise date using modern radiometric dating yield a date of 538.8 ± 0.2 million years ago." supporting the view that 539 and 538.8 are functionally equivalent and just depend on the level of precision you want, although 538.8 isn't really false precision assuming you trust the dating method and its error estimate. While it doesn't specifically comment on a 541 figure, it does mention "Nevertheless, there are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata – which would mean that dates from other sections, ranging from 544 or 542 Ma, are more suitable. Nil Einne (talk) 15:08, 14 July 2023 (UTC)[reply]
I want to note that our article Cambrian references the 541 Ma claim with International Commission on Stratigraphy's chronostratigraphic chart. The older version of that chart indeed says that the Cambrian period began 541.0 Ma ago, but since February 2022, the chart has used 538.8 Ma as the beginning of the Cambrian. Valtaisa varpunen (talk) 15:46, 19 July 2023 (UTC)[reply]

Collocyte, collencyte, collenocyte

Hello, I'm a boring bastard from Wiktionary (the free dictionary project, defining words), and I would like some expertise from biologists. Look at our entry for collocyte. You can also see the etymology (word origin) there, meaning basically "glue cell".

So: it seems that there might be three words: collocyte (see above), collencyte and collenocyte. I am personally sceptical about the middle one because it doesn't sound like a typical formation: these Latin and Greek words would not usually allow something like ..nc.. in the middle. HOWEVER...

We have this interesting slice of pie: Collocyte#Confusion_between_collocytes_and_collencytes Is that really correct? Is anybody experienced and confident enough to help our little dictionary project with these words?

Thanks, from your word nerd, Equinox ◑ 11:02, 14 July 2023 (UTC)[reply]

They all seem to be words relating to slightly different things, sometimes coined for very niche subjects. "The problem is so marked that to avoid confusion many workers are abandoning traditional terms in favour of new words."

A treatise on zoology, Vol 2, p. 52, as cited in the Collocyte#Confusion_between_collocytes_and_collencytes section on collencytes seems to be relatively authoritative on sponges, if a century out of date. The article says the term was borrowed from Collenchyma. My Shorter Oxford English Dictionary 3rd ed., only lists collenchyma and none of the words you are asking about. Britannica has its own section on collencytes in the Sponges article (subscription or UK Public Library membership needed.)

A source-book of biological names and terms, p. 63 (free registration needed, 1 hour borrow) gives the etymology of collenocyte, kolla, glue, + engchyma, something poured in, calling it a "poorly-formed word". So there would appear to be three separate words with separate individual meanings and etymologies. MinorProphet (talk) 12:29, 14 July 2023 (UTC)[reply]

Spectroscopic electron configurations

The NIST Basic Atomic Spectroscopic Date pages give the following ground states for thulium, ytterbium, lutetium, and hafnium:

69 Tm	(1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2  5p6) 4f13 6s2 
70 Yb	(1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2  5p6) 4f14 6s2
71 Lu	(1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2  5p6) 5d1 6s2
72 Hf	 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2  5p6  5d2 6s2

The bold text and irregular spacing is mine.

Why is it that for Lu and Hf the 4f14 orbital is demoted so that it appears after the 4d10 orbital rather than after the 5p6 orbital as in Tm and Yb? Sandbh (talk) 12:44, 14 July 2023 (UTC)[reply]

So, there's a LOT of conflicting conventions when writing electron configurations. It appears from my quick looking over that NIST website is that everything in the parenthesis are "non-valence" electrons; i.e. sublevels from which electrons are NOT normally lost during normal chemical reactions. Lutetium, for example, has a maximum oxidation state of 3+; Lu³⁺ is presumably /1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6/ By contrast, Thulium also has a 3+ oxidation state, and that means that once the two 6s2 electrons are removed, a third electron has to come from somewhere, Th³⁺ must therefore be /1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f12 5s2 5p6/ (bold for highlight), implying that the 4f sublevel is acting as a valence level, given that electrons can easily be removed from hit. Basically, for Tm and Yb, the valence electrons are 4f and 6s, while in Lu and Hf the valence electrons are 6s and 5d, with 4f electrons not participating in bonding. This is common among both transition (d block) and post-transition (f block) elements, where the d and/or f sublevels function as valence levels. This sort of thing is known as "pseudo-octet" configurations, see here for some examples.--Jayron 32 18:09, 14 July 2023 (UTC)[reply]

Thank you @Jayron32:

There may be an explanation that I hadn't appreciated. This is that the spectroscopic configuration of e.g. neutral Lu does not necessarily denote the order in which electrons leave. That is to say, as electrons leave or are ionized away the remaining sub-orbitals may change their order. Continuing the example, it is known that Lu⁴⁺ is [Xe]4f13. IOW, the electrons that have left appear to be 4f, 5d and 6s. OTOH, looking at the spectroscopic configuration of Lu as (1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6) 5d1 6s2 it appears that the leaving order should be two s, one d, and then (presumably) a 5p electron. Whereas in actuality, a 4f electron appears to leave.

The Tm example is a good one. Spectroscopically it is (1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6) 4f13 6s2. Tm3+ would be (1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6) 4f12, as you suggest but for the position of the 4f12 sub-orbital.

Does the above seem plausible?

I don't yet know if it's right (I could be wrong) but it appears to do the least amount of injury to the known experimental facts. Sandbh (talk) 05:15, 15 July 2023 (UTC)[reply]

I am not aware of a Lutetium 4+ being a common oxidation state; according to Lutetium, the maximum common chemically stable oxidation state is Lu³⁺. This only lists 3+, I'm not sure where you are seeing data showing a Lu⁴⁺ state, so I can't comment on your proposed configuration. If you could provide a source for that information, it may help someone answer your question. --Jayron 32 10:49, 17 July 2023 (UTC)[reply]