Talk:Noble metal/Archive 1

Request for merge
Re the request for a merge with Precious metal requested by User:Abdull:
 * Disagree - "Precious metal" is a term clearly understood in the field of commerce, where the term Noble metal is not known. Conversely, Noble metal has a precise scientific use not covered by "precious metal". However, both articles certainly need a rethink and clarification. --mervyn 09:46, 1 May 2006 (UTC)


 * Disagree - Noble metal is a chemical term with a particular meaning. As explained in the article Platinum essentially isn't a noble metal, but it is in fact a precious metal.  This would be an excellent place to talk about concepts of chemical non-reactivity of noble metals, vs the value of precious metals (which would fit in the other article).  Rich0 02:52, 7 May 2006 (UTC)


 * Disagree - Noble metal and precious metal have very different connotations, and merging them would diminish the distictions these terms are meant to convey.

Copyedit needed

 * Note: Incorrect use of "e.g." as well as poor grammar in latter portion of article should be corrected.


 * "e.g." (meaning: "for example") is used correctly in the article to describe gold. "i.e." (meaning: "that is," i.e., "in other words") would be incorrect.

Inkypan (talk) 04:25, 1 March 2009 (UTC)

Confusion
The entries for this article seem to contradict one another. Copper oxidizes quite readily (short timescale), whereas gold does not. But if copper is considered a noble metal, according to the basic definition given, how can it be considered to not oxidize/corrode? Another confusion is palladium: does it corrode (carbon monoxide) or does it stay basically resistant to surface changes? It's either one or the other. Maybe someone could clarify or merge some of these ideas? There seems to be some disagreement.


 * I understood Palladium to be considered the 'least noble' of the noble metals as it undergoes oxidation more readily than the others. I do not have my books here, but I will try to research this more and put up citations.

Clarification
A noble metal should be a metal! And therefore e.g. Arsenic or Silicium can not be considered as a possible noble metal. And Antimony is in the "gray range". Therefore these elements are also denoted by semi-metalls or metalloids. Why Mercury is not called a noble metall, I can only guess, has only historical reasons. Perhaps because it is liquid at room temperature. :) When the term "noble metal" was coined, preciser a wording with this name, there were only Au, Ag and Hg as possible candidates known. The platin-metalls was first known in the middle ages (in europe). Perhaps Polonium will be given the status of a noble metal if macroscopic pieces of Po-209 are availible! Human naming terminology if often irrational. ;) Regards, Achim1999 (talk) 23:24, 26 June 2009 (UTC)

Merge this article with Platinum group??
What about merging this article with Platinum group? Platinum group (or better platinum group metals) is a well defined concept, so we would not have the definitional issues etc. Anyway, I have never heard of the term "noble metal." And I've worked with the area for many years. The IUPAC goldbook (http://goldbook.iupac.org/list_chemistry.html) does not define Noble metals." "Noble metals" is not mentioned in the Dictionary of Inorganic Chemistry Encyclopedia of Inorganic Chemistry). For all we know, the article was initiated by someone unfamiliar with chemistry. Views?--Smokefoot (talk) 00:53, 27 June 2009 (UTC)

I think this is a really different topic as the topic "Platinum group". I agree totaly that a problem maybe that this term "noble metal" may not be defined scientifically strict! See the difficulties of defining "semi-noble metal". Moreover I'm convincient that this wording hasn't a scientific origin but was common use in society and has a many hundreds (if not thousands) year history. Cite
 * And I've worked with the area for many years.

You worked obviously not in this area of noble metals. :) Therefore I am also a bit relucant to give this chemical qualitatively reliable source of Holeman/Wiberg as first reference for definition. But I'm too lazy to do a research on how and when this phrase "noble metal" was coined. Maybe it is a good idea to point out that this metal denotion did not spring of from chemists, they later only has tried to find a good scientific reasoning that only these metalls in question was given the name "noble metal" -- difficult to attack by water and air. Regards, Achim1999 (talk) 10:05, 27 June 2009 (UTC)
 * Well I was thinking that the term "noble" is a term used by lay people and chemists alike, but a wide range of metals could be called noble. Thus the term noble metal is appropriate for Wiktionary (http://en.wiktionary.org/wiki/Wiktionary:Main_Page). Otherwise this article will continue to collect anectdotes and possible devolve vs evolve.  Let's see if we hear anything from people that edit chemical pages.--Smokefoot (talk) 13:05, 27 June 2009 (UTC)

The term "noble" is used by chemist in the wording/term "noble gas". :-) The notion of "noble metal" is typically NOT used by chemists. This is also the reason you have not heard of it, I assume. Else I would also have found a chemical definition. But maybe this term is in wider use in metallurgy? Anyway, it seems to be in common use if we make a Google-count of the term. Regards, Achim1999 (talk) 14:50, 27 June 2009 (UTC)


 * "Noble gas" is in the Oxford English Dictionary, whereas "noble metal" is not.


 * Ben (talk) 15:15, 27 June 2009 (UTC)

I just checked the german WP, because I add there the section "Chemisches Verständnis von Edelmetallen". Moreover there seems to be a strong correlation with "precious metal" but I believe this is not the same, as aluminium never is denoted as a "noble metal". Regards, Achim1999 (talk) 15:28, 27 June 2009 (UTC)
 * Well there are lots of archane and semi-useless articles in Wikipedia, and this will become one more. No harm done. We also have an article on toxic metal, and I expect someone will contribute an article on maleable metal, silvery metal, and volatile metal, etc.--Smokefoot (talk) 17:17, 27 June 2009 (UTC)

Hmm ... then I should stop correcting and updating the article? At least the question stays: what is meant by "noble metal" if your child asks you. ;) BTW: In german there is an important difference in "edles Metall" and "Edelmetall". The first seems to be equivalent to "precious metal" and the later could be to "noble metal". The spanish WP-article seems to be an older german version and the french WP-article is a really stub to be judged. Regards, Achim1999 (talk) 17:37, 27 June 2009 (UTC)
 * There's a good chance that I am wrong, so thanks for ignoring my sarcasm.  Noble metal would be a good subsection on electrochemistry, Metal, corrosion but it just seems that the article invites fluffiness.  It occurs to me that tantalum is highly noble in an operational sense (being acid stable), but it is a pretty good reductant from a redox potential, it happens to form a passivating oxide.  I think that your talents would have more impact editing on those themes.  But who knows.  Happy editing,--Smokefoot (talk) 18:27, 27 June 2009 (UTC)

Of cource, therefore exist Ta-crucibles. Also Ti is very resistant to sea-water due to its firm TiO2 film. And now-a-days even wedding rings made of Ti are offered. :) I will add a link from corrosion to noble metal in its see-also-section. Regards, Achim1999 (talk) 18:45, 27 June 2009 (UTC)


 * FWIW, Wiktionary says the following about noble metals:

"noble metal (plural noble metals)

1. (inorganic chemistry, alchemy) A metal resistant to corrosion, oxidizing and tarnishing, such as gold, platinum, copper and mercury.

Usage notes So termed because they resist being made base. Metals that chemist recognize as the platinum group of elements."
 * It should be noted however that the Wiktionary entry for noble metal contains no citations supporting where this sense or usage note came from (which is often the case in Wiktionary) and nothing on the discussion page for the entry. I add this here for everyone's information in the discussion.  I'll add my own comments on the merge later on. N2e (talk) 20:56, 27 June 2009 (UTC)

Additional informational comment (I will continue to reserve my opinion on the merge for later): I am not a chemist. The term noble metal or metal, noble does not occur in the indexes of either of the two chemistry textbooks I own, one from high school and another from Freshman chemistry (101/102?) in university. Interestingly noble gas, which does appear in both indexes, is spelled "nobel" in one text and "noble" in the other -- so obviously both spellings are verifiable in standard American textbooks.

I concur with the point made earlier that noble metal may very well be more of a vernacular, everyday usage in English rather than a strictly-defined usage in the discipline of Chemistry. But it is a term that is commonly used in English, and thus might very well merit a Wikipedia article on it, whether Chemists use the term or do not. N2e (talk) 21:08, 27 June 2009 (UTC)

I just stumpled about this (sadly german) reference:. This web-side seems to offer "university-protocolls made by students", doesn't it? I wonder how reliable this is. Regards, Achim1999 (talk) 22:15, 27 June 2009 (UTC)

Chemical article discovered: Bahlawane, N.; Kohse-Höinghaus, K.; Park, J.-S.; Gordon, R.G.:  (2005)    ALD process for the preparation of noble-metal-free monolithic catalysts    In: Proc. Electrochem. Soc. , S. 583-590. The Electrochemical Society - Band 2005-09.

Title contains wording "noble-metal-free" in a scientifc journal article. Achim1999 (talk) —Preceding undated comment added 22:22, 27 June 2009 (UTC).

Wow! I just find a hopefully very reliable source on the internet! Never thought of this! I immediately have added it to the article. There is a phrase: who searches, who will find. :) Regards, Achim1999 (talk) 22:31, 27 June 2009 (UTC)

removing subsections / paragrahs
Before removing whole paragraphs, please firstly give reliable informations why everything in there should be removed:


 * "Antimony and Polonium are considered to be only metalloids and thus ruled out to get "noble metal" status. Also most chemists and metallurgists consider metals which are not nobler than copper as "semi-noble", only. :Notice the reaction O2 + 2 H2O + 4 e− ⇄ 4 OH−(aq) :+0.40 V which is possible by wet air."

Why is Copper not called a noble metal? Because of the reaction given above! BTW: I have one very reliabale, inorganic reference calling Copper a "semi noble metal".

Thanks & Regards, Achim1999 (talk) 11:41, 28 June 2009 (UTC)

Hopefully resolved the dispute by reformulation and omitting the term "semi-noble". Regards, Achim1999 (talk) 12:00, 28 June 2009 (UTC)

To Do
Would be nice if someone else can resolve the remaining 3 citation/clarification needing demands -- not always me ;). Thanks & Regards, Achim1999 (talk) 00:07, 28 June 2009 (UTC)

Someone should also add the [Category: Noble metal] at least to the other 6 Platin metals. Regards, Achim1999 (talk) 00:16, 28 June 2009 (UTC)

All done! Achim1999 (talk) 18:56, 28 June 2009 (UTC)

some words to passivity
Cr, Nb, Ta, Ti are also very resistant to corossion. Need a few sentences to their chemical behaviour, don't we? Regards, Achim1999 (talk) 21:07, 28 June 2009 (UTC)

Two (or more) definitions?
It seems to me that one of the problems here is that there are multiple definitions of "noble metal".
 * You can define a noble metal by opposition to a "base metal", as in aurifaction, turning base metals into gold. This would give the definition as those metals used for coinage or fine jewellry: originally silver and gold, also platinum, palladium and rhodium these days.


 * The definition which is given in the article, is simply calling these metals which should be called "noble metals"! And I took the burden, do look up 5 such references and give them all here, and choose the most reliable as first!


 * I wonder what you want. A collection from contradictive sources of different quality and reliability?


 * The definition to call "noble metals" these which are resistant to corossion is no precise defintion as long as you don't tell what kind of corossion you mean! Therefore I add "from moist air" after the oxidization. And even then it is elastic wording, because "to be resistant" is only relatively! Therefore we have only the naming of the metals as the only really exact definition currently.

IUPAC only uses the term "noble metal" once in the Gold Book, here. Physchim62 (talk) 22:02, 29 June 2009 (UTC)
 * You can define a noble metal by analogy with a noble gas, i.e. an unreactive metal. You have to be careful, as many metals (e.g. titanium) are unreactive in bulk but very reactive when finely divided (Ti is pyrophoric, for example). Electrode potentials are a poor guide here, as they only show the values for the species given: I can't believe that polonium forms Po4+ ions in aqueous solution, for example. The RSC uses thirteen untarnishable metals for its President's Badge of Office.


 * Please call them explicitely, these thirteen untarnishable metals! As we both know, it depends on the conditions. Thus I am curious which metals these are. Perhaps they also count Cr, Ti, Nb and Ta, the 8 classical noble metals and Hg, add up to 13; but I guess only. —Preceding unsigned comment added by Achim1999 (talk • contribs) 21:53, 30 June 2009 (UTC)


 * I think the Electrode potentials are a good guide because they explain why these metals are called noble metals and no others. Have you a better objectivly quantitive explanation? It also matches very well the "resistant to corossion" words in the beginning. I know, I know. Hence I alreday had question on this talk-page "why not say some words about Ti, Cr, Nb & Ta" and because I got no quick response, I already added a sentecne about Ti, NB & Ta in the article (before you comment this)! :)


 * Well, to potential/reactions: I wrote "simplified reactions" E.g. I know that Re -> ReO_2, or better
 * Re + 2 H_20 ↔ ReO_2 + 4H^ + 4e^-, not Re -> Re^4+, but fill free to give the full reactions.


 * Regarding Po, I wrote I took the data from CRC. And there is the only reaction :given (from pure metal state): Po ↔ Po ^4+ + 4e^- with 0.76 V, the only other is
 * Po^2^- ↔ Po ^4+ + 2e^- 0.9 V. So, if you know that Po ↔ Po ^2+ + 2e^- with -0.14 V
 * exists in aquious solution then give the reference and add it. Period.
 * Or better correct the article by deleting the Po-entry in the table and give in the comment to the editing your reference. This would have been most helpful! Thanks. :) —Preceding unsigned comment added by Achim1999 (talk • contribs) 21:42, 30 June 2009 (UTC)

Re polonium: "Po dissolves readily [in dilute HCl] to yield pink solutions of PoII" (from Greenwood & Earnshaw). It seems that this article is currently repeating a substantial amount of original research which has been brought over (in good faith) from German Wikipedia. The simplest way to help our German colleagues would be to clean up the article here! Physchim62 (talk) 22:45, 29 June 2009 (UTC)


 * Which original research do you mean? Call it explicitly! I added no OR nor did I copy (by sense) any statements from the german article! Moreover I searched up all references currently in the article and added them -- there was not a single before -- in the german WP there are NO references at all. It was pure luck, that I discovered the Encylopedia Britania online edition and there is the same picture as in the german article. And this I used als. Yes, this was my copy from the german WP if you want to call it so. :)
 * Regards, Achim1999 (talk) 23:27, 29 June 2009 (UTC)

untarnishable metals in President's Badge of Office
I like to cite from the above given URL: "The rim of the wheel is gold, and the twelve spokes are of non-tarnishable metals. The metals used are: palladium, nickel, titanium, iridium, niobium, tungsten, platinum, molybdenum, tantalum, rhodium, zirconium and cobalt".

Well, obviously, they accept H_2S as an attacking gas and thus can't use Ag & Hg. I wonder why they dislike Chromium. Some of the above metals (Ni & Co) are attacked slowly or even fast by certain deluted non-oxidizing acids. But if we demand this, non-attackable by non-oxidizing acids, this would rule out all metalls with negative Electrode potential, but you will immediately call this OR as long as none can give a reference. :-) But this is what probaly is needed at least to be called to be "resistant to corission". Some people call this last set of metals the "semi-noble metals". Regards, Achim1999 (talk) 22:11, 30 June 2009 (UTC)

More noble than what?
If "noble metal" is a vocab from human culture and not primarily a chemical discovery, then we should ask: what is the original "not noble metal"? During the Bronze Age, "metal" and "copper" might have been even synonyms. Every metal more noble than copper thus became a "noble metal". Mercury doesn't count because it is cheap and not even solid. —Preceding unsigned comment added by 84.134.240.129 (talk) 22:59, 2 July 2009 (UTC)
 * As I understand the concept, it is all about resistance of a metal to oxidation, measured through a specific tool (electrochemical reaction) - nothing to do with metal price or acceptance in the society. Materialscientist (talk) 01:28, 3 July 2009 (UTC)
 * In Chemistry, yes, of course. But the term "noble metal" is much older than Chemistry. I only wanted to point out why the border line between noble metal and non-noble metal is at a potential of around 0.4 V and not at a potential of exactly 0 V, where Chemistry and Physics would probably make a distinction and coin a new technical term. Is there a special border for resistance of a metal to oxidation at a potential of around 0.4 V? —Preceding unsigned comment added by 84.134.188.87 (talk) 06:26, 3 July 2009 (UTC)
 * If you take the german word "Edelmetall" and look up in a typical dictionary, you get the only translation "precious metal". This is a social valuable metal and you may read its WP-article here. On the other hand, both, "precious metal" and "noble metal" are translated to the german "Edelmetall". And yes, "noble metal" in french, (I just took a view into the french WP, I don't speak french,) looks like the english meaning of "noble metal". Thus it looks like to me that "noble metal" is indeed a term meaning resistant to oxidation and corossion.
 * I read somewhere that the word "noble metal" origins at least from about 1400 in Europe, but I have no reference for the date. It would be very interesting to get a linguist who can tell you something about the wording history. I was told from talk that in english the term "semi-noble metal" is extremly rarely used. This denotes in german "Halbedelmetalle" which are characterized having a standard-potential > 0, i.e. can not be attacked by non-oxydizing acids. On the other hand, I got not hand on any formal quanitative definition in chemistry of one of these two terms. Perhaps there is indeed no, and only a certain set of metals is accepted to be denoted as "noble metals", which must be/was enlarged by the discovery of the platinium-group metals, earliest(!) around the mid of 18th century. It would be nice to get a reference from this time.

A still have not heard the statements of any metallurgist, perhaps this is in wider use there. Regards,Achim1999 (talk) 10:25, 3 July 2009 (UTC)

Eletcrochemical potential of Polonium
I just checked the "Chemistry of Elements", 1988 edition, from Earnshaw and Greenwood

There is indeed stated, that Po is easily solubale in deluted HCl to give Po^2+, but no standard potential is given. On the other hand, in the same book, two potential-diagrams for Po in acid and in alcalic solution for various redox-reactions of the metal are given. Sadly the acid-diagram contradicts the above statement be stating it has a 0.65 V potential. As reference is given A. J. Bard, "Encyclopedia of the Electrochemical reactions". I looked up this 1975 reference. Very dissatisfactory, there is stated that tries to measure the potentials reliable were unsuccessfully. They gave only a list of slightly contradictive measurement results. The experimentators handle at most about 10^-8 g polonium and I guess only Po-210. I think as long as we can't give a more recent, and of cource reliable source resp. measurment, this value should be as unknown marked. Any comments? BYW: Forgive me my many typos, grammar and stylish slips. ;) Regards, Achim1999 (talk) 10:49, 1 July 2009 (UTC)

I like to cite from ''The Polonium Halides. Part I. Polonium Chlorides. By K. W. BAGNALLR,. W. M. D’EYE, and J. H. FREEMAN''
 * Since polonium is very soluble in hydrochloric acid and is probably 'largely involved in complex-formation (e.g., as PoCl62-), the acid should be a suitable solvent

see. And a further reference accessable online: 

Regards, Achim1999 (talk) 14:30, 3 July 2009 (UTC)


 * Regarding polonium, I did a miscalculation in above paragraph. Taking the data from CRC:
 * Po ↔ Po ^4+ + 4e^- with 0.76 V
 * Po^2^- ↔ Po ^4+ + 2e^- with 0.9 V.
 * Thus we get in the case of reversibility of these reactions:
 * Po ↔ Po ^2+ + 2e^- with 0.62 V
 * because 2*0.62 + 2*0.9 = 4*0.76
 * which is in nice agreement wich a value about 0.65 V.

Regards, Achim1999 (talk) 18:24, 3 July 2009 (UTC)

Is copper noble or not?
I came to this article to find out whether copper is classified as a noble metal and found no answer. The text says that it is and, elsewhere, that it isn't. The illustration near the top says it isn't. Very confusing! Cognita (talk) 03:52, 15 June 2013 (UTC)
 * I also found this confusing. Copper is considered noble by some definitions. I edited the image caption and the table to reflect this. Bcharles (talk) 15:59, 4 August 2013 (UTC)

Is silver a noble metal?
Contradiction problem: The article does not explicitly list silver as a noble metal in the list contained in the lead paragraph. The photo in the article does however silver as a noble metal. Someone who knows more than me, and can cite a verifiable source, should clarify the matter and either update the lead paragraph or delete the incorrect photo. N2e (talk) 23:38, 9 November 2008 (UTC)


 * The problem is not just with silver. The photo shows eleven noble metals; the list claims there are only seven.  They cannot both be correct.


 * Silver is definitely considered a noble metal in many contexts and has many of the properties associated with the descriptor "noble" (in the context of metals, at least.) I've added it, and if anyone has any problems with this addition, please state them here before reverting the change.--Scyldscefing (talk) 05:31, 15 March 2009 (UTC)


 * I am no chemist, but don't both silver and copper tarnish in the air? How can they therefore be noble metals?--Antigravity711 (talk) 08:27, 26 November 2014 (UTC)

Citations?
Although citations are indeed given for the standard electrode potentials, it is not clear what source actually uses this as a benchmark. I think the problem starts from 2009 already with this uncited addition of the table from User:Achim1999. Also, chemical intuition revolts. By electrode potential, ruthenium appears way lower than silver, yet for sure silver is more reactive (tarnishing with sulfur). So, not saying outright that it's original research, but there is a burden of proof that is not being met. Physchim62 also raised concerns about original research in 2009. The problem is also there in our articles on the superheavies (which is kind of my fault before I realised the problem). So I remove the text pending actual citations that use this as a benchmark.

For similar reasons I also remove the periodic table. There is not clear definition of decisive categories, I agree with Smokefoot above that this is mostly well-intentioned synthesis. Which I contributed to, and repent of. Double sharp (talk) 09:41, 12 August 2020 (UTC)

Is ___ a noble metal?
There seem to be a lot of "Is [metal] a noble metal?" sections on this talk page, so let's put all future "Is [metal] a noble metal?" posts here. 50.5.106.139 (talk) 04:31, 13 November 2016 (UTC)


 * First off: Aluminum. Al is mentioned in the article that seems to indicate that it's like other noble metals. But I've heard that Al is actually very reactive, and the Al on the surface oxidizes but stays there so the metal beneath can't. It doesn't rust and lose structure like Fe, but is reactive. (talk) 04:31, 13 November 2016 (UTC)
 * Citation: "Corrosion resistance can be excellent because a thin surface layer of aluminium oxide forms when the bare metal is exposed to air, effectively preventing further oxidation, in a process termed passivation." (Wikipedia Al article.) 50.5.106.139 (talk) 04:46, 13 November 2016 (UTC)


 * It would seem "valve metals" aren't very noble. Ref: http://www.ak-tremel.chemie.uni-mainz.de/1353.php  The fact that they oxidize, under otherwise fairly normal conditions (such as the protective oxide layer on Al, Ti, etc.), and can be oxidized electrochemically (anodization), or dissolved in acids (e.g., Ti dissolves in HCl to give a pink Ti(3+) solution), would discount them from being "noble" (given that "noble" isn't a very precise term to begin with).  — Preceding unsigned comment added by 2605:A000:140A:400A:5C53:42CD:72A5:B849 (talk) 05:50, 19 February 2017 (UTC)

It would seem to me that if we're going to consider tellurium, selenium isn't too far-fetched to include; it is often considered a heavy metal in the environmental literature. Also Hs through Lv are likely not to be as noble as expected if you could make enough; it's just like how the actinides should get less reactive from Th to Lr Ac to No changed this as my view on the group 3 thing changed Double sharp (talk) 12:12, 4 October 2020 (UTC), but actually get more reactive, because the increasing violence of the radiation they emit provides the necessary activation energy. Double sharp (talk) 03:42, 9 February 2018 (UTC)

My two cents
--Smokefoot (talk) 22:11, 23 October 2020 (UTC)
 * If this article, I would not mention radioactive elements. Who cares?  Nobility is a practical, qualitative description used by chemists. And we do not fabricate with radioactive metals. The focus on radioactive elements detracts from Wikipedia-Chem, because it comes across as nerdy lawyering (IMHO).
 * For myself, I dont consult physicists on what is noble, they consult me, so there would be no physics subsection.
 * The corrosion, geo, and oxide sections are discussing very intertwined topics. The main geo-fact (iMHO) is that these elemments are often found in nature in native form.
 * Chem of Materials is a weak journal to support the lede's claim definition.


 * Thank you. Nice to hear from you.
 * Yes, I'd thought about the limited relevance of radioactive elements, too. I'll probably get rid of them. DONE
 * I'll look again at that physics section; it may be that physicists have their own defintion of a noble metal cf. semimetal in the chemistry sense v in the physics sense. DONE
 * That's a good point about them being found in nature in native form; I'll add something along these lines. DONE
 * Chemistry of Materials is a reliable, peer-reviewed source published by the American Chemistry Society. Do you have a suggestion for a "better" definition and citation?
 * --- Sandbh (talk) 03:00, 24 October 2020 (UTC)
 * --- Sandbh (talk) 03:00, 24 October 2020 (UTC)

Reason for titanium's non-membership of noble metals.
The article states:
 * titanium, niobium, and tantalum are not included as noble metals despite the fact that they are very resistant to corrosion.

...but doesn't say why.

I would guess it's because thorium's non-reactivity is due to its oxide layer - it apparently corrodes quickly when the oxide layer cannot be maintained (e.g. due to erosion - the example I heard of was a high flowrate bend in a heat exchanger.) Also, as stated above by "many metals (e.g. titanium) are unreactive in bulk but very reactive when finely divided (Ti is pyrophoric, for example)."

I don't have sources. This is just a drive-by comment. --Chriswaterguy talk 09:00, 10 July 2014 (UTC)
 * The article contains a lot of "synthesis", the well-intentioned product of editors earnestly writing about a topic that is pretty poorly undefined. So dont fret about whether this flakey article is complete or not. --Smokefoot (talk) 17:19, 10 July 2014 (UTC)

FYI: Tantalum and niobium are soluble in fluoride solutions, so the part where it says they are "resistant to all acids" is simply false. While hydrofluoric acid is not a strong mineral acid (according to the Brønsted–Lowry theory), the fluoride ion has chemical wiles that are able to seduce many transition metals, forming complexes, and thus dissolving them. (Silicon is one of the non-transition-metal elements that's quite characteristic of this; the complex in fact forms an acid, hexafluorosilicic acid.) 2605:A000:140A:400A:5C53:42CD:72A5:B849 (talk) 05:53, 19 February 2017 (UTC)
 * Same thing happens for otherwise very resistant group 4 metals, too. Double sharp (talk) 07:38, 4 February 2021 (UTC)

Holleman Wiberg citation
In previous revisions of this article this citation was used to support Ag as a noble metal as well. I have strong suspicions that its current usage that does not include Ag is inaccurate. I have not the German version, but the English version clearly includes Ag on p. 1133 and 1248.

This refusal to include Ag is not a matter of almost all chemists agreeing. It seems, AFAIK, to be rather confined only to Rayner-Canham. Which is correctly noted in the article as only his contention. But why then is that version of the set the one in the lede? Indeed, is there even such a well-defined topic at all? Double sharp (talk) 07:27, 4 February 2021 (UTC)
 * P.S. I actually think his exclusion of silver makes sense, if you accept H2S as an attacking gas (so as to cut out mercury also). But I don't really see the literature consensus for it even among those who define and explain what they use the term for. (Regarding other exclusions, I would wonder if anyone has excluded Os for the smell of its oxidation product when finely divided. Certainly I would be tempted to do so.) Double sharp (talk) 09:27, 11 April 2021 (UTC)