To clarify, by noble metals in this case I mean all platinum metals + gold, no copper, silver or rhenium. These noble metals are described as having low reactivity, but which one is least reactive or most "noble" of them all?

I am chemistry noob and I have feeling this might greatly depend on what type of element/molecule is trying to react with given noble metal so to make this simple I will narrow this question down to oxidation by diatomic oxygen in air at room and elevated temperatures.

I will now writte my current knowledge and thoughts on why some noble metals might be more noble than other. Its just window into my head so you can see what I think so you are able to correct what is wrong, approve what is right and expand on what is missing.

My current understanding of what makes noble metal noble, is that its somewhere in middle of periodic table, their electron shells are half full. Elements on the left side are strongly reactive because their shells are only sparsely filled, so they give away electrons easily, they have low ionisation energy.

Elements on the right side, excluding noble gasses, are strongly reactive because they have almost full electron shell and they have tendency to tear out and "steal" electrons from less electronegative elements. That is why noble metals are in middle,they don't want to give away or get inside electrons very much.

That would be my first trait of noble metal, the more in middle it is, the more noble it is. Since periodic table is 18 numbers wide, 18 is even number so there is no integer exactly in middle, the middle is between 9 and 10.

That means elements in group 9 and 10 should be more noble than those in 8 and 11 group because they are closer to middle, so rhodium, palladium, iridium, and platinum should be more noble than gold, ruthenium, and osmium.

Another thing that I believe is that the heavier elements don't hold their electrons as strongly as lighter elements because the inner electron shells shield the valence electron shell from attractive force of nucleus. If this is true, then noble metals in period 5 should be more noble than those in period 6, that is ruthenium, rhodium, and palladium more noble than osmium, iridium, platinum, and gold.

Third factor in my opinion is first ionisation energy, the higher it is, the more it should be resistant to reacting with oxygen. From highest to lowest first ionisation energy: Gold = 9.2255 eV, iridium = 8.967 eV, platinum = 8.9587 eV, osmium = 8.4382 eV, palladium = 8.3369 eV, rhodium = 7.4589 eV, ruthenium = 7.3605 eV. If this is true, gold should be most noble, and ruthenium least noble.

I read one study about oxidation resistance at high temperatures, the study said rhodium was most resistant, even more than iridium or platinum. I also heard Platinum doesn't oxidise in air at any temperature bellow melting point, that seems contradictory to the paper I read.Furthermore, spark plugs are mode from iridium and platinum but not rhodium.

If we had all these noble metals side by side in identical shape, lets say cube with highly polished sides, and we heated them in air atmosphere to 100, 500, and 1000 degrees Celsius, which one would oxidise least?

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    $\begingroup$ The spark plug application may depend on availability/cost as well as chemistry. $\endgroup$ – Oscar Lanzi Apr 5 '18 at 12:26
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    $\begingroup$ @OscarLanzi - quite true, and add resistance to erosion by the spark as well. Way more variables than just 'reactivity' going on for any engineering application. $\endgroup$ – Jon Custer Apr 5 '18 at 13:13
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    $\begingroup$ Let's see. Pt (78) has 1st ionization energy = 8.96 eV, Pd (46) = 8.34 eV. Uh ... How do heavier atoms hold their electrons most loosely within their groups? Relativistic effects can blow up the expected periodic trends. $\endgroup$ – Oscar Lanzi Apr 5 '18 at 14:06
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    $\begingroup$ The answer to your general question (as usual) is "depends". And the setup in your last sentence is very specific, and arbitrary on the border to irrelevant. What is it you really want to know? What are you trying to do? $\endgroup$ – Karl Apr 5 '18 at 16:17
  • $\begingroup$ I want to know,what noble metal is most oxidation resistant in air at 100,500 and 1000 degrees celsius.I gave three temperatures becose I think some noble metals might loose their "nobility" faster than others as temperature rises.If that is not the case,then it can simply be reduced to,what noble metal is most oxidation resistant in air. $\endgroup$ – wav scientist Apr 5 '18 at 16:29

Ruthenium, Rhodium, Palladium, Osmium, Iridium, Platinum, Gold

... I will narrow this question down to oxidation by diatomic oxygen in air at room and elevated temperatures.

According to this criterion, gold is the most oxidation resistant of the above metals.

First of all, gold oxide cannot be obtained by reaction of gold and oxygen ($\ce{O2}$) at standard pressure. Gold oxide ($\ce{Au2O3}$) decomposes to gold and oxygen above $\pu{160°C}$.

  • Ruthenium reacts with oxygen (air) above $\pu{700°C}$ to give $\ce{RuO4}$. Ruthenium powder burns in air.
  • Rhodium reacts with oxygen (air) above $\pu{700°C}$ to give $\ce{Rh2O3}$.
  • Palladium powder burns in air.
  • Osmium powder reacts with oxygen (air) at room temperature to give $\ce{OsO4}$.
  • Iridium reacts with oxygen at high temperatures to give $\ce{IrO2}$ or $\ce{IrO3}$. Iridium powder burns in air.
  • Platinum powder burns in air.
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  • $\begingroup$ So up to 160 celsius,Gold is most oxidation resistant? $\endgroup$ – wav scientist Apr 5 '18 at 19:24
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    $\begingroup$ Correct. It does not directly react with oxygen. $\endgroup$ – aventurin Apr 5 '18 at 19:25
  • $\begingroup$ Does the Gold start to oxidise at 160 C or is it merely the temperature point at which the Gold oxide starts to separate into Gold and oxygen? What is even this separation thing,it reminds me of self-healing materials,except instead of healing a crack,it "heals" oxidation.Also,is the fact that Gold is most oxidation resistant result of Gold having the highest first ionisation energy of the noble metals? $\endgroup$ – wav scientist Apr 5 '18 at 19:33
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    $\begingroup$ Gold does not directly react with oxygen. Neither above nor below 160°C. $\endgroup$ – aventurin Apr 5 '18 at 19:35
  • $\begingroup$ Whereas Osmium is the biggest pain in the butt. If you leave the pure stuf alone in a room it will happily start making $OsO_4$ and turning the walls green ... $\endgroup$ – Gwyn Jul 16 at 23:53

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