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?
