Editing Noble metal (section)

===Oxides===
{| class="wikitable" style="font-size:90%; float:right; margin-left:20px"
|+ Oxide melting points, °C
|-
! Element  !! I !! II !! III !! IV !! VI !! VII !! VIII
|-
| Copper || 1232 || 1326 ||  ||  ||  ||  ||
|-
| Ruthenium ||  ||  ||  || d1300 ||  ||  || 25
|-
| Rhodium ||  ||  || d1100 || d1050 ||  ||  ||
|-
| Palladium ||  || d750{{#tag:ref|Palladium oxide PdO can be reduced to palladium metal by exposing it to hydrogen in ambient conditions<ref name="HW2001"/>|group=n}} ||  ||  ||  ||  ||
|-
| Silver || d200 || d100{{#tag:ref|Ag<sub>4</sub>O<sub>4</sub> is a mixed oxidation state compound silver in the oxidation state of +1 and +3.|group=n}} ||  ||  ||  ||  ||
|-
| Rhenium ||  ||  ||  || d1000 || d400 || 327 ||
|-
| Osmium ||  ||  ||  || d500 ||  ||  || 40
|-
| Iridium ||  ||  ||  || d1100 ||  ||  || 
|-
| Platinum ||  ||  ||  || 450 ||  ||  ||
|-
| Gold ||  ||  || d150 ||   ||   ||   ||
|-
| Mercury ||  || d500 ||   ||   ||   ||   ||
|-
| Strontium‡ ||  || 2430 ||   ||   ||   ||   ||
|-
| Molybdenum‡ ||  || ||  ||  || 801 ||  ||
|-
| Antimony<sup>MD</sup> ||  || ||655   ||   ||    ||   ||
|-
| Lanthanum‡ ||  ||  ||2320   ||   ||   ||   ||
|-
| Bismuth‡ ||  || || 817  ||   ||   ||    ||
|-
|colspan="8"|d = decomposes; ‡ = not a noble metal; <sup>MD</sup> = metalloid
|}

As long ago as 1890, Hiorns observed as follows:

:"'''Noble Metals.''' Gold, Platinum, Silver, and a few  rare metals. The members of this class have little or no tendency to unite with oxygen in the free state, and when placed in water at a red heat do not alter its composition. The oxides are readily decomposed by heat in consequence of the feeble affinity between the metal and oxygen."<ref>Hiorns AH 1890, ''[https://archive.org/details/mixedmetalsormet00hiorrich Mixed metals or metallic alloys]'', p. 7</ref>

Smith, writing in 1946, continued the theme:
:"There is no sharp dividing line [between 'noble metals' and 'base metals'] but perhaps the best definition of a noble metal is a metal whose oxide is easily decomposed at a temperature below a red heat."{{#tag:ref|Incipient red heat corresponds to 525 °C<ref>Hiorns RH 1890, Mixed metals or metallic alloys, MacMillian, New York, p. 5</ref>|group=n}}<ref>{{cite book |last=Smith |first=JC |date=1946 |title= The chemistry and metallurgy of dental materials |location= Oxford|publisher= Blackwell|page=40}}</ref>

:"It follows from this that noble metals...have little attraction for oxygen and are consequently not oxidised or discoloured at moderate temperatures."
Such nobility is mainly associated with the relatively high electronegativity values of the noble metals, resulting in only weakly polar covalent bonding with oxygen.<ref name="Kepp"/> The table lists the melting points of the oxides of the noble metals, and for some of those of the non-noble metals, for the elements in their most stable oxidation states.