The Eight Precious Metals Considered Noble Metals
Most metallic chemical elements are known to easily oxidize and corrode, especially at high temperatures. Metals that have excellent resistance to oxidation and corrosion, even at high temperatures, are called noble metals. While there is no strict definition for this group of metals, it usually includes those that are extremely rare.
Thus, metals that are labeled "precious" are also considered noble metals (note, however, that noble metals are not necessarily precious metals). There are nine known precious metals - gold, platinum, iridium, palladium, osmium, silver, rhodium, ruthenium, and rhenium. With the exception of the latter, all are considered noble metals.
Using the "aqua regia" test (aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid, which is highly corrosive), here's how the eight precious-noble metals react:
• Gold, platinum, palladium, and osmium dissolve.
• Ruthenium dissolves only in the presence of oxygen.
• Rhodium dissolves only when it is in a fine, pulverized form.
• Iridium and silver do not dissolve under any condition.
In another definition, noble metals may refer to electrically conductive elements. Here, the term "noble" serves as a modifying word, such that the electrical conductivity of materials is graded from noble to active. Using this definition for noble metals then, we will find that silver is less noble than, say, graphite (although graphite is an elemental form of carbon and, therefore, is not considered a metal).
Below is a comparison of the chemical nobility of the precious metals (again, excluding rhenium), with details of their specific electronic configurations, simplified reactions (as per the pH diagram), and electrode potentials. The list is presented in order of increasing atomic number.
1. Ruthenium - VIIIb/5; Ru -> Ru2+ + 2 e-; 0.455 V
2. Rhodium - VIIIb/5; Rh -> Rh2+ + 2 e-; 0.600 V
3. Palladium - VIIIb/5; Pd -> Pd2+ + 2 e-; 0.987 V
4. Silver - Ib/5; Ag -> Ag+ + e-; 0.7996 V
5. Osmium - VIIIb/6; Os + 4 H2O -> OsO4 + 8 H+ + 8 e-; 0.838 V
6. Iridium - VIIIb/6; Ir -> Ir3+ + 3 e-; 1.156 V
7. Platinum - VIIIb/6; Pt -> Pt2+ + 2 e-; 1.18 V
8. Gold - Ib/6; Au -> Au3+ + 3 e-; 1.498 V
The second item appearing after the name of the metal (the three items are separated by semi-colons) represents the metal's reaction in water. In the pH diagram, the pH symbol is labeled on the horizontal axis to signify the -log function of the concentration of H+ ion. The lines, which represent equilibrium for the concentration, are drawn for ions at unit activity. Other concentrations may be represented by additional lines. The voltage potential is represented by a vertical axis, which is labeled Eh, where "h" stands for hydrogen.
Physics has an even more strict definition for noble metals. Here, it is required that the electronic structure's d-bands are filled. If this definition is followed, only gold and silver (among the precious metals) qualify as noble metals. Also note that the varying reactivity of the precious metals can readily be observed while preparing their surfaces in the vacuum regime called ultra high vacuum.
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