noble metal

The noble metals including mercury and rhenium together with the non-noble metal copper ordered according their position in the periodic table of the elements

Noble metals are metals that are resistant to corrosion and oxidation in moist air, unlike most base metals.^{[citation needed]} They tend to be precious, often due to the rarity in the Earth's crust. The noble metals are considered to be (in order of increasing atomic number)^[1]

ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold .

Other sources include mercury^[2][3][4] or even rhenium^[5] as a noble metal. On the other hand, neither titanium nor niobium nor tantalum are called noble metals despite the fact that they are very resistant to corrosion.

Noble metals should not be confused with precious metals (although many noble metals are precious).

Introduction

Palladium, osmium, platinum, gold and mercury can be dissolved in aqua regia, a highly concentrated mixture of hydrochloric acid and nitric acid, but iridium and silver can not. Ruthenium can be dissolved in aqua regia only when in the presence of oxygen, while rhodium must be in a fine pulverized form. Niobium and tantalum are resistant to acids, including aqua regia. ^[6]

This term can also be used in a relative sense, considering "noble" as an adjective for the word "metal". A "galvanic series" is a hierarchy of metals (or other electrically conductive materials, including composites and semimetals) that runs from noble to active, and allows designers to see at a glance how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver (even though it is alchemically more base) and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.^[7]

In chemistry and physics , the definition of a noble metal is even more strict. It is required that the d-bands of the electronic structure are filled. Taking this into account, only copper, silver and gold are noble metals, as all d-like band are filled and don't cross the Fermi level.^[8] For platinum two d-bands cross the Fermi level, changing its chemical behaviour; it is used as a catalyst. The different reactivity can easily be seen while preparing clean metal surfaces in ultra high vacuum; surfaces of "physical defined" noble metals (e.g., gold) are easy to clean and stay clean for a long time, while those of platinum or palladium, for example, are covered by carbon monoxide very quickly.^[9]

Electrochemistry

Metalic elements, including several non-noble metals, sorted by their chemical "nobility" (noble metals bolded): ^[10]

The column group denotes its position in the periodic table, hence electronic configuration. The simplified reactions, listed in the next column, can also be read in detail from the Pourbaix diagrams of the considered element in water. Finally the column potential indicates the electric potential of the element measured against a H-electrode in aqueous, pH 7 solution. All missing elements in this table are either not metals or have a negative standard potential.

Antimony and polonium are considered metalloids and thus can not be noble metals. Also chemists and metallurgists consider copper and bismuth not noble metals because they easily oxidize due to the reaction O₂ + 2 H₂O + 4 e⁻ ⇄ 4 OH⁻(aq) +0.40 V which is possible in moist air.

Silver and copper film over and oxidize easily and readily, thus the copper sheets with a patina of oxidation used in architectural designs and the resultant market for a myriad of silver polishing compounds. The film over of Silver is due to its high sensibility to hydrogen sulfide. Chemically patina is caused by an attack of oxygen in wet air (solvability!) and by CO₂ afterward.^[6] On the other hand, rhenium coated mirrors are said to be very durable,^[6] despite the fact that rhenium and technetium are said to tarnish slowly in moist atmosphere.^[12]

See also

• Base metal
• Precious metal

References

Notes

External links

• noble metal - chemistry Encyclopedia Britannica, online edition
• To see which bands cross the Fermi level, the Fermi surfaces of almost all the metals can be found at the Fermi Surface Database
• The following article might also clarify the correlation between band structure and the term noble metal: Hüger, E.; Osuch, K. (2005). "Making a noble metal of Pd". EPL (Europhysics Letters) 71: 276. doi:10.1209/epl/i2005-10075-5.