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Noble metal
 
 
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Noble metals are metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
s that are resistant to corrosion
Corrosion

Corrosion means the breaking down of essential properties in a material due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen....
 or oxidation, unlike most base metal
Base metal

In chemistry, the term base metal is used informally to refer to a metal that oxidation or corrosion relatively easily, and reacts variably with diluted hydrochloric acid to form hydrogen....
s. They tend to be precious metal
Precious metal

A precious metal is a rare metallic chemical element of high economics value. Chemically, the precious metals are less reactivity than most elements, have high lustre , are softer or more ductility, and have higher melting points than other metals....
s, often due to rarity in the crust of the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
. The noble metals are considered to be (in order of increasing atomic number
Atomic number

In chemistry and physics, the atomic number is the number of protons found in the atomic nucleus of an atom. It is conventionally represented by the symbol Z....
)

Some of the noble metals can be dissolved in aqua regia
Aqua regia

Aqua regia is a highly corrosive, fuming yellow or red solution. The mixture is formed by freshly mixing concentrated nitric acid and concentrated hydrochloric acid, usually in a volumetric ratio of 1:3 respectively....
, a highly concentrated mixture of acid
Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
s.

The term can also be used in a relative sense. A "Galvanic series
Galvanic series

The galvanic series determines the noble metal of metals and semi-metals. When two metals are submerged in an electrolyte, while electrically connected, the less noble will experience galvanic corrosion....
" is a hierarchy of metals (or other electrically conductive materials, including composites and semimetal
Semimetal

A semimetal is a material with a small overlap in the energy of the Electrical conduction Electronic band structure and valence bands.However, the bottom of the conduction band is typically situated in a different part of momentum space than the top of the valence band....
s) 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.






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Noble metals are metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
s that are resistant to corrosion
Corrosion

Corrosion means the breaking down of essential properties in a material due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen....
 or oxidation, unlike most base metal
Base metal

In chemistry, the term base metal is used informally to refer to a metal that oxidation or corrosion relatively easily, and reacts variably with diluted hydrochloric acid to form hydrogen....
s. They tend to be precious metal
Precious metal

A precious metal is a rare metallic chemical element of high economics value. Chemically, the precious metals are less reactivity than most elements, have high lustre , are softer or more ductility, and have higher melting points than other metals....
s, often due to rarity in the crust of the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
. The noble metals are considered to be (in order of increasing atomic number
Atomic number

In chemistry and physics, the atomic number is the number of protons found in the atomic nucleus of an atom. It is conventionally represented by the symbol Z....
)
  • Ruthenium
    Ruthenium

    Ruthenium is a chemical element that has the symbol Ru and atomic number 44. A rare transition metal of the platinum group of the periodic table, ruthenium is found associated with platinum ores and used as a catalyst in some platinum alloys....
     
  • Rhodium
    Rhodium

    Rhodium is a chemical element that is a rare, silvery-white, hard transition metal and a member of the platinum group. Rhodium is found in platinum ores and is used in alloys with platinum and as a catalyst....
     
  • Palladium
    Palladium

    Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it palladium after the 2 Pallas, which in turn, was named after the epithet of the Greek mythology goddess Athena, acquired by her when she slew Athena#Pallas_Athena....
     
  • Osmium
    Osmium

    Osmium is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is the densest natural element....
  • Iridium
    Iridium

    Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 ?C....
  • Platinum
    Platinum

    Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements....
  • Gold
    Gold

    Gold is a chemical element with the symbol Au and atomic number 79. It is a highly sought-after precious metal, having been used as money, as a store of value, in jewelry, in sculpture, and for ornamentation since the beginning of recorded history....


Some of the noble metals can be dissolved in aqua regia
Aqua regia

Aqua regia is a highly corrosive, fuming yellow or red solution. The mixture is formed by freshly mixing concentrated nitric acid and concentrated hydrochloric acid, usually in a volumetric ratio of 1:3 respectively....
, a highly concentrated mixture of acid
Acid

An acid is traditionally considered any chemical compound that, when dissolved in water, gives a solution with a hydrogen ion Activity greater than in pure water, i.e....
s.

The term can also be used in a relative sense. A "Galvanic series
Galvanic series

The galvanic series determines the noble metal of metals and semi-metals. When two metals are submerged in an electrolyte, while electrically connected, the less noble will experience galvanic corrosion....
" is a hierarchy of metals (or other electrically conductive materials, including composites and semimetal
Semimetal

A semimetal is a material with a small overlap in the energy of the Electrical conduction Electronic band structure and valence bands.However, the bottom of the conduction band is typically situated in a different part of momentum space than the top of the valence band....
s) 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
Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek language ??afe?? : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead....
 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
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
 and stainless steel
Stainless steel

In metallurgy, stainless steel is defined as a steel alloy with a minimum of 10% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel , but it is not stain-proof....
 in conditions of varying pH
PH

pH is a measure of the Acid or Base of a solution. It is defined as the cologarithm of the Activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations....
.

In physics the definition of a noble metal is even more strict. It is required that the d-bands
Atomic orbital

An atomic orbital is a mathematical function that describes the wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus....
 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. For platinum two d-bands cross the Fermi level, changing its chemical behaviour; it is used (in contrast to gold, for example) as a catalyst. The different reactivity can easily be seen while preparing clean metal surfaces in ultra high vacuum
Ultra high vacuum

Ultra high vacuum is the vacuum regime characterised by pressures lower than about 10-7 pascal or 100 nanopascals . UHV requires the use of special materials in creating the vacuum system, extreme cleanliness to maintain the vacuum system, and baking of the entire system to remove water and other trace gases that adsorb on the su...
; surfaces of 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
Carbon monoxide

Carbon monoxide, with the chemical formula CO, is a colorless and odorless, tasteless, yet highly toxic gas. Its molecules consist of one carbon atom covalent bond to one oxygen atom....
 very quickly.

See also

  • Base metal
    Base metal

    In chemistry, the term base metal is used informally to refer to a metal that oxidation or corrosion relatively easily, and reacts variably with diluted hydrochloric acid to form hydrogen....
  • Precious metal
    Precious metal

    A precious metal is a rare metallic chemical element of high economics value. Chemically, the precious metals are less reactivity than most elements, have high lustre , are softer or more ductility, and have higher melting points than other metals....
  • Synthesis of noble metals
    Synthesis of noble metals

    Synthesis of noble metals refers to the realization of the age-old dream of alchemy: to artificially produce noble metals. This synthesis is only possible with methods utilizing nuclear physics, currently involving either nuclear reactors or particle accelerators....
Silver and copper corrode 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.

External links

  • To see which bands cross the Fermi level, the Fermi surface
    Fermi surface

    In condensed matter physics, the Fermi surface is an abstract boundary useful for predicting the thermal, electrical, magnetic, and optical properties of metals, semimetals, and doped semiconductors....
    s of almost all the metals can be found at the
  • The following article might also clarify the correlation between band structure and the term noble metal: