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Home > What is Copper

What is Copper?




Copper, silver and gold are in group 11 of the periodic table, and they share certain attributes: they have one s-orbital electron on top of a filled d-electron shell and are characterized by high ductility and electrical conductivity. The filled d-shells in these elements do not contribute much to the interatomic interactions, which are dominated by the s-electrons through metallic bonds. Contrary to metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are relatively weak. This explains the low hardness and high ductility of single crystals of copper.[1] At the macroscopic scale, introduction of extended defects to the crystal lattice, such as grain boundaries, hinders flow of the material under applied stress thereby increasing its hardness. For his reason, copper is usually supplied in a fine-grained polycrystalline form, which has greater strength than monocrystalline forms.

The low hardness of copper partly explains its high electrical (59.6×106 S/m) and thus also high thermal conductivity, which are the second highest among pure metals at room temperature.[3] This is because the resistivity to electron transport in metals at room temperature mostly originates from scattering of electrons on thermal vibrations of the lattice, which are relatively weak for a soft metal.[1] The maximum permissible current density of copper in open air is approximately 3.1×106 A/m2 of cross-sectional area, above which it begins to heat excessively.[4] As with other metals, if copper is placed against another metal, galvanic corrosion will occur.[5]
Together with osmium (bluish), caesium (yellow) and gold (yellow), copper is one of only four elemental metals with a natural color other than gray or silver.[6] Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells is such that it corresponds to orange light. The same mechanism accounts for the yellow color of gold.[1]


Copper forms a rich variety of compounds with oxidation states +1 and +2, which are often called cuprous and cupric, respectively.[7] It does not react with water, but it slowly reacts with atmospheric oxygen forming a layer of brown-black copper oxide. In contrast to the oxidation of iron by wet air, this oxide layer stops the further, bulk corrosion. A green layer of verdigris (copper carbonate) can often be seen on old copper constructions, such as the Statue of Liberty, the largest copper statue in the world build using repoussé and chasing.[8] Hydrogen sulfides and sulfides react with copper to form various copper sulfides on the surface. In the latter case, the copper corrodes, as is seen when copper is exposed to air containing sulfur compounds.[9] Oxygen-containing ammonia solutions give water-soluble complexes with copper, as do oxygen and hydrochloric acid to form copper chlorides and acidified hydrogen peroxide to form copper(II) salts. Copper(II) chloride and copper comproportionate to form copper(I) chloride.[10]



Copper can be found as either native copper or as part of minerals. Native copper is a polycrystal, with the largest single crystals found to date measuring 4.4×3.2×3.2 cm.[14] The largest mass of elemental copper weighed 420 tonnes and was found in 1857 on the Keweenaw Peninsula in Michigan, US.[15] There are many examples of copper-containing minerals: chalcopyrite and chalcocite are copper sulfides, azurite and malachite are copper carbonates and cuprite is a copper oxide.[3] Copper is present in the Earth's crust at a concentration of about 50 parts per million (ppm),[15] and is also synthesised in massive stars.[16]




The price of copper has historically been unstable,[20] and it quintupled from the 60-year low of US$0.60/lb (US$1.32/kg) in June 1999 to US$3.75 per pound (US$8.27/kg) in May 2006. It dropped to US$2.40/lb (US$5.29/kg) in February 2007, then rebounded to US$3.50/lb (US$7.71/kg) in April 2007.[21] In February 2009, weakening global demand and a steep fall in commodity prices since the previous year's highs left copper prices at US$1.51/lb.[22]
The concentration of copper in ores averages only 0.6%, and most commercial ores are sulfides, especially chalcopyrite (CuFeS2) and to a lesser extent chalcocite (Cu2S).[23] These minerals are concentrated from crushed ores to the level of 10–15% copper by froth flotation or bioleaching.[24] Heating this material with silica in flash smelting removes much of the iron as slag. The process exploits the greater ease of converting iron sulfides into its oxides, which in turn react with the silica to form the silicate slag, which floats on top of the heated mass. The resulting copper matte consisting of Cu2S is then roasted to convert all sulfides into oxides:[23]
2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2
The cuprous oxide is converted to blister copper upon heating:
2 Cu2O → 4 Cu + O2

Cu2+ + 2 e– → Cu




Electronics and related devices



Architecture and industry

Because of the waterproof nature of copper, it has been used as the roofing material of many buildings since ancient times. The green colour on these buildings is due to a long-term chemical reaction: copper is first oxidised to copper(II) oxide, then to cuprous and cupric sulfide and finally to copper(II) carbonate, also called verdigris, which is highly corrosion-resistant.[62] The copper used in this application is phosphorus deoxidized copper (Cu-DHP).[63] Lightning rods use copper as a means to divert electric current throughout the ground instead of destroying the main structure.[64] Copper has excellent brazing and soldering properties and can be welded; the best results are obtained with gas metal arc welding.[65]


Copper in alloys

See also: List of copper alloys



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