copper

Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Copper is one of the few metals that can occur in nature in a directly usable metallic form (native metals). This led to very early human use in several regions, from circa 8000 BC. Thousands of years later, it was the first metal to be smelted from sulfide ores, circa 5000 BC; the first metal to be cast into a shape in a mold, c. 4000 BC; and the first metal to be purposely alloyed with another metal, tin, to create bronze, c. 3500 BC.

In the Roman era, copper was mined principally on Cyprus, the origin of the name of the metal, from ''aes cyprium'' (metal of Cyprus), later corrupted to ''cuprum'' (Latin). ''Coper'' (Old English) and ''copper'' were derived from this, the later spelling first used around 1530.

Commonly encountered compounds are copper(II) salts, which often impart blue or green colors to such minerals as azurite, malachite, and turquoise, and have been used widely and historically as pigments.

Copper used in buildings, usually for roofing, oxidizes to form a green verdigris (or patina). Copper is sometimes used in decorative art, both in its elemental metal form and in compounds as pigments. Copper compounds are used as bacteriostatic agents, fungicides, and wood preservatives.

Copper is essential to all living organisms as a trace dietary mineral because it is a key constituent of the respiratory enzyme complex cytochrome c oxidase. In molluscs and crustaceans, copper is a constituent of the blood pigment hemocyanin, replaced by the iron-complexed hemoglobin in fish and other vertebrates. In humans, copper is found mainly in the liver, muscle, and bone. The adult body contains between 1.4 and 2.1 mg of copper per kilogram of body weight.

Characteristics

Physical

Copper, silver, and gold are in group 11 of the periodic table; these three metals have one s-orbital electron on top of a filled d- electron shell and are characterized by high ductility, and electrical and thermal conductivity. The filled d-shells in these elements contribute little to interatomic interactions, which are dominated by the s-electrons through metallic bonds. Unlike metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are relatively weak. This observation explains the low hardness and high ductility of single crystals of copper. 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 this reason, copper is usually supplied in a fine-grained polycrystalline form, which has greater strength than monocrystalline forms.

The softness of copper partly explains its high electrical conductivity () and high thermal conductivity, second highest (second only to silver) among pure metals at room temperature. This is because the resistivity to electron transport in metals at room temperature originates primarily from scattering of electrons on thermal vibrations of the lattice, which are relatively weak in a soft metal. The maximum permissible current density of copper in open air is approximately of cross-sectional area, above which it begins to heat excessively.

Copper is one of a few metallic elements with a natural color other than gray or silver. 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 corresponds to orange light.

As with other metals, if copper is put in contact with another metal, galvanic corrosion will occur.

Chemical

Copper does not react with water, but it does slowly react with atmospheric oxygen to form a layer of brown-black copper oxide which, unlike the rust that forms on iron in moist air, protects the underlying metal from further corrosion ( passivation). A green layer of verdigris (copper carbonate) can often be seen on old copper structures, such as the roofing of many older buildings and the Statue of Liberty. Copper tarnishes when exposed to some sulfur compounds, with which it reacts to form various copper sulfides.

Isotopes

There are 29 isotopes of copper. and are stable, with comprising approximately 69% of naturally occurring copper; both have a spin of . The other isotopes are radioactive, with the most stable being with a half-life of 61.83 hours. Seven metastable isotopes have been characterized; is the longest-lived with a half-life of 3.8 minutes. Isotopes with a mass number above 64 decay by β⁻, whereas those with a mass number below 64 decay by β⁺. , which has a half-life of 12.7 hours, decays both ways.

and have significant applications. is used in Cu-PTSM as a radioactive tracer for positron emission tomography.

Occurrence

Copper is produced in massive stars and is present in the Earth's crust in a proportion of about 50 parts per million (ppm). In nature, copper occurs in a variety of minerals, including native copper, copper sulfides such as chalcopyrite, bornite, digenite, covellite, and chalcocite, copper sulfosalts such as tetrahedite-tennantite, and enargite, copper carbonates such as azurite and malachite, and as copper(I) or copper(II) oxides such as cuprite and tenorite, respectively. The largest mass of elemental copper discovered weighed 420 tonnes and was found in 1857 on the Keweenaw Peninsula in Michigan, US. Native copper is a polycrystal, with the largest single crystal ever described measuring . Copper is the 25th most abundant element in Earth's crust, representing 50 ppm compared with 75 ppm for zinc, and 14 ppm for lead.

Typical background concentrations of copper do not exceed in the atmosphere; in soil; in vegetation; 2 μg/L in freshwater and in seawater.

Production

Most copper is mined or extracted as copper sulfides from large open pit mines in porphyry copper deposits that contain 0.4 to 1.0% copper. Sites include Chuquicamata, in Chile, Bingham Canyon Mine, in Utah, United States, and El Chino Mine, in New Mexico, United States. According to the British Geological Survey, in 2005, Chile was the top producer of copper with at least one-third of the world share followed by the United States, Indonesia and Peru. Copper can also be recovered through the in-situ leach process. Several sites in the state of Arizona are considered prime candidates for this method. The amount of copper in use is increasing and the quantity available is barely sufficient to allow all countries to reach developed world levels of usage. An alternative source of copper for collection currently being researched are polymetallic nodules, which are located at the depths of the Pacific Ocean approximately 3000–6500 meters below sea level. These nodules contain other valuable metals such as cobalt and nickel.

Reserves and prices

Copper has been in use at least 10,000 years, but more than 95% of all copper ever mined and smelted has been extracted since 1900. As with many natural resources, the total amount of copper on Earth is vast, with around 10¹⁴ tons in the top kilometer of Earth's crust, which is about 5 million years' worth at the current rate of extraction. However, only a tiny fraction of these reserves is economically viable with present-day prices and technologies. Estimates of copper reserves available for mining vary from 25 to 60 years, depending on core assumptions such as the growth rate. Recycling is a major source of copper in the modern world. Because of these and other factors, the future of copper production and supply is the subject of much debate, including the concept of peak copper, analogous to peak oil.

The price of copper has historically been unstable, and its price increased from the 60-year low of US$0.60/lb (US$1.32/kg) in June 1999 to $3.75 per pound ($8.27/kg) in May 2006. It dropped to $2.40/lb ($5.29/kg) in February 2007, then rebounded to $3.50/lb ($7.71/kg) in April 2007. In February 2009, weakening global demand and a steep fall in commodity prices since the previous year's highs left copper prices at $1.51/lb ($3.32/kg). Between September 2010 and February 2011, the price of copper rose from £5,000 a metric ton to £6,250 a metric ton.

Methods

The concentration of copper in ores averages only 0.6%, and most commercial ores are sulfides, especially chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄) and, to a lesser extent, covellite (CuS) and chalcocite (Cu₂S). Conversely, the average concentration of copper in polymetallic nodules is estimated at 1.3%. The methods of extracting copper as well as other metals found in these nodules include sulphuric leaching, smelting and an application of the Cuprion process. For minerals found in land ores, they are concentrated from crushed ores to the level of 10–15% copper by froth flotation or bioleaching. 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 oxides, which in turn react with the silica to form the silicate slag that floats on top of the heated mass. The resulting ''copper matte,'' consisting of Cu₂S, is roasted to convert the sulfides into oxides:
:2 Cu₂S + 3 O₂ → 2 Cu₂O + 2 SO₂
The cuprous oxide reacts with cuprous sulfide to converted to ''blister'' copper upon heating:
:2 Cu₂O + Cu₂S → 6 Cu + 2 SO₂
The Sudbury matte process converted only half the sulfide to oxide and then used this oxide to remove the rest of the sulfur as oxide. It was then electrolytically refined and the anode mud exploited for the platinum and gold it contained. This step exploits the relatively easy reduction of copper oxides to copper metal. Natural gas is blown across the blister to remove most of the remaining oxygen and electrorefining is performed on the resulting material to produce pure copper:
:Cu²⁺ + 2 e⁻ → Cu

Recycling

Like aluminium, copper is recyclable without any loss of quality, both from raw state and from manufactured products. In volume, copper is the third most recycled metal after iron and aluminium. An estimated 80% of all copper ever mined is still in use today. According to the International Resource Panel's Metal Stocks in Society report, the global per capita stock of copper in use in society is 35–55 kg. Much of this is in more-developed countries (140–300 kg per capita) rather than less-developed countries (30–40 kg per capita).

The process of recycling copper is roughly the same as is used to extract copper but requires fewer steps. High-purity scrap copper is melted in a furnace and then reduced and cast into billets and ingots; lower-purity scrap is refined by electroplating in a bath of sulfuric acid.

Alloys

Numerous copper alloys have been formulated, many with important uses. Brass is an alloy of copper and zinc. Bronze usually refers to copper- tin alloys, but can refer to any alloy of copper such as aluminium bronze. Copper is one of the most important constituents of silver and karat gold solders used in the jewelry industry, modifying the color, hardness and melting point of the resulting alloys. Some lead-free solders consist of tin alloyed with a small proportion of copper and other metals.

The alloy of copper and nickel, called cupronickel, is used in low-denomination coins, often for the outer cladding. The US five-cent coin (currently called a ''nickel'') consists of 75% copper and 25% nickel in homogeneous composition. Prior to the introduction of cupronickel, which was widely adopted by countries in the latter half of the 20th century, alloys of copper and silver were also used, with the United States using an alloy of 90% silver and 10% copper until 1965, when circulating silver was removed from all coins with the exception of the Half dollar - these were debased to an alloy of 40% silver and 60% copper between 1965 and 1970. The alloy of 90% copper and 10% nickel, remarkable for its resistance to corrosion, is used for various objects exposed to seawater, though it is vulnerable to the sulfides sometimes found in polluted harbors and estuaries. Alloys of copper with aluminium (about 7%) have a golden color and are used in decorations. '' Shakudō'' is a Japanese decorative alloy of copper containing a low percentage of gold, typically 4–10%, that can be patinated to a dark blue or black color.

Compounds

Copper forms a rich variety of compounds, usually with oxidation states +1 and +2, which are often called ''cuprous'' and ''cupric'', respectively. Copper compounds, whether organic complexes or organometallics, promote or catalyse numerous chemical and biological processes.

Binary compounds

As with other elements, the simplest compounds of copper are binary compounds, i.e. those containing only two elements, the principal examples being oxides, sulfides, and halides. Both cuprous and cupric oxides are known. Among the numerous copper sulfides, important examples include copper(I) sulfide and copper(II) sulfide.

Cuprous halides with fluorine, chlorine, bromine, and iodine are known, as are cupric halides with fluorine, chlorine, and bromine. Attempts to prepare copper(II) iodide yield only copper(I) iodide and iodine.
:2 Cu²⁺ + 4 I⁻ → 2 CuI + I₂

Coordination chemistry

Copper forms coordination complexes with ligands. In aqueous solution, copper(II) exists as . This complex exhibits the fastest water exchange rate (speed of water ligands attaching and detaching) for any transition metal aquo complex. Adding aqueous sodium hydroxide causes the precipitation of light blue solid copper(II) hydroxide. A simplified equation is:
:Cu²⁺ + 2 OH⁻ → Cu(OH)₂
Aqueous ammonia results in the same precipitate. Upon adding excess ammonia, the precipitate dissolves, forming tetraamminecopper(II):
: + 4 NH₃ → + 2 H₂O + 2 OH⁻
Many other oxyanions form complexes; these include copper(II) acetate, copper(II) nitrate, and copper(II) carbonate. Copper(II) sulfate forms a blue crystalline pentahydrate, the most familiar copper compound in the laboratory. It is used in a fungicide called the Bordeaux mixture.

Polyols, compounds containing more than one alcohol functional group, generally interact with cupric salts. For example, copper salts are used to test for reducing sugars. Specifically, using Benedict's reagent and Fehling's solution the presence of the sugar is signaled by a color change from blue Cu(II) to reddish copper(I) oxide. Schweizer's reagent and related complexes with ethylenediamine and other amines dissolve cellulose. Amino acids form very stable chelate complexes with copper(II). including in the form of metal-organic biohybrids (MOBs). Many wet-chemical tests for copper ions exist, one involving potassium ferrocyanide, which gives a brown precipitate with copper(II) salts.

Organocopper chemistry

Compounds that contain a carbon-copper bond are known as organocopper compounds. They are very reactive towards oxygen to form copper(I) oxide and have many uses in chemistry. They are synthesized by treating copper(I) compounds with Grignard reagents, terminal alkynes or organolithium reagents; in particular, the last reaction described produces a Gilman reagent. These can undergo substitution with alkyl halides to form coupling products; as such, they are important in the field of organic synthesis. Copper(I) acetylide is highly shock-sensitive but is an intermediate in reactions such as the Cadiot-Chodkiewicz coupling and the Sonogashira coupling. Conjugate addition to enones and carbocupration of alkynes can also be achieved with organocopper compounds. Copper(I) forms a variety of weak complexes with alkenes and carbon monoxide, especially in the presence of amine ligands.

Copper(III) and copper(IV)

Copper(III) is most often found in oxides. A simple example is potassium cuprate, KCuO₂, a blue-black solid. The most extensively studied copper(III) compounds are the cuprate superconductors. Yttrium barium copper oxide (YBa₂Cu₃O₇) consists of both Cu(II) and Cu(III) centres. Like oxide, fluoride is a highly basic anion and is known to stabilize metal ions in high oxidation states. Both copper(III) and even copper(IV) fluorides are known, K₃CuF₆ and Cs₂CuF₆, respectively.

Some copper proteins form oxo complexes, which also feature copper(III). With tetrapeptides, purple-colored copper(III) complexes are stabilized by the deprotonated amide ligands.

Complexes of copper(III) are also found as intermediates in reactions of organocopper compounds. For example, in the Kharasch–Sosnovsky reaction.

History

A timeline of copper illustrates how this metal has advanced human civilization for the past 11,000 years.

Prehistoric

Copper Age

Copper occurs naturally as native metallic copper and was known to some of the oldest civilizations on record. The history of copper use dates to 9000 BC in the Middle East; a copper pendant was found in northern Iraq that dates to 8700 BC. Evidence suggests that gold and meteoric iron (but not smelted iron) were the only metals used by humans before copper. The history of copper metallurgy is thought to follow this sequence: First, cold working of native copper, then annealing, smelting, and, finally, lost-wax casting. In southeastern Anatolia, all four of these techniques appear more or less simultaneously at the beginning of the Neolithic c. 7500 BC.

Copper smelting was independently invented in different places. It was probably discovered in China before 2800 BC, in Central America around 600 AD, and in West Africa about the 9th or 10th century AD. Investment casting was invented in 4500–4000 BC in Southeast Asia and carbon dating has established mining at Alderley Edge in Cheshire, UK, at 2280 to 1890 BC. Ötzi the Iceman, a male dated from 3300 to 3200 BC, was found with an axe with a copper head 99.7% pure; high levels of arsenic in his hair suggest an involvement in copper smelting. Experience with copper has assisted the development of other metals; in particular, copper smelting led to the discovery of iron smelting. Production in the Old Copper Complex in Michigan and Wisconsin is dated between 6000 and 3000 BC.Pleger, Thomas C. "A Brief Introduction to the Old Copper Complex of the Western Great Lakes: 4000–1000 BC",
Proceedings of the Twenty-Seventh Annual Meeting of the Forest History Association of Wisconsin
', Oconto, Wisconsin, 5 October 2002, pp. 10–18. Natural bronze, a type of copper made from ores rich in silicon, arsenic, and (rarely) tin, came into general use in the Balkans around 5500 BC.

Bronze Age

Alloying copper with tin to make bronze was first practiced about 4000 years after the discovery of copper smelting, and about 2000 years after "natural bronze" had come into general use. Bronze artifacts from the Vinča culture date to 4500 BC. Sumerian and Egyptian artifacts of copper and bronze alloys date to 3000 BC. The Bronze Age began in Southeastern Europe around 3700–3300 BC, in Northwestern Europe about 2500 BC. It ended with the beginning of the Iron Age, 2000–1000 BC in the Near East, and 600 BC in Northern Europe. The transition between the Neolithic period and the Bronze Age was formerly termed the Chalcolithic period (copper-stone), when copper tools were used with stone tools. The term has gradually fallen out of favor because in some parts of the world, the Chalcolithic and Neolithic are coterminous at both ends. Brass, an alloy of copper and zinc, is of much more recent origin. It was known to the Greeks, but became a significant supplement to bronze during the Roman Empire.

Ancient and post-classical

In Greece, copper was known by the name (χαλκός). It was an important resource for the Romans, Greeks and other ancient peoples. In Roman times, it was known as ''aes Cyprium'', ''aes'' being the generic Latin term for copper alloys and ''Cyprium'' from Cyprus, where much copper was mined. The phrase was simplified to ''cuprum'', hence the English ''copper''. Aphrodite ( Venus in Rome) represented copper in mythology and alchemy because of its lustrous