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In the Periodic Table of Elements, the chemical elements are organised according to their chemical and physical properties. In the case of copper, silver and gold, these form a vertical triad, called Group 11, within the group of transition metals. They are also found together at the foot of the reactivity series, which means that the metals are not easily oxidised to form positive ions. Hence, attack by most reagents (such as oxygen or acids) is slow; this resistance increases from copper through silver to gold.
Conversely, reduction of their compounds to the pure metal is easiest for gold, and diminishes through silver to copper1. Because of this resistance to chemical attack, the metals are unreactive, including to atmospheric components such as oxygen and water2, and have therefore been used for making coins and medals3. Indeed they are often referred to as the 'coinage' or 'noble' metals.
Examination of the electronic configurations of these metals found in the box above shows that atoms of these metals have a single electron in the outermost shell4, like the alkali metals, and would therefore be expected to give univalent ions. This is so for copper and silver, but not for gold, for reasons which will be explained below.
Discovery and Refinement of the Elements
Gold, silver and copper were the first metals that early humans came across. They would have been found in streambeds and rivers, washed out of the rocks. They are called 'native' metals, because they are sufficiently unreactive to be found in the ground in their elemental forms. The more reactive metals combine with non-metals, especially oxygen and sulphur, in rocks called 'ores'. (However, small amounts of unreacted iron have been found in the form of meteorites. This was discovered by the North American Indians long before anyone learned how to smelt iron from its ore).
The first discovery of metals may have been as long ago as 8000 BC, but another 2,000 years passed before man learned how to smelt copper and tin from their ores. This discovery may have been made accidentally by someone using a circle of stones to contain a fire. If the stones had happened to contain copper and/or tin compounds, the metals would flow out as liquids and quickly harden into lumps.
The Bronze Age started when copper and tin were mixed, probably accidentally, to make the alloy we now know as bronze - a much harder metal than either copper or tin alone. Bronzes of differing hardnesses could be made by mixing together different proportions of copper and tin and, because of this, bronze became the most important metal of its time.
However, finding the copper and tin was still a problem. Once found, mining it was laborious, difficult and dangerous. The process was simplified when early man realised that he could light a fire under metal-bearing rock. Once the rock was red hot, he could crack it open by pouring cold water over it.
The process was further refined when early man realised that the metals could be even more easily extracted from their ores if charcoal, from burned wooden embers, was mixed in with the ores before heating. This meant the fire could reach sufficiently high temperatures to extract iron from its ores, catapulting man into the Iron Age in around 2000 BC.
World Production of Copper, Silver and Gold
The 2004 figures for the world production of these metals was: gold 2,490 tonnes; silver 19,690 tonnes; and copper, 14,500,000 tonnes.
Properties and Uses of Copper
Copper is often described as a 'salmon pink' metallic element, and was probably the first metal to be used by humans for practical purposes. Though nowadays it is found in combination with other substances, when we first became interested in its properties about 5000 BC, there was probably enough natural metallic copper5 around to be found readily.
Copper is malleable and ductile - both easily beaten into shape or drawn out into wires. As a good conductor of both heat and electricity, it is a useful component of electrical circuitry. Indeed, its chief use, both in pure form and as alloys, is in the electrical industry.
Most people will have heard of phrase 'copper-bottomed' which, nowadays, is used figuratively to describe anything that is 'absolutely certain or absolutely trustworthy'. It derives from the late 18th Century British invention of sheathing the hulls of wooden ships with copper metal, to prevent infestation by marine organisms such as 'shipworm' and barnacles. When in contact with sea water, copper produces a toxic film, composed mainly of oxychloride. As well as protecting the wooden hulls, copper sheathing also conferred additional speed and manoeuvrability to ships of the Royal Navy; and is thus credited with giving Admiral Lord Nelson a tactical advantage over the French at the Battle of Trafalgar.
As one of only two coloured pure metals (the other being gold), copper is widely used as adornment, for example in the form of bracelets. It is worth noting here that an old folk remedy advocates the wearing of copper bracelets to relieve arthritic pain. It is claimed that copper is absorbed through the skin to relieve joint pain. Although many such people do wear copper bracelets, research has shown that people with arthritis do have sufficient copper in their bodies for normal health. Thus it is difficult to understand scientifically why these devices work.
Because of its attractiveness, durability and corrosion resistance, copper is often specified as the material of choice for roofing of commercial and institutional buildings. Because of its corrosion resistance, it is also used for making water pipes and for making stills in the brewing industry. More prosaically, copper is used for making the rivets on Levis.
Compounds of Copper
Copper is the most reactive of the three metals, and forms two series of compounds, those with an oxidation number of one and those with an oxidation number of two. (These are referred to as copper (I) and copper (II) compounds respectively.)
The copper (I) ion in solution disproportionates to become copper (II) ions and metallic copper:
2Cu+ > Cu2+ + Cu
The equilibrium constant for this is given by:
K =[Cu2+]/[Cu+] =106
and so the equilibrium is well over to the right hand side, meaning there will be a greater concentration of copper (II) ions than copper (I) ions in solution.
Compounds of copper (I) are uncommon, apart from the oxide (red copper oxide) and the halides.
The halides (except the fluoride) are all white insoluble solids, prepared usually by hydrolysis of the complex halogen acids:
2Cu(s) + 8HCl(conc) → 2H3[CuCl4](s) + H2(g)
H3[CuCl4](s) + H20 → 3HCl(aq) + Cu2Cl2(s)
Copper(I) salts are all insoluble in water, but they will dissolve in solutions of 'complexing agents' such as concentrated hydrochloric acid, aqueous ammonia and cyanides. Any of these solutions will react with other complexing agents such as ammoniacal copper(I) chloride dissolves carbon monoxide to scrub flue gases etc.
Copper (II) Compounds. These are the most common compounds of copper. The soluble ones and some others, such as the 'basic carbonate' and hydroxide, are green or blue. The blue colour is due to the hydrated ion [Cu(H2O)4]2+; the anhydrous Cu2+ ion is colourless.
Other familiar copper (II) compounds include copper (II) oxide (CuO) which is black, copper (II) sulphate, which is blue and the copper (II) halides (CuX2) of which the fluoride is colourless, the chloride is yellow/green, and the bromide is black but forms a green/blue solution. Copper (II) iodide is unknown since the iodide reduces Cu2+ to Cu+:
2Cu2+ + 4I- → Cu2I2 + I2.
Copper (II) sulphate crystallises as the pentahydrate, CuSO4.5H2O which, on heating, loses first four molecules of water, followed by the fifth one at a higher temperature. The product, 'anhydrous' copper (II) sulphate, is a white solid, but turns blue when water is added. Hence anhydrous copper (II) sulphate is used as a test for moisture.
Uses of Copper Compounds
Being relatively unreactive and aesthetically pleasing at the same time, copper is frequently used for roofing. However, the metal is sufficiently reactive to be attacked by chemicals present in the atmosphere. Indeed, the distinctive green patina of copper is a major feature in the skylines of most European cities. For example, the famous green roofs, domes and spires of Copenhagen are due to copper (II) chloride, caused by the reaction of copper metal with sodium chloride present in sea-spray. Further inland, in the industrial towns, the roofs have a bluer appearance because of attack by sulphur oxides (SO2 and SO3) from combustion of fossil fuels forming copper sulphate; while in rural areas the green roofs are due mainly to attack by carbon dioxide forming copper (II) carbonate.
Copper forms a hydrated basic carbonate of formula CuCO3.Cu(OH)2 known as malachite. This is an attractive bright green mineral with a long history of use. In the 5th Millennium BC, Egyptians were using it as a pigment, a cosmetic and in medicine. As a pigment/cosmetic, malachite was used by the ancient Egyptians to produce the green eye shadow depicted in tomb paintings. Interestingly, the Egyptian word for these eyeshadow kits was quite similar to the phrase 'to protect'. Some archaeologists speculate that this may allude to the antibacterial properties of malachite. Malachite is particularly effective against staphylococci, which cause skin diseases. Similarly, since the 1950s malachite has been used in fish hatcheries to control parasitic water mould, which inhibit the incubation rate of fertilized eggs.
The principle use of malachite today is as a source for copper metal and as a green pigment for artists.
Many copper (II) compounds are toxic and are thus widely used as weed-killers, insecticides, algicides and fungicides. Thus, Bordeaux mixture is a mixture of copper sulphate and hydrated lime, invented in the vineyards in the Bordeaux region of France, and is used by gardeners to control fungal infections.
Copper in Nutrition
Copper is an essential nutrient required for normal metabolic processes, and it is obtained as a result of dietary intake and absorption. It is estimated that a safe and adequate intake of copper is between 2 and 3 mg per day. This can be obtained from foods such as calf or lamb liver, whelks, crabs, certain cereals, nuts and cocoa. Copper has several beneficial roles in the body. These include the formation of the connective tissue protein, collagen; and in maintaining the body's immune system.
Like any other chemical, too much can be deleterious ('The dose makes the poison' - Paracelsus). It is possible to create a 'living histogram' by growing wheat seedlings in steadily increasing concentrations of copper ions; the average heights of the seedlings increases to a maximum, and then declines as the copper ion concentration increases beyond the healthy dosage level.
In humans, a build-up of copper in the body tissues is responsible for a condition called Wilson's Disease – a genetic condition which affects one in 30,000 people worldwide. This is thought to be associated with a reduction in the body's ability to manufacture ceruloplasmin, a protein which normally carries copper around in the blood.
Diagnosis of this condition can be difficult, but can be aided by finding a characteristic rusty brown ring around the cornea of the eye, called the Kayser-Fleischer ring.
At present, there is no cure for Wilson's Disease, and it can cause brain damage, liver failure and eventually death if left unmanaged. Management is achieved with a low-copper diet and vitamin B6 supplements. Drugs are also available to help remove excess copper from body tissues.
There appears to be anatomical similarities between animals deficient in copper and people with ischaemic heart disease (IHD). In particular, a low dietary copper intake correlates with an increase in low density lipoprotein (LDL - 'bad' cholesterol) and a decrease in high density lipoprotein (HDL - 'good' cholesterol).
It is thought that a relatively high incidence of oral cancer in some Asian countries may be due to a high copper content of betel (Areca) nuts. Chewing of betel nuts has been practised in many Asian and Pacific Rim countries for thousands of years. The nut is prepared by slicing it onto a leaf of betel pepper smeared with lime and sometimes spices. It is known that high copper levels are often associated with pre-cancerous fibrotic conditions (eg Oral Submucous Fibrosis, OSF), and it has been shown that betel nuts have a very high copper content compared to other commonly eaten nuts such as peanuts. It is speculated that soluble copper compounds stimulate the enzyme lysyl oxidase, which in turn stimulates the activity of fibroblast cells, thus leading to cross-linking of the collagen of the oral tissues.
Copper deficiency is very common in ruminant animals (eg: cows) which inadvertently tend to eat a lot of dirt. Dirt contains a lot of molybdenum, which can interfere with copper uptake, resulting in loss of hair growth. This was graphically described in the James Herriott book, 'Vet in Harness', where Mrs Dalby's stirks7 had the appearance of wearing spectacles. The treatment was to administer copper sulphate solution per os.
Properties and Uses of Silver
Silver was probably discovered by prehistoric man after gold and copper. Like gold, it remains bright and untarnished when heated in air, and so was classed as a noble metal by alchemists, who named it Luna and symbolised it with the crescent moon.
Like copper, silver is found in both the native state and in sulphide ores such as argentite or silver glance (Ag2S), which has been worked in Mexico since 1557. Other important ores include ruby silver, 3Ag2S.Sb2S3 and Horn silver, AgCl. As with copper ores, silver ores are found mixed with large amounts of worthless rock where the silver content may be 1% or less. Since the end of the 19th Century, it has been obtained principally as a by-product in the extraction of lead and copper from ores distributed in America, Europe, Asia and Australia.
Like copper and gold, it is a good conductor of heat and electricity and can be hammered into a very thin sheet or drawn into fine wire. In fact, pure silver has the highest electrical and thermal conductivity of all metals, followed by gold and copper.
Compounds of Silver
One of the most important compounds of silver is silver nitrate, used for making silver bromide. This is a light-sensitive salt used for making photographic film and preparing indelible inks.
Silver nitrate is used extensively in qualitative and quantitative chemical analysis. For example, it gives precipitates with the halides except fluoride, thiocyanates, chromates, orthophosphates and most ions of organic acids. It is particularly useful in identifying the halide anion, since the silver chloride precipitate is white, silver bromide is cream and silver iodide is yellow.
Silver nitrate is also used in the gravimetric8 determination of chloride, and the volumetric determination of the chloride, bromide, cyanide and thiocyanate anions.
Silver iodide, AgI, has been used to cause clouds to produce rain.
Although silver is present in the human body at a level of 2mg per 70kg, it has no known role.
Use of Silver Salts in Photography
Although it was known as early as the 17th Century that silver salts were photosensitive9, photography only became possible when Sir John William Herschel10 found that silver chloride was soluble in sodium thiosulphate.
Nowadays, photographic film consists of a celluloid matrix coated with gelatin in which is dissolved silver bromide. (This is now used instead of the original silver chloride due to its greater photosensitivity). On exposure to light, the silver in silver bromide is reduced to metallic silver, whilst the bromide anion is oxidised to bromine. At this stage, the minute particles of metallic silver are too small to be seen by the naked eye, but may be observed under an electron microscope. The number of such silver particle 'nuclei' depends on the intensity of light falling on the film. The film is then 'developed' with a reducing agent, which causes the unchanged silver bromide immediately surrounding the nuclei to be reduced to metallic silver, thus giving a visible blackening of the film. The film is then 'fixed' by washing ion sodium thiosulphate solution ('photographer's hypo'). Here, any unchanged bromide is dissolved to form the complex ion:
AgBr + 2S2O → [Ag(S2O3)2]2 + Br?-
Properties and Uses of Gold
Gold is a soft, bright and very dense yellow metal; it has twice the density of silver. Apart from its resistance to tarnishing and corrosion - which makes it unique amongst metals - gold is a good conductor of heat and electricity. It is also very soft and malleable, and so can be shaped and moulded easily.
The inertness of gold is exemplified by the fact that it is insoluble in most acids, with the exceptions of selenic acid and aqua regia, which is a fuming yellow mixture of concentrated hydrochloric and nitric acids. This resistance to corrosion, combined with its outstanding ability to conduct electricity, makes gold ideal for use in electrical components. Indeed, of the 1,500 tonnes of gold mined each year, some 200 tonnes goes into the manufacture of televisions, video cassette recorders, cell phones and computers.
Thin films of gold display some unusual optical properties. When manufactured into ultra-thin sheets, gold is transparent. At this thickness, it allows the passage of green light, but reflects infra-red light. Thus windows coated with a thin layer of gold will admit light while reflecting heat. For this reason, the cockpit windows of modern aircraft are coated with gold, as are the windows of modern office buildings and parts of space vehicles.
As with silver, gold is highly resistant to bacteria, so dentists have used it to repair or replace damaged or decayed teeth. In recent years, gold has proven to be an ideal material for surgical stents - tiny wire tubes inserted inside damaged veins or arteries for reinforcement.
Gold holds its value better than currency11, and so it is widely invested in by banks.
Gold is sometimes said to be the world's most precious metal; the market rate for it in late 2006 was over £280 per ounce. Pure Welsh gold, however - supplies of which are expected to be exhausted in early 2007 - is the world's most valuable precious metal at an estimated £850 per ounce, three times that of ordinary gold and exceeding that of platinum which, at £600 per ounce, is officially the dearest precious metal.
Welsh gold was originally mined by ancient man because of its ease of extraction from rock. Unlike South African gold, which is mixed in with the rock and yields just a quarter of an ounce for every tonne mined, pure Welsh gold forms in seams, like coal, and has been known to yield up to 30 ounces per tonne.
Pure Welsh gold has been mined from the Welsh mountains for centuries, and was used to make the jewellery worn by Celtic chieftains. Welsh gold was used to make the regalia of the future King Edward VII's investiture as Prince of Wales. The late Queen Mother (of the United Kingdom) had her wedding band fashioned from Welsh gold. The nugget used for her ring was sufficiently large to produce wedding rings for Queen Elizabeth II in 1947, Princess Margaret in 1960, the Princess Royal in 1973 and Diana, Princess of Wales in 1981. Welsh gold has also been used to fashion wedding rings for the Duchess of York, the Countess of Wessex and Camilla Parker-Bowles for her wedding to Prince Charles.
The Welsh actress, Catherine Zeta Jones also commissioned a wedding band of Welsh gold for her wedding to Michael Douglas in 2000.
Pure Welsh gold is distinguishable by the hallmark Aur Cymru and the engraving of a Welsh maiden.
Compounds of Gold
Gold(I) Compounds. These all tend to disproportionate into metallic gold and gold (III) compounds. Those which are insoluble in water, such as gold (I) sulphide, Au2S, are fairly stable, whereas others, such as gold(I) oxide, Au2 readily decompose on gentle heating. One of the most stable gold(I) compounds is gold(I) cyanide, AuCN, which is formed when the ion [Au(CN)2]-, is allowed to react with hydrochloric acid. Gold(I) iodide, AuI, is also formed by the slow loss of iodine from gold(III) iodide. (AuI3)n.
Gold(III) Compounds These are also known as auric compounds. All the gold (III) halides, except the fluoride, show evidence of the existence of double molecules of formula Au2X6 (cf the chlorides of aluminium and iron(III)). In this situation, the two gold atoms are bridged by two chlorine atoms, with a coordinate (dative) bond between one gold atom and a chlorine atom, so that the covalency of gold is brought up to four.
Figure: Structure of gold (III) Halides
X X X \ / \ / Au Au / \ / \ X X X
Gold(III) chloride dissolves in hydrochloric acid to form tetrachlorgold(III) acid, HAuCl4. Here again, the gold(III) has a covalency of four in the ion, [AuCl4]-.
Colloidal Gold. Colloidal gold, which consists of gold nanoparticles, has been known since ancient times, and was originally used as a method of staining glass. It is produced from tetrachlorogold(III) acid by, firstly, addition of alkali, whereupon successive replacement of chlorine atoms by hydroxyl groups occurs, to form the unstable tetrahydroxygold(III) ion, [Au(OH)4]-.
AuCl4]- → [AuCl3OH]- ... → [Au(OH)4]-
This is used as a test for all gold compounds, because they are all easily reduced in alkaline solution to metallic gold which, being in colloidal form, may appear red, blue or intermediate colours due to light scattering.
Nowadays, colloidal gold finds application in a wide variety of areas, including medicine, electronics and nanotechnology.
In the medical fields, an interesting application of gold nanoparticles is in 'immunogold labeling' of antibodies, for detection of antigens under the electron microscope. Here, gold particles are conjugated to antibodies then allowed to react with the antigen requiring detection, which may be a virus or bacteria. The method of use is very similar to conventional systems employing other types of label, for example radio-isotopes or enzymes, but is much simpler and has fewer of the disadvantages of other techniques. The gold conjugates are non-hazardous. The method works partly due to the high electron density of the gold particles, facilitating detection by both transmission and scanning electron microscopy.
A characteristic of the transition metals is that their atoms are often similar in size and behaviour. This means that the lattice structure may not be altered very much if one atom type is substituted for another, and this is what happens when alloys are formed. Alloys may be considered like mixtures of metals in solid solution; that is, where the atoms of each are able to intermingle completely and take part in the same regular arrangement or 'lattice'. They are usually made by melting the component metals together.
More often than not, the physical and chemical properties of alloys are very different than those expected of a simple mixture of the component metals. In general, alloying makes metals harder and less malleable; for example, brass is much tougher than either of its constituents, copper or zinc. Thus the purpose of alloying is to produce a metallic material having the properties required for the specific desired purpose.
The two best known alloys of copper are the bronzes, which contain tin, and the brasses, which contain zinc.
Normally, bronze contains 90% of copper and 10% of tin, but small amounts of other metals may be added. Bell-metal has a tin content of 20%.
Bronze is the traditional material for sculpture. Phosphor-bronze contains up to 12% phosphorus and is as hard as many steels. The gun-metals are used in ordnance and contain small amounts of zinc to facilitate machining. In the aluminium bronzes the tin is replaced by aluminium, and these are desirable for their cheapness and greater tensile strength.
Ordinary brass contains 60% copper and 40% zinc. Cartridge brass, however, contains only 30% zinc and is more ductile. Clock brass contains a small amount of lead. Ship propellers require high tensile strength, something achieved by adding small amounts of manganese and iron. Generally, both bronzes and brasses are harder and more resistant to corrosion than pure copper.
Silver is hardened for use in jewellery and table silver by alloying with small amounts of copper or nickel. Old British silver coins have a composition of 92.5% silver and 7.5% copper.
German silver contains approximately 60% copper, 20% nickel and 20% zinc, and is used in the manufacture of jewellery and cutlery.
Gold is soluble in mercury, with which it forms a type of alloy called an amalgam, so gold jewellery and ornaments should be kept well away from mercury! Amalgams were used in dentistry for fillings. Gold is often intentionally alloyed with other metals, notably copper, silver or zinc to increase its hardness.
Antibacterial Properties of Group 11 Elements
Copper has a history of medical use that goes back to the dawn of recorded history. The antibacterial properties of malachite have already been alluded to. Furthermore, the ancient Egyptians also used verdigris – a basic copper ethanoate (a blue-green coating formed on copper in contact with air and vinegar fumes12) – for treating eye infections.
A manuscript, known as the Smith papyrus13 which dates from 2600 - 2200BC, records the use of copper salts as sterilizers for drinking water and for dressing wounds.
The Greeks, who obtained their copper from Cyprus14 also used copper substances medicinally. Thus Hippocrates, the 'father of modern medicine' recommended red copper oxide ('flowers of copper') mixed with honey as an antiseptic to treat leg ulcers associated with varicose veins. Pliny opined that copper oxide was excellent for clearing the stomach, due to its purgative properties. Copper is also the active ingredient in Bordeaux mixture, used to protect grape vines from fungal infection.
Much more recently, scientists at the University of Southampton, UK, discovered that methicillin-resistant Staphylococcus aureus (MRSA) die with an hour or so of coming into contact with copper surfaces, whereas they will survive for days on stainless steel. It has further been discovered that brass – an alloy of copper with zinc – also performs quite well in this role, killing the bacteria in 5 hours, compared to 90 minutes for copper. This phenomenon is being exploited by University Hospital, Birmingham NHS Trust, UK, who are replacing all their metal fittings (door handles, taps, toilet flushes and handrails) with copper or brass ones, in an 18-month trial to see if it reduces the present high levels of MRSA infections.
Silver is long famed for its antiseptic properties. Early records indicate that the Phoenicians used silver vessels to keep water, wine and vinegar pure during long voyages. More recently, the American pioneers moving west put silver and copper coins into water barrels to keep it pure. Nowadays, some authorities are considering using it to sanitise swimming pools in place of chlorine.
The phrase 'to be born with a silver spoon in one's mouth' refers to the fact that babies fed using silver spoons in the 1800s were generally healthier than those fed using utensils made from cheaper metals15.
There is interest in the antimicrobial properties of silver nanoparticles and structured nanosurfaces, these being among the first commercial applications of the emerging science of nanotechnology. In 2005, scientists in the United Kingdom began experimenting with surgical dressings containing metallic silver thread, in the hope that it can eradicate MRSA cross-infection in hospitals.
Copper, Silver and Gold as Coinage Metals
The abundance of gold in the earth's crust is about 0.0000005% or, expressing this more simply, 100 tonnes of rock would yield, on average, about 0.5g of gold. Silver is 20 times more abundant than this and copper about 20 times more abundant than silver. Hence, at one time, in England, coins of these different metals could be made of about the same size with the values of 10 shillings, 6d. and 0.25d (one farthing), respectively.
However, rapid industrial expansion from the 18th century caused frequent fluctuations in the relative prices of metals and, after about 1816, only gold coins in Britain contained the nominal value of the metal. Had this not been so, an 'old' penny (1d.) which weighed 1 ounce (28.47g) in 1797, would have needed to have weighed 5 ounces (over 142g) by 1930, thus causing much damage to trouser pockets! In any event, between 1860 and 1971 (when decimal coinage was introduced) 'old' bronze pennies weighed one-third of an ounce (9.48g).
Nowadays, the 'copper' coins (1p and 2p coins) are made of copper-plated steel; 'silver' coins of copper-nickel alloy and £1 coins are of nickel-brass alloy.