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Thallium

Gist

Thallium (Tl) is a highly toxic, silvery-white post-transition metal, atomic number 81, found in trace amounts in the Earth's crust and sulfide ores. Although once used in rodenticides and some medical applications, its use has been largely banned due to severe poisoning risks, mimicking symptoms of other diseases, and potential for criminal misuse. Thallium poisoning can be treated with Prussian Blue, and the element is still used in some specialized electronic and optical applications. 

Thallium blood concentration levels are normal below 2 µg/L, and toxic at concentrations greater than 200 µg/L. Long-term effects of thallium exposure can include difficulty walking, various involuntary movement disorders, and impairment of thought and mood. Neurological damage resolves slowly and may be permanent.

Summary

Thallium is a chemical element; it has symbol Tl and atomic number 81. It is a silvery-white post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists William Crookes and Claude-Auguste Lamy discovered thallium independently, in 1861, in residues of sulfuric acid production. Both used the newly developed method of flame spectroscopy, in which thallium produces a notable green spectral line. Thallium, from Greek thallós, meaning "green shoot" or "twig", was named by Crookes. It was isolated by both Lamy and Crookes in 1862, Lamy by electrolysis and Crookes by precipitation and melting of the resultant powder. Crookes exhibited it as a powder precipitated by zinc at the International Exhibition, which opened on 1 May that year.

Thallium tends to form the +3 and +1 oxidation states. The +3 state resembles that of the other elements in group 13 (boron, aluminium, gallium, indium). However, the +1 state, which is far more prominent in thallium than the elements above it, recalls the chemistry of alkali metals and thallium(I) ions are found geologically mostly in potassium-based ores and (when ingested) are handled in many ways like potassium ions (K+) by ion pumps in living cells.

Commercially, thallium is produced not from potassium ores, but as a byproduct from refining of heavy-metal sulfide ores. Approximately 65% of thallium production is used in the electronics industry and the remainder is used in the pharmaceutical industry and in glass manufacturing. It is also used in infrared detectors. The radioisotope thallium-201 (as the soluble chloride TlCl) is used in small amounts as an agent in a nuclear medicine scan, during one type of nuclear cardiac stress test.

Soluble thallium salts (many of which are nearly tasteless) are highly toxic and they were historically used in rat poisons and insecticides. Because of their nonselective toxicity, use of these compounds has been restricted or banned in many countries. Thallium poisoning usually results in hair loss.

Details

Thallium (Tl) is a chemical element, metal of main Group 13 (IIIa, or boron group) of the periodic table, poisonous and of limited commercial value. Like lead, thallium is a soft, low-melting element of low tensile strength. Freshly cut thallium has a metallic lustre that dulls to bluish gray upon exposure to air. The metal continues to oxidize upon prolonged contact with air, generating a heavy nonprotective oxide crust. Thallium dissolves slowly in hydrochloric acid and dilute sulfuric acid and rapidly in nitric acid.

Rarer than tin, thallium is concentrated in only a few minerals that have no commercial value. Trace amounts of thallium are present in sulfide ores of zinc and lead; in the roasting of these ores, the thallium becomes concentrated in the flue dusts, from which it is recovered.

British chemist Sir William Crookes discovered (1861) thallium by observing the prominent green spectral line generated by selenium-bearing pyrites that had been used in the manufacture of sulfuric acid. Crookes and French chemist Claude-Auguste Lamy independently isolated (1862) thallium, showing it to be a metal.

Two crystalline forms of the element are known: close-packed hexagonal below about 230 °C (450 °F) and body-centred cubic above. Natural thallium, the heaviest of the boron group elements, consists almost entirely of a mixture of two stable isotopes: thallium-203 (29.5 percent) and thallium-205 (70.5 percent). Traces of several short-lived isotopes occur as decay products in the three natural radioactive disintegration series: thallium-206 and thallium-210 (uranium series), thallium-208 (thorium series), and thallium-207 (actinium series).

Thallium metal has no commercial use, and thallium compounds have no major commercial application, since thallous sulfate was largely replaced in the 1960s as a rodenticide and insecticide. Thallous compounds have a few limited uses. For example, mixed bromide-iodide crystals (TlBr and TlI) that transmit infrared light have been fabricated into lenses, windows, and prisms for infrared optical systems. The sulfide (Tl2S) has been employed as the essential component in a highly sensitive photoelectric cell and the oxysulfide in an infrared-sensitive photocell (thallofide cell). Thallium forms its oxides in two different oxidation states, +1 (Tl2O) and +3 (Tl2O3). Tl2O has been used as an ingredient in highly refractive optical glasses and as a colouring agent in artificial gems; Tl2O3 is an n-type semiconductor. Alkali halide crystals, such as sodium iodide, have been doped or activated by thallium compounds to produce inorganic phosphors for use in scintillation counters to detect radiation.

Thallium imparts a brilliant green coloration to a bunsen flame. Thallous chromate, formula Tl2CrO4, is best used in the quantitative analysis of thallium, after any thallic ion, Tl3+, present in the sample has been reduced to the thallous state, Tl+.

Thallium is typical of the Group 13 elements in having an s2p1 outer electron configuration. Promoting an electron from an s to a p orbital allows the element to be three or four covalent. With thallium, however, the energy required for s → p promotion is high relative to the Tl–X covalent bond energy that is regained on formation of TlX3; hence, a derivative with a +3 oxidation state is not a very energetically favoured reaction product. Thus, thallium, unlike the other boron group elements, predominantly forms singly charged thallium salts having thallium in the +1 rather than the +3 oxidation state (the 6s2 electrons remain unused). It is the only element to form a stable singly charged cation with the outer electron configuration (n-1)d10ns2, which is, unusually enough, not an inert gas configuration. In water the colourless, more stable thallous ion, Tl+, resembles the heavier alkali metal ions and silver; the compounds of thallium in its +3 state are easily reduced to compounds of the metal in its +1 state.

In its oxidation state of +3, thallium resembles aluminum, although the ion Tl3+ appears to be too large to form alums. The very close similarity in size of the singly charged thallium ion, Tl+, and the rubidium ion, Rb+, makes many Tl+ salts, such as the chromate, sulfate, nitrate, and halides, isomorphous (i.e., have an identical crystal structure) to the corresponding rubidium salts; also, the ion Tl+ is able to replace the ion Rb+ in the alums. Thus, thallium does form an alum, but in doing so it replaces the M+ ion, rather than the expected metal atom M3+, in M+M3+(SO4)2∙12H2O.

Soluble thallium compounds are toxic. The metal itself is changed to such compounds by contact with moist air or skin. Thallium poisoning, which may be fatal, causes nervous and gastrointestinal disorders and rapid loss of hair.

Element Properties
atomic number  :  81
atomic weight  :  204.37
melting point  :  303.5 °C (578.3 °F)
boiling point  :  1,457 °C (2,655 °F)
specific gravity  :  11.85 (at 20 °C [68 °F])
oxidation states  :  +1, +3.

Additional Information:

Appearance

A soft, silvery-white metal that tarnishes easily.

Uses

The use of thallium is limited as it is a toxic element. Thallium sulfate was employed as a rodent killer – it is odourless and tasteless – but household use of this poison has been prohibited in most developed countries.

Most thallium is used by the electronics industry in photoelectric cells. Thallium oxide is used to produce special glass with a high index of refraction, and also low melting glass that becomes fluid at about 125K.

An alloy of mercury containing 8% thallium has a melting point 20°C lower than mercury alone. This can be used in low temperature thermometers and switches.

Biological role

Thallium has no known biological role. It is very toxic and there is evidence that the vapour is both teratogenic (disturbs the development of an embryo or foetus) and carcinogenic. It can displace potassium around the body affecting the central nervous system.

Natural abundance

Thallium is found in several ores. One of these is pyrites, which is used to produce sulfuric acid. Some thallium is obtained from pyrites, but it is mainly obtained as a by-product of copper, zinc and lead refining.

Thallium is also present in manganese nodules found on the ocean floor.

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Mercury (Element)

Gist

Mercury is a chemical element with the symbol Hg and atomic number 80. It is a heavy, silvery-white metal that is unique for being the only metallic element that remains liquid at standard temperature and pressure, though other metals like gallium melt just above room temperature. Also known as quicksilver, it forms alloys called amalgams with other metals and is found in nature in deposits of cinnabar. Mercury is toxic and can cause serious health problems, particularly to the nervous system, and bioaccumulates in the environment and food chains. 

Mercury has historically been used in thermometers, barometers, electrical switches, fluorescent lamps, dental amalgams, and some batteries, pharmaceuticals, and cosmetics. However, due to its toxicity, many of these uses are being phased out in favor of alternatives, with the Minamata Convention on Mercury working to eliminate its use in products and industries globally. 

Summary

Mercury is a chemical element; it has symbol Hg and atomic number 80. It is commonly known as quicksilver. A heavy, silvery d-block element, mercury is the only metallic element that is known to be liquid at standard temperature and pressure; the only other element that is liquid under these conditions is the halogen bromine, though metals such as caesium, gallium, and rubidium melt just above room temperature.

Mercury occurs in deposits throughout the world mostly as cinnabar (mercuric sulfide). The red pigment vermilion is obtained by grinding natural cinnabar or synthetic mercuric sulfide. Exposure to mercury and mercury-containing organic compounds is toxic to the nervous system, immune system and kidneys of humans and other animals; mercury poisoning can result from exposure to water-soluble forms of mercury (such as mercuric chloride or methylmercury) either directly or through mechanisms of biomagnification.

Mercury is used in thermometers, barometers, manometers, sphygmomanometers, float valves, mercury switches, mercury relays, fluorescent lamps and other devices, although concerns about the element's toxicity have led to the phasing out of such mercury-containing instruments. It remains in use in scientific research applications and in amalgam for dental restoration in some locales. It is also used in fluorescent lighting. Electricity passed through mercury vapor in a fluorescent lamp produces short-wave ultraviolet light, which then causes the phosphor in the tube to fluoresce, making visible light.

Details

Mercury (Hg) is a chemical element, liquid metal of Group 12 (IIb, or zinc group) of the periodic table.

Element Properties

atomic number  :  80
atomic weight  :  200.592
melting point  :  -38.83 °C (-37.89 °F)
boiling point  :  356.62 °C (673.91 °F)
specific gravity  :  13.5 at 20 °C (68 °F)
valence  :  1, 2.

Properties, uses, and occurrence

Mercury was known in Egypt and also probably in the East as early as 1500 bce. The name mercury originated in 6th-century alchemy, in which the symbol of the planet was used to represent the metal; the chemical symbol Hg derives from the Latin hydrargyrum, “liquid silver.” Although its toxicity was recognized at an early date, its main application was for medical purposes.

Mercury is the only elemental metal that is liquid at room temperature. (Cesium melts at about 28.5 °C [83 °F], gallium at about 30 °C [86 °F], and rubidium at about 39 °C [102 °F].) Mercury is silvery white, slowly tarnishes in moist air, and freezes into a soft solid like tin or lead at −38.83 °C (−37.89 °F). It boils at 356.62 °C (673.91 °F).

It alloys with copper, tin, and zinc to form amalgams, or liquid alloys. An amalgam with silver is used as a filling in dentistry. Mercury does not wet glass or cling to it, and this property, coupled with its rapid and uniform volume expansion throughout its liquid range, made it useful in thermometers. (Mercury thermometers were supplanted by more accurate electronic digital thermometers in the early 21st century.) Barometers and manometers also used its high density and low vapour pressure. However, mercury’s toxicity has led to its replacement in these instruments. Gold and silver dissolve readily in mercury, and in the past this property was used in the extraction of these metals from their ores.

The good electrical conductivity of mercury makes it exceptionally useful in sealed electrical switches and relays. An electrical discharge through mercury vapour contained in a fused silica tube or bulb produces a bluish glow rich in ultraviolet light, a phenomenon exploited in ultraviolet, fluorescent, and high-pressure mercury-vapour lamps. Some mercury is used in the preparation of pharmaceuticals and agricultural and industrial fungicides.

In the 20th century the use of mercury in the manufacture of chlorine and sodium hydroxide by electrolysis of brine depended upon the fact that mercury employed as the negative pole, or cathode, dissolves the sodium liberated to form a liquid amalgam. In the early 21st century, however, mercury-cell plants for manufacturing chlorine and sodium hydroxide have mostly been phased out.

Mercury occurs in Earth’s crust on the average of about 0.08 gram (0.003 ounce) per ton of rock. The principal ore is the red sulfide, cinnabar. Native mercury occurs in isolated drops and occasionally in larger fluid masses, usually with cinnabar, near volcanoes or hot springs. Extremely rare natural alloys of mercury have also been found: moschellandsbergite (with silver), potarite (with palladium), and gold amalgam. Over 90 percent of the world’s supply of mercury comes from China; it is often a by-product of gold mining.

Cinnabar is mined in shaft or open-pit operations and refined by flotation. Most of the methods of extraction of mercury rely on the volatility of the metal and the fact that cinnabar is readily decomposed by air or by lime to yield the free metal. Mercury is extracted from cinnabar by roasting it in air, followed by condensation of the mercury vapour. Because of the toxicity of mercury and the threat of rigid pollution control, attention is being directed toward safer methods of extracting mercury. These generally rely on the fact that cinnabar is readily soluble in solutions of sodium hypochlorite or sulfide, from which the mercury can be recovered by precipitation with zinc or aluminum or by electrolysis.

Mercury is toxic. Poisoning may result from inhalation of the vapour, ingestion of soluble compounds, or absorption of mercury through the skin.

Natural mercury is a mixture of seven stable isotopes: 196Hg (0.15 percent), 198Hg (9.97 percent), 199Hg (16.87 percent), 200Hg (23.10 percent), 201Hg (13.18 percent), 202Hg (29.86 percent), and 204Hg (6.87 percent). Isotopically pure mercury consisting of only mercury-198 prepared by neutron bombardment of natural gold, gold-197, has been used as a wavelength standard and for other precise work.

Principal compounds

The compounds of mercury are either of +1 or +2 oxidation state. Mercury(II) or mercuric compounds predominate. Mercury does not combine with oxygen to produce mercury(II) oxide, HgO, at a useful rate until heated to the range of 300 to 350 °C (572 to 662 °F). At temperatures of about 400 °C (752 °F) and above, the reaction reverses with the compound decomposing into its elements. Antoine-Laurent Lavoisier and Joseph Priestley used this reaction in their study of oxygen.

There are relatively few mercury(I) or mercurous compounds. The mercury(I) ion, Hg22+, is diatomic and stable. Mercury(I) chloride, Hg2Cl2 (commonly known as calomel), is probably the most important univalent compound. It was used in antiseptic salves. Mercury(II) chloride, HgCl2 (also called bichloride of mercury or corrosive sublimate), is perhaps the commonest bivalent compound. Although extremely toxic, this odourless, colourless substance has a wide variety of applications. In agriculture it is used as a fungicide, in medicine it was sometimes employed as a topical antiseptic in concentrations of one part per 2,000 parts of water, and in the chemical industry it serves as a catalyst in the manufacture of vinyl chloride and as a starting material in the production of other mercury compounds. Mercury(II) oxide, HgO, provides elemental mercury for the preparation of various organic mercury compounds and certain inorganic mercury salts. This red or yellow crystalline solid is also used as an electrode (mixed with graphite) in zinc-mercuric oxide electric cells and in mercury batteries. Mercury(II) sulfide, HgS, is a black or red crystalline solid used chiefly as a pigment in paints, rubber, and plastics.

Additional Information:

Appearance

A liquid, silvery metal.

Uses

Mercury has fascinated people for millennia, as a heavy liquid metal. However, because of its toxicity, many uses of mercury are being phased out or are under review.

It is now mainly used in the chemical industry as catalysts. It is also used in some electrical switches and rectifiers.

Previously its major use was in the manufacture of sodium hydroxide and chlorine by electrolysis of brine. These plants will all be phased out by 2020. It was also commonly used in batteries, fluorescent lights, felt production, thermometers and barometers. Again, these uses have been phased out.

Mercury easily forms alloys, called amalgams, with other metals such as gold, silver and tin. The ease with which it amalgamates with gold made it useful in recovering gold from its ores. Mercury amalgams were also used in dental fillings.

Mercuric sulfide (vermilion) is a high-grade, bright-red paint pigment, but is very toxic so is now only used with great care.

Biological role

Mercury has no known biological role, but is present in every living thing and widespread in the environment. Every mouthful of food we eat contains a little mercury.

Our daily intake is less than 0.01 milligrams (about 0.3 grams in a lifetime), and this we can cope with easily. However, in much higher doses it is toxic and one form of mercury – methylmercury – is particularly dangerous. It can accumulate in the flesh of fish and be eaten by people, making them ill.

Natural abundance

Mercury rarely occurs uncombined in nature, but can be found as droplets in cinnabar (mercury sulfide) ores. China and Kyrgyzstan are the main producers of mercury. The metal is obtained by heating cinnabar in a current of air and condensing the vapour.

Gold

Gist

Gold is a chemical element with the symbol Au (from the Latin aurum) and atomic number 79, known for its bright yellow color and high density. It's a transition metal, belonging to the noble metals, which means it is highly unreactive, resistant to corrosion, and a good conductor of heat and electricity. Gold is extremely malleable and ductile, allowing it to be hammered into thin sheets or drawn into wires, and it is widely used in jewelry, electronics, finance, and dentistry. 

Gold is called "Au" because the symbol comes from its Latin name, aurum, which means "shining dawn" or "glow of sunrise". The chemical symbol is a short, two-letter abbreviation derived from this Latin word, which was a common practice for elements known since antiquity, as Latin was a widely known language among educated people.

Summary

Gold is a chemical element; it has chemical symbol Au (from Latin aurum) and atomic number 79. In its pure form, it is a bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal, a group 11 element, and one of the noble metals. It is one of the least reactive chemical elements, being the second lowest in the reactivity series, with only platinum ranked as less reactive. Gold is solid under standard conditions.

Gold often occurs in free elemental (native state), as nuggets or grains, in rocks, veins, and alluvial deposits. It occurs in a solid solution series with the native element silver (as in electrum), naturally alloyed with other metals like copper and palladium, and mineral inclusions such as within pyrite. Less commonly, it occurs in minerals as gold compounds, often with tellurium (gold tellurides).

Gold is resistant to most acids, though it does dissolve in aqua regia (a mixture of nitric acid and hydrochloric acid), forming a soluble tetrachloroaurate anion. Gold is insoluble in nitric acid alone, which dissolves silver and base metals, a property long used to refine gold and confirm the presence of gold in metallic substances, giving rise to the term "acid test". Gold dissolves in alkaline solutions of cyanide, which are used in mining and electroplating. Gold also dissolves in mercury, forming amalgam alloys, and as the gold acts simply as a solute, this is not a chemical reaction.

A relatively rare element when compared to silver (though thirty times more common than platinum), gold is a precious metal that has been used for coinage, jewelry, and other works of art throughout recorded history. In the past, a gold standard was often implemented as a monetary policy. Gold coins ceased to be minted as a circulating currency in the 1930s, and the world gold standard was abandoned for a fiat currency system after the Nixon shock measures of 1971.

In 2023, the world's largest gold producer was China, followed by Russia and Australia. As of 2020, a total of around 201,296 tonnes of gold exist above ground. If all of this gold were put together into a cube shape, each of its sides would measure 21.7 meters (71 ft). The world's consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry. Gold's high malleability, ductility, resistance to corrosion and most other chemical reactions, as well as conductivity of electricity have led to its continued use in corrosion-resistant electrical connectors in all types of computerized devices (its chief industrial use). Gold is also used in infrared shielding, the production of colored glass, gold leafing, and tooth restoration. Certain gold salts are still used as anti-inflammatory agents in medicine.

Details

Gold (Au), chemical element, is a dense lustrous yellow precious metal of Group 11 (Ib), Period 6, of the periodic table of the elements. Gold has several qualities that have made it exceptionally valuable throughout history. It is attractive in colour and brightness, durable to the point of virtual indestructibility, highly malleable, and usually found in nature in a comparatively pure form. The history of gold is unequaled by that of any other metal because of its perceived value from earliest times.

Element Properties

atomic number  :  79
atomic weight  :  196.96657
melting point  :  1,063 °C (1,945 °F)
boiling point  :  2,966 °C (5,371 °F)
specific gravity  : 19.3 at 20 °C (68 °F)
oxidation states  :  +1, +3.

Properties, occurrences, and uses

Gold is one of the densest of all metals. It is a good conductor of heat and electricity. It is also soft and the most malleable and ductile of the elements; an ounce (31.1 grams; gold is weighed in troy ounces) can be beaten out to 187 square feet (about 17 square metres) in extremely thin sheets called gold leaf.

Because gold is visually pleasing and workable and does not tarnish or corrode, it was one of the first metals to attract human attention. Examples of elaborate gold workmanship, many in nearly perfect condition, survive from ancient Egyptian, Minoan, Assyrian, and Etruscan artisans, and gold continues to be a highly favoured material out of which to craft jewelry and other decorative objects.

Because of its unique qualities, gold has been the one material that is universally accepted in exchange for goods and services. In the form of coins or bullion, gold has occasionally played a major role as a high-denomination currency, although silver was generally the standard medium of payments in the world’s trading systems. Gold began to serve as backing for paper-currency systems when they became widespread in the 19th century, and from the 1870s until World War I the gold standard was the basis for the world’s currencies. Although gold’s official role in the international monetary system had come to an end by the 1970s, the metal remains a highly regarded reserve asset, and approximately 45 percent of all the world’s gold is held by governments and central banks for this purpose. Gold is still accepted by all nations as a medium of international payment.

Gold is widespread in low concentrations in all igneous rocks. Its abundance in Earth’s crust is estimated at about 0.005 part per million. It occurs mostly in the native state, remaining chemically uncombined except with tellurium, selenium, and bismuth. The element’s only naturally occurring isotope is gold-197. Gold often occurs in association with copper and lead deposits, and, though the quantity present is often extremely small, it is readily recovered as a by-product in the refining of those base metals. Large masses of gold-bearing rock rich enough to be called ores are unusual. Two types of deposits containing significant amounts of gold are known: hydrothermal veins, where it is associated with quartz and pyrite (fool’s gold); and placer deposits, both consolidated and unconsolidated, that are derived from the weathering of gold-bearing rocks.

Veins enriched in gold form when the gold was carried up from great depths with other minerals, in an aqueous solution, and later precipitated. The gold in rocks usually occurs as invisible disseminated grains, more rarely as flakes large enough to be seen, and even more rarely as masses or veinlets. Crystals about 2.5 cm (1 inch) or more across have been found in California. Masses, some on the order of 90 kg (200 pounds), have been reported from Australia.

Alluvial deposits of gold found in or along streams were the principal sources of the metal for ancient Egypt and Mesopotamia. Other deposits were found in Lydia (now in Turkey) and the lands of the Aegean and in Persia (Iran), India, China, and other lands. During the Middle Ages the chief sources of gold in Europe were the mines of Saxony and Austria. The era of gold production that followed the Spanish discovery of the Americas in the 1490s was probably the greatest the world had witnessed to that time. The exploitation of mines by slave labour and the looting of palaces, temples, and graves in Central and South America resulted in an unprecedented influx of gold that literally unbalanced the economic structure of Europe. From Christopher Columbus’s discovery of the New World in 1492 to 1600, more than 225,000 kg (8,000,000 ounces) of gold, or 35 percent of world production, came from South America. The New World’s mines—especially those in Colombia—continued into the 17th and 18th centuries to account for 61 and 80 percent, respectively, of world production; 1,350,000 kg (48,000,000 ounces) were mined in the 18th century.

Russia became the world’s leading producer of gold in 1823, and for 14 years it contributed the bulk of the world supply. During the second era of expanding production (1850–75), more gold was produced in the world than in all the years since 1492, primarily because of discoveries in California and Australia. A third marked increase (1890–1915) stemmed from discoveries in Alaska, Yukon Territory (now Yukon), and South Africa. A major factor in the increase of the world’s supply of gold was the introduction in 1890 of the cyanide process for the recovery of gold from low-grade ores and ores containing minute, particle-sized gold. Gold production continued to rise throughout the 20th century, partly because of the improvement in recovery methods and partly because of the continual growth and expansion of South Africa’s gold-mining operations.

In the late 20th century, four countries—South Africa, Russia, the United States, and Australia—accounted for two-thirds of the gold produced annually throughout the world. In the early 21st century, China was the world leader in gold production. During this period, Australia, the United States, Russia, Canada, and South Africa also continued to supply large amounts of the precious metal.

Because pure gold is too soft to resist prolonged handling, it is usually alloyed with other metals to increase its hardness for use in jewelry, goldware, or coinage. Most gold used in jewelry is alloyed with silver, copper, and a little zinc to produce various shades of yellow gold or with nickel, copper, and zinc to produce white gold. The colour of these gold alloys goes from yellow to white as the proportion of silver in them increases; more than 70 percent silver results in alloys that are white. Alloys of gold with silver or copper are used to make gold coins and goldware, and alloys with platinum or palladium are also used in jewelry. The content of gold alloys is expressed in 24ths, called karats; a 12-karat gold alloy is 50 percent gold, and 24-karat gold is gold that is more than 99 percent pure.

Because of its high electrical conductivity (71 percent that of copper) and inertness, the largest industrial use of gold is in the electric and electronics industry for plating contacts, terminals, printed circuits, and semiconductor systems. Thin films of gold that reflect up to 98 percent of incident infrared radiation have been employed on satellites to control temperature and on space-suit visors to afford protection. Used in a similar way on the windows of large office buildings, gold reduces the air-conditioning requirement and adds to the beauty. Gold has also long been used for fillings and other repairs to teeth.

Gold is one of the noblest—that is, least chemically reactive—of the transition elements. It is not attacked by oxygen or sulfur, although it will react readily with halogens or with solutions containing or generating chlorine, such as aqua regia. It also will dissolve in cyanide solutions in the presence of air or hydrogen peroxide. Dissolution in cyanide solutions is attributable to the formation of the very stable dicyanoaurate ion, [Au(CN)2]−.

Like copper, gold has a single s electron outside a completed d shell, but, in spite of the similarity in electronic structures and ionization energies, there are few close resemblances between gold on the one hand and copper on the other.

Compounds

The characteristic oxidation states of gold are +1 (aurous compounds) and +3 (auric compounds). The state +1 is generally quite unstable, and most of the chemistry of gold involves the state +3. Gold is more easily displaced from solution by reduction than any other metal. Even platinum will reduce Au3+ ions to metallic gold.

Among the relatively few gold compounds of practical importance are gold(I) chloride, AuCl; gold(III) chloride, AuCl3; and chlorauric acid, HAuCl4. In the first compound, gold is in the +1 oxidation state, and in the latter two, the +3 state. All three compounds are involved in the electrolytic refining of gold. Potassium cyanoaurate, K[Au(CN)2], is the basis for most gold-plating baths (the solution employed when gold is plated). Several organic compounds of gold have industrial applications. For example, gold mercaptides, which are obtained from sulfurized terpenes, are dissolved in certain organic solutions and used for decorating china and glass articles.

Additional Information:

Appearance

A soft metal with a characteristic yellow colour. It is chemically unreactive, although it will dissolve in aqua regia (a mixture of nitric and hydrochloric acids).

Uses

Most mined gold is stored as bullion. It is also, however, used extensively in jewellery, either in its pure form or as an alloy. The term ‘carat’ indicates the amount of gold present in an alloy. 24-carat is pure gold, but it is very soft. 18- and 9-carat gold alloys are commonly used because they are more durable.

The metal is also used for coinage, and has been used as standard for monetary systems in some countries.

Gold can be beaten into very thin sheets (gold leaf) to be used in art, for decoration and as architectural ornament. Electroplating can be used to cover another metal with a very thin layer of gold. This is used in gears for watches, artificial limb joints, cheap jewellery and electrical connectors. It is ideal for protecting electrical copper components because it conducts electricity well and does not corrode (which would break the contact). Thin gold wires are used inside computer chips to produce circuits.

Dentists sometimes use gold alloys in fillings, and a gold compound is used to treat some cases of arthritis.

Gold nanoparticles are increasingly being used as industrial catalysts. Vinyl acetate, which is used to make PVA (for glue, paint and resin), is made using a gold catalyst.

Biological role

Gold has no known biological role, and is non-toxic.

Natural abundance

Gold is one of the few elements to occur in a natural state. It is found in veins and alluvial deposits. About 1500 tonnes of gold are mined each year. About two-thirds of this comes from South Africa and most of the rest from Russia.

Seawater contains about 4 grams of gold in 1,000,000 tonnes of water. Overall this is a huge amount of gold stored in the oceans but, because the concentration is so low, attempts to reclaim this gold have always failed.

Close Quotes - II

1. To draw you must close your eyes and sing. - Pablo Picasso

2. Each morning sees some task begun, each evening sees it close; Something attempted, something done, has earned a night's repose. - Henry Wadsworth Longfellow

3. It is vain for the coward to flee; death follows close behind; it is only by defying it that the brave escape. - Voltaire

4. I usually tried to stay in the net for 45 minutes, half an hour longer than most batsmen would stick at the county nets. There was a reason for this so-called gluttony of practice: it was a conscious effort to make myself concentrate for long periods of time in circumstances as close to the real thing as I could make them. Geoffrey Boycott

5. Boys used to call me Soda in school days. Soda means 'serving officers daughters association.' I miss those days when I had a very protected life: one could get close and bond with other army people that they gradually would become your extended family. - Anushka Sharma

6. If I get stuck, I look at a book that tells me how someone else did it. I turn the pages, and then I say, 'Oh, I forgot that bit,' then close the book and carry on. Finally, after you've figured out how to do it, you read how they did it and find out how dumb your solution is and how much more clever and efficient theirs is! - Richard P. Feynman

7. I don't have many friends; I'm very much a loner. As a child I was very isolated, and I've never been really close to anyone. - Anthony Hopkins

8. The just is close to the people's heart, but the merciful is close to the heart of God. - Khalil Gibran.

Q: Why is a math book always unhappy?
A: Because it always has lots of problems.
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Q: What do dogs and story tellers have in common?
A: They both have tails!
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Q: What is it called when someone gets suffocated by a book?
A: Literally murder!
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Q: Have you seen the Bruce Willis movie where an entire library gets destroyed?
It's called "Die Hardcover".
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Q: Why can't lawyers trick a librarian?
A: Because librarians can read the fine print.
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Q: What did the surfer say to the librarian?
A: Is my book over dude?
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Q: Why should you be careful about what you say around a librarian?
A: Because they can read lips.
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Q: What has a spine but, no bones?
A: A book.
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Q: Who's the biggest liar in school?
A: The Lie-brarian.
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Q: What did the librarian say to John Cusack?
A: Shhhhh! Don't Say Anything.
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Q: What did the librarian say to the astronaut?
A: Find space for a book.
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