Gold - Au

Chemical properties of gold - Health effects of gold - Environmental effects of gold

Atomic number	79
Atomic mass	196.9655 g.mol^-1
Electronegativity ccording to Pauling	2.4
Density	19.3 g.cm^-3 at 20°C
Melting point	1062 °C
Boiling point	2000 °C
Vanderwaals radius	0.144 nm
Ionic radius	0.137 nm (+1)
Isotopes	7
Electronic shell	[ Xe ] 4f¹⁴ 5d¹⁰ 6s¹
Energy of first ionisation	888 kJ.mol^-1
Energy of second ionisation	1974.6 kJ.mol^-1
Standard potential	+1,68 V ( Au⁺/ Au )
Discovered	c.a. 3000 BC

Gold - Au

Gold

Gold is metallic, with a yellow colour when in a mass, but when finely divided it may be black, ruby, or purple. It is the most malleable and ductile metal; 1 ounce (28 g) of gold can be beaten out to 300 square feet. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents.

Gold is usually alloyed in jewellery to give it more strength, and the term carat describes the amount of gold present (24 carats is pure gold). It is estimated that all the gold in the world, so far refined, could be placed in a single cube 60 ft. on a side.

The most common gold compounds are auric chloride (AuCl₃) and chlorauric acid (HAuCl₄). A mixture of one part nitric acid with three of hydrochloric acid is called aqua regia (because it dissolved gold, the King of Metals). It is unaffected by air and most reagents.

Applications

Gold is used as buillon and in jewellery, glass and electronics. Jewellery consumes around 75% of all gold produced. Gold for jewellery can be given a range of hues depending on the metal with which is alloyed (white, red, blue, green etc.). Colloidal gold is added to glass to colour it red or purple, and metallic gold is applied as a thin film on the windows of large building to reflect the heat of the Sun's ray. Gold electroplating is used to in the electronic industry to protect their copper components and improve their solderability.

Gold in the environment

Glod is widely distributes on the earth at a background level of 0.03 g/1000 kg (0.03 ppm by weight). Its interness and its high density causes it to concentrate in streambeds, either in small flakes or in larger nuggets, from which it may be recovered by panning. It is found free in nature and associated with quartz, pyrite and other minerals.

Most gold is mined and comes from gravels and quarts veins or is associated with pyrites deposits. Two thirds of the world's supply comes from South Africa, and ²/₃ of USA production is from South Dakota and Nevada. Other main mining areas are Canada and Russia. Gold is found in sea water, but no effective economic process has been designed (yet) to extract it from this source. World production is around 2500 tonnes per year, but reserves are estimated to be ten of thousand of tonnes.

Health effects of gold

Effects of exposure: Inhalation: May cause irritation if exposure is prolonged or excessive. Ingestion: No adverse effects expected. Skin: May cause irritation and allergic reaction. Eye: May cause irritation.

Gold is used to cure rheumatoid arthritis, under a treatment called Chrysoteraphy. It is prescribed when treatment with non-steroid antu-infiammatory drugs is failing to give relief.

Environmental effects of gold

Gold has not been evaluated for its ecotoxicity. However, the biodegradation of gold under aerobic conditions is expected to be very poor and there is no evidence to suggest it creates ecological problems when released into the environment. Since gold is insoluble, it is believed to have minimal bioaccumulation and bioavailability characteristics.

Gold's natural color is further enhanced by alloying it with small amounts of other metals. Jewelers create yellow, rose, green and white golds by using different alloys. More copper results in a soft rose color; additional silver creates green gold; and palladium produces white.

Properties: Gold is very durable and look no further than the nearest museum where gold jewelry, coins and artifacts from ancient civilizations attest to the metal's enduring beauty and permanence. Jewelers throughout the ages have preferred gold to all other metals for its beauty and ease of workmanship. Gold can be melted, or shaped, to create any design. It can be alloyed with a number of other metals to increase its strength and produce a variety of colors.

Colors of Gold: Gold comes in a variety of colors. Because gold is alloyed with other metals to increase its strength, it can also be made in a variety of colors. For example, yellow gold is created by alloying the metal with copper and silver; using copper only creates pink gold; white gold contains platinum or palladium, zinc and copper; green gold contains silver, copper and zinc.

24K Gold: Pure gold, or 24-karat, is generally considered too soft for use in jewelry, so it is alloyed with other metals to increase its strength.

18K Gold: 18K Gold is 18/24ths, or three-quarters pure gold. Jewelry of this fineness is marked "18k" or "750," meaning 75 percent gold.

14K Gold: In the United States, 14-karat gold is used most commonly for jewelry. Fourteen-karat gold is 14/24ths, or slightly more than one-half pure gold. Jewelry of this fineness is marked "14k" or "585," the European designation meaning 58.5 percent gold.

10K Gold: Nothing less than 10-karat gold can be legally marked or sold as gold jewelry in the United States. These pieces are marked "10k" or "417," the European designation meaning 41.7 percent gold.

Stamping: Look for trademarks accompanying the quality mark. When a piece of jewelry is stamped with a quality mark, law requires that it be stamped with a hallmark or trademark as well. Sometimes the jewelry is also marked with its country of origin. These designations are designed to assure you that you are buying genuine gold jewelry of the karatage marked.

Silver, iron, platinum, gold, and copper are all metals, which generally are malleable and ductile, conduct electricity and heat, and have a metallic luster. Some of their properties can be attributed to the way electrons are arranged in the material.

If an energy level (like the 3d band) holds many more electrons (than other energy levels) then the excitation of electrons from this highly occupied level to above the Fermi level will become quite important. Gold fulfills all the requirements for an intense absorption of light with energy of 2.3 eV (from the 3d band to above the Fermi level). The color we see is yellow, as the corresponding wavelengths are re-emitted. Copper has a strong absorption at a slightly lower energy, with orange being most strongly absorbed and re-emitted. In silver, the absorption peak lies in the ultraviolet region, at about 4 eV. As a result, silver maintains high reflectivity evenly across the visible spectrum, and we see it as a pure white. The lower energies (which in this case contain energies corresponding to the entire visible spectrum of color) are equally absorbed and re-emitted.

Silver and aluminum powders appear black because the white light that has been re-emitted is absorbed by nearby grains of powder and no light reaches the eye.

Transmitted color of gold

Gold is so malleable that it can be beaten into gold leaf less than 100 nm thick, revealing a bluish-green color when light is transmitted through it. Gold reflects yellow and red, but not blue or blue-green. The direct transmission of light through a metal in the absence of reflection is observed only in rare instances.

Colored gold alloys

When two metals are dissolved in each other (as is the case with alloys), the color is often a mixture of the two. For example, copper dissolved in gold changes the color from a yellow-gold to a red-gold. Silver dissolved in gold creates a green-gold color. White gold contains palladium and silver. The color of gold jewelry can be attributed to the addition of different amounts of several metals (such as copper, silver, zinc, and so on). Some of these color changes can be explained by shifts in the energy levels relative to the Fermi level.

Some alloys form intermetallics, where strong covalent bonds replace metallic bonding. Bonding is localized, so there is no sea of electrons.

When indium or gallium is added to gold, a blue color can result. The cause of color in these intermetallics is different than that of yellow gold.

Surface colors

Many metals create the illusion of being colored. The color can be attributed to a very thin surface coating, such as a paint or dye, or thin oxide layers can create interference colors (see butterflies) similar to those in oil or soap bubbles.

Some metals like aluminum (left) and titanium (right) can be anodized to create colors. The anodizing process creates a thin oxide layer. Aluminum and titanium films produced by anodizing can form thin, transparent coatings that produce interference effects in reflected light. The colors are applied using a bath, a brush, or a sponge, with the voltage applied determining the final color. Another anodizing application, particularly in aluminum, is the formation of thick, porous coatings that can absorb dyes to produce intense colors.

Stainless steel can also be colored, the color varying with oxide thickness, but the anodizing process is not used. In Sky Church, the purple area consists of stainless steel, coated to create interference effects. The panels appearing to flutter above the path of the monorail speeding below are made from painted aluminum.

Gold can be colored by creating surface oxide layers. Because gold does not oxidize in its pure form, base metals have to be added to create blue, brown, and black gold. The "Hearts" collection, in blue gold, is by Ludwig Muller of Switzerland. Crosses of black gold may be colored in different ways, as shown above. This antique engraved Czechoslovakian glass shows color produced by colloidal suspensions. When gold is in metallic colloidal form, as in the 10-nm-diameter particles in "ruby glass," the very complex "Mie scattering theory" has to be used to explain the unexpected red color illustrated; the yellow glass in this figure is colored by Mie scattering from metallic colloidal silver particles.