Gold

I

Introduction

Gold, symbol Au (from Latin aurum, “gold”), soft, dense, bright yellow metallic element. Gold is one of the transition elements of the periodic table (see Periodic Law); its atomic number is 79. Gold has long held a special place in human culture as one of the most valuable of all substances.

II

Properties of Gold

Pure gold is the most malleable and ductile of all the metals. It can easily be beaten or hammered to a thickness of 0.000013 cm (0.000005 in), and 29 g (1.02 oz) could be drawn into a wire 100 km (62 mi) long. It is one of the softest metals (hardness, 2.5 to 3) and is a good conductor of heat and electricity. Gold is bright yellow and has a high luster. Finely divided gold, like other metallic powders, is black; colloidally suspended gold ranges in color from ruby red to purple (see Colloid).

Gold is extremely inactive in chemical terms. It is unaffected by air, heat, moisture, and most solvents. It will, however, dissolve in aqueous mixtures containing various halogens such as chlorides, bromides, or some iodides. It will also dissolve in some oxidizing mixtures, such as cyanide ion with oxygen, and in aqua regia, a mixture of hydrochloric and nitric acids. The chlorides and cyanides are important compounds of gold. Gold melts at about 1064°C (about 1947°F), boils at about 2856°C (about 5173°F), and has a specific gravity of 19.3; its atomic weight is 196.97.

III

Occurrence of Gold

Gold is found in nature in quartz veins and secondary alluvial deposits as a free metal or in a combined state. It is widely distributed although it is rare, being 75th in order of abundance of the elements in Earth’s crust. It is almost always associated with varying amounts of silver; the naturally occurring gold-silver alloy is called electrum. Gold occurs, in chemical combination with tellurium, in the minerals calaverite and sylvanite along with silver, and in the mineral nagyagite along with lead, antimony, and sulfur. It occurs with mercury as gold amalgam. It is generally present to a small extent in iron pyrites; galena, the lead sulfide ore that usually contains silver, sometimes also contains appreciable amounts of gold. Gold also occurs in seawater to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in seawater is more than 9 billion metric tons, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.

---

---

IV

Uses of Gold

The metal has been known and highly valued from earliest times, not only because of its beauty and resistance to corrosion, but also because gold is easier to work than all other metals. In addition, gold was easier to obtain in pure form than the other metals. Because of its relative rarity, gold became used as currency and as a basis for international monetary transactions (see Dollar; Gold Standard). The unit used in weighing gold is the troy ounce; 1 troy ounce is equivalent to 31.1 grams.

The major portion of the gold produced is used in coinage and jewelry (see Metalwork). For these purposes it is alloyed with other metals to give it the necessary hardness. The gold content in alloys is expressed in carats (see Carat). Coinage gold is alloyed with copper or silver. Green gold used in jewelry contains copper and silver; white gold contains zinc and nickel, or platinum metals.

Gold is also used in the form of gold leaf in the arts of gilding and lettering. Purple of Cassius, a precipitate of finely divided gold and stannic hydroxide formed by the interaction of auric chloride and stannous chloride, is used in coloring ruby glass. Chlorauric acid is used in photography for toning silver images. Potassium gold cyanide is used in electrogilding. Gold is also used in dentistry. Radioisotopes of gold are used in biological research and in the treatment of cancer (see Isotopic Tracer).

The special properties of gold as a highly efficient electrical conductor that resists corrosion have made it very important in the manufacture of electronics and personal computers. Gold is used in components such as connectors, switches, and relay contacts. The gold is often electroplated onto metal computer parts and is made more durable in the form of alloys with nickel or cobalt. Although only small amounts of gold may be used in individual electronic devices such as cell phones, personal digital assistants, or GPS locators, the great number of such devices in use means valuable gold is lost if such electronics are not recycled. Thin films of gold are also used as a lubricant in place of oil for mechanical parts on space probes and satellites that must function in the cold vacuum of space while exposed to intense radiation. In addition, parts of both piloted and unpiloted space vehicles are often protected with a special gold-plated polyester film that reflects heat from the Sun into space.

	More from Encarta
	•  Important Notice: MSN Encarta to be discontinued. Learn more. •  Presidential Myths Quiz •  Coffee break: Recharge your brain