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.
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 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 the crust of the earth. 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.
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.
On December 31, 1974, the U.S. federal government lifted a 41-year ban on the private ownership of gold. Around that time gold was being traded on the London bullion market at record highs approaching $200 an ounce. After subsequent sharp decreases, prices rose to a high of $850 in January 1980. They then dropped considerably and in the early 1990s settled at about $370 an ounce.
