166 | The History and Use of Our Earth’s Chemical Elements Gold is classed as a heavy, noble metal located just below copper and silver in group 11 of

166 | The History and Use of Our Earth’s Chemical Elements Gold is classed as a heavy, noble metal located just below copper and silver in group 11 of

the periodic table. Gold is a good conductor of electricity as well as an excellent heat reflector of infrared radiation, which makes it an efficient thin coating on glass in skyscrapers to reflect the heat of sunlight.

The purity of gold is measured in “carats” (one carat is equal to one part in twenty-four). The purest gold is rated at 24 carats, but it is much too soft to be used for jewelry. Good jewelry is made from 18-carat gold that is 18 parts gold and six parts alloy metal. Thus, an 18-carat gold ring is about 75% pure gold and contains about 25% of another metal, such as nickel or copper, to make it harder and more durable. Other alloy metals mixed with gold are silver, platinum, and palladium—all used to increase gold’s strength and reduce its cost. Some less expensive jewelry contains 14 or 10 carats of gold (14/24 or 10/24) as well as some other alloy metals.

Abundance฀and฀Source Gold is the 72nd most abundant element and is widely spread around the world, but it is

not evenly distributed through the surface of the Earth. It is usually found in a few concentrat-

ed regions, sometimes in pure flake and nugget metallic forms. Most of it exists in conjunction with silver ore, quartz (SiO 2 ), and the ores of tellurium, zinc, and copper. About one milligram of gold exists in every ton of seawater (this is about 10 parts of gold per trillion parts of seawater, which amounts to a total of about 79 million tons of gold in solution). No economical method of extracting gold from seawater has been developed to recover this “treasury of the sea.”

Free metallic gold is found in veins of rocks and in ores of other metals. Alluvial gold (placer deposits) is found in the sand and in the gravel at the bottom of streams where it has been deposited as a result of the movement of water over eons.

Most gold is recovered from quartz veins called “loads” and from ores that are crushed. The crushed ores are treated with a cyanide solution that dissolves the gold. This cyanide–gold solution is filtered and then treated with zinc to extract the gold. Disposal of the cyanide resi- due after the gold has been removed has become an environmental problem over the years. Gold can also be extracted from ores by using mercury to form an amalgam-like alloy, which is heated to drive off the mercury and recover the gold.

Gold is recovered economically in South Africa, Russia, Canada, Australia, Mexico, China, and India and in the states of California, Utah, Alaska, Nevada, and South Dakota in the United States. Small, scattered deposits of gold are found in several other states, including Florida, Arkansas, Washington, Oregon, Texas, Georgia, and the Carolinas. All the gold that has been refined in the world would form a cube with a volume of over 8,000 cubic meters.

History Gold is considered one of the first metals used by humans. Along with other free metals,

mankind discovered gold thousands of years ago. Most likely, early humans found pebble-like nuggets of metals, including gold, which they admired for their colors. There is evidence that gold was known in ancient Egypt about 6,000 years ago, and gold is mentioned several times in the Old Testament of the Bible.

The word “metal” is from the Greek word meaning “to hunt for” or “to search for,” which is how early humans found these rare, heavy metals. The search for gold was one of the con- tributing factors in the exploration and settlement of the New World by Europeans in the

167 Common฀Uses

Guide to the Elements |

Gold’s chemical and physical properties make it a very versatile element. Its noncorrosive nature provides protection as plating for other metals. Its malleability and ductile qualities mean it can be formed into many shapes, including very thin sheets (gold leaf) and very thin gold diode wires. Gold has the ability to carry electricity with little resistance, making it an excel- lent component for all kinds of electronic equipment. Gold leaf finds many uses in surgery, space vehicles, and works of art. Gold electronic switches do not create a dangerous spark when engaged, and they last for a long time. The element’s reflective surface provides protection from infrared heat radiation as a coating on the visors of aerospace personnel, as well as on large win- dow expanses in buildings. Its color and durability make it a major metal for the jewelry indus- try. Gold has been used to replace teeth for many ages, and the teeth usually last longer than the person wearing them. Gold is also the worldwide monetary standard, although the United States abandoned the gold standard in the 1930s. Even so, gold is still traded as a commodity. Small amounts of other metals are added to gold coinage for hardening purposes so that the coins will not wear out with use. Gold bars (bullion) are stored in the treasuries of most countries. Some countries maintain huge stockpiles of gold for both monetary and industrial uses.

Two forms of gold provide medical treatments. The radioactive isotope Au-198, with a short half-life of 2.7 days, is used to treat cancer and is produced by subjecting pure gold to neutrons within a nuclear reactor. A gold salt, a solution called sodium thiosulfate (AuNa 3 O 6 Cl 4 ), is injected as an internal treatment for rheumatoid arthritis. However, since gold and some of its compounds are toxic when ingested, this treatment may cause complications such as skin rashes and kidney failure. It is a less popular treatment, particularly with the development of newer and more effective medications.

An interesting bit of gold’s history occurred when Ernest Rutherford (1871–1937) and his junior assistants used a very thin gold leaf that was only 1/50,000 of an inch thick (about 2,000 atoms thick) to perform a classic experiment. Rutherford bombarded the gold foil with alpha particles (helium nuclei), and most of the particles passed through the foil and were detected by photographic plates that were placed behind the gold foil. Some alpha particles were deflected sideways by the nuclei of the atoms in the gold foil, and some were sent in different directions and were also recorded on photographic plates. A few particles squarely hit a few gold atoms and bounced back toward the source of the alpha particles. Because the vast majority of the particles passed through the foil as though nothing could stop them, this experiment demonstrated that the gold atom was mostly empty space, with a small, dense, positively charged nucleus, surrounded by orbiting electrons. (See the section of the book titled “Atomic Structure” for more on the Rutherford experiment.)

Examples฀of฀Compounds Because gold is a rather inert metal, it does not form many compounds. Even its oxidation

states of +1 and +3 do not react with oxygen to form metallic oxides, as do most other metals. Examples of two possible compounds of gold follow:

Auric chloride: Au + 2Cl 2 → AuCl 4 . It is important to note that the Cl 2 used here must be hot chlorine gas. Chlorauric acid: Au + 4HCl + HNO 2 → HAuCl 4 + NO + 2H 2 O. The HCl plus the HNO 2 (hydrochloric and nitric acids) are combined to produce agua regia acid, which is the only