For many thousands of years, humans have known and valued the metallic element gold (Au).Too soft for use in making weapons or tools it was for its beauty and rarity that man searched for this metal.
Gold’s atomic number of 79 puts the element in period 6 and group 11 (also known as coinage metals) of the periodic table. Its atomic mass is 196.96655 and its density is 19.32 grams per cubic centimeter. Gold melts at 1064.43 °C (1337.5801 K, 1947.9741 °F) and boils at 2807.0 °C (3080.15 K, 5084.6 °F). It has an ionization energy of 9.226 eV. Naturally occurring gold consists of a single isotope, this is the stable isotope gold-137.
This extremely malleable and ductile metal is ideal for manufacturing of jewelry and ornaments. Beating a single ounce of gold can produce a thin sheet with an area of 300 square feet. It is possible to read text through a thinly beaten piece of gold. Thin sheets of gold, known as gold leaf, are used in gilding many decorative objects.
Some gold occurs in its natural state and the finding of gold nuggets amongst the gravel of rivers and streams have caused many a gold rush. A lot of gold mined today is associated with other minerals. The un-regulated extraction of gold, by forming an amalgam with mercury, has led to extensive mercury poisoning of some watercourses. The gold mines in South Africa produce two thirds of the world’s gold supply. Within the United States, the majority of gold production comes from mines in South Dakota and Nevada. Our oceans contain gold, with one milligram of the metal in a ton of seawater. At present, the cost of extraction of this gold exceeds its value.
Gold reacts with very few chemical reagents. Aqua regia, made of one part nitric acid and three parts hydrochloric acid, is so called because it can dissolve gold.
Pure gold is too soft for most uses. To increase its hardness gold is alloyed with a number of different metals. Commonly, gold alloys contain silver, platinum or copper. The purity of gold is measured in carats. Pure gold is twenty-four-carat gold, while eighteen-carat gold contains eighteen parts gold to six parts of other metal.
As gold reacts with so few other elements, it resists tarnishing in air. It is also good conductor of electricity. These properties allow gold’s use by the electronics industry to make electrical contacts and printed circuit boards.
Medicine utilizes a number of gold treatments. Doctors treat some cancers with the isotope gold-198, which has a half-life of 2.96517 days and decays by beta decay. The gold compounds, gold sodium thiosulfate, sodium aurothiomalate, aurothioglucose and Auranofin are used to treat a number of autoimmune diseases such as rheumatoid arthritis and Lupus erythematosus.
Medical uses of gold compounds Past present and future Simon P. Fricker
Los Alamos National Laboratory’s Chemical Division Periodic Table of Elements
Jefferson Labs Science Education