Gold prospects: gold can be found anywhere by anyone.
Bassanio knew the way to Portia's heart was not through gold. The same cannot be said for the rest of the world. From the mountains of Minais Gerais in Brazil and up to the Western Cordillera of British Columbia--gold has lured people to migrate across country just for the prospect of returning home with riches.
Gold has been used by a number of diverse civilizations throughout history. It continues to be internationally recognized as a symbol of wealth and artistic merit. Egyptian Pharaoh King Tutankhamun of the 14th century BC was encased in a coffin of pure gold surrounded by priceless golden objects. Gold is still the metal of choice in the 21st century for declaring one's undying love and commitment.
Metallic gold is malleable, ductile, inert, dense and conducive to heat and electricity. These properties, combined with its shiny, yellow lustre, enhance gold's desirability.
Gold is malleable and ductile--so soft that 1 ounce of gold can be beaten and flattened out to 300 square feet. As such, gold is a prime metal of choice for jewelry and other artistic artifacts. Because gold is so soft, other metals are often required to alloy with gold to give it more strength. The purity of gold relates to its colour and is characterized by karats. Gold and silver both belong to the copper family. They have the same number of valence electrons, the same atomic structure and have nearly identical atomic size. This means that silver alloys with gold naturally. Pure gold is bright yellow and is called 24-karat gold. As the amount of silver increases, the colour becomes paler and the purity is reduced to 20-karat for an 80 percent gold/20 percent silver alloy. Other similar atoms like copper, palladium, rhodium and iridium have been substituted into gold to increase its hardness.
Gold is considered relatively inert and stable. It is unreactive in air, unreactive towards acids, does not rust, tarnish, decay of decompose. It exists as a free metal in the earth, and it can lay underground or on the bottom of the sea untouched for centuries. Therefore, gold has been deposited throughout the world. It is generally found in only minute quantities, but gold can be found anywhere by anyone.
The origin of gold ore deposits is still up for debate. Depending on the age of the surrounding rock, popular theories include the possibilities that gold washes in from rivers and streams from volcanic mountains and that hot spring fluids deposit gold inside the rocks.
Geochemistry has recently solved the riddle of the origin of South African gold nuggets found in the Witwatersrand Basin. (l) In this particular case, if the gold is older than the surrounding rock, it must have washed in from the surrounding mountains and highlands, and the rock then built up around the gold. If the rock is older than the gold, then the hydrothermal model stating that the gold seeped in with fluids into crevices in the rock makes more sense. A team of geochemists, led by Jason Kirk from the University of Arizona, used radiochemistry to determine the age of the gold. Rhenium and osmium are both naturally found in gold. Rhenium naturally decays into osmium with a half-life of 42.3 billion years! By dissolving gold grains and measuring the ratio of rhenium to osmium, the gold was aged at a quarter of a billion years older than the rock. The geochemists also concluded that the rhenium-to-osmium ratio means that the gold came from the Earth's mantle and not its crust. They proposed that the Witwatersrand Basin gold originated from volcanic rocks and not from granite in the Earth's crust. Gold-panning enthusiasts take this finding as a golden opportunity for uncovering other gold deposits.
Gold weighs in at 19.3 grams per cubic centimetre. It is 19 times heavier than water, making gold one of the heaviest metals. As a result, gold deposits are found settled at the bottom of river beds or concentrated in soil. The host rocks are subject to weathering and chemical erosion. Eventually, they are broken down to expose the quartz and gold. Weathering
also fragments the quartz, releasing any gold contained in it. Since gold is so heavy, it sinks. This specific attribute has led to the success of gold panning in both modern and ancient times. Gold can be panned in water or the surrounding debris and sifted and sorted easily.
Gold's density and stability lend to its accessibility. Its softness and conductive properties lead to its wide utility. Its colour adds beauty. All of its inherent chemistry designates gold as the world's most treasured metal.
(1.) Charles Choi, "Origin of World's Largest Gold Deposit Found," United Press International Science News, September 23, 2002.
RELATED ARTICLE: Divide and protect structure of the 25-atom gold nanocluster.
A report published in the July 8 issue of the journal Proceedings of the National Academy of Sciences (PNAS) is the first to describe the principles behind the stability and electronic properties of tiny nanoclusters of metallic gold. The study, which confirms the "divide and protect" bonding structure, resulted from the work of researchers at four universities on two continents.
"While gold nanoparticles are being used by so many researchers--chemists, materials scientists and biomedical engineers--no one understood their molecular and electronic structures until now," said Robert Whetten, a professor in the Georgia Institute of Technology's School of Physics and School of Chemistry and Biochemistry. "This research opens a new window for nanoparticle chemistry."
Gold and sulfur atoms tend to aggregate in specific numbers and highly symmetrical geometries. Sometimes these clusters are called "superatoms" because they can mimic the chemistry of single atoms of a completely different element. Researchers commonly use gold nanoparticles because they are stable and exhibit distinct optical, electronic, electrochemical and bio-labeling properties. However, understanding the physicochemical properties of such clusters is a challenge requiring knowledge of their atomic structures. A significant advance came in late 2007, when Stanford University researchers reported the first-ever total structure determination of a 102-atom gold cluster. The X-ray structure study revealed that pairs of organic sulfur ("thiolate") groups extracted gold atoms from the gold layer to form a linear thiolate-gold-thiolate bridge while interacting weakly with the metal surface below. These gold-thiolate complexes formed a sort of protective crust around the nanoparticles.
"This discovery contradicted what most chemists believed was going on--which was that the sulfur atom merely sat atop the uppermost gold layer, bound to three adjacent metal atoms," said Whetten.
With the experimentally determined structural coordinates, an international team of researchers conducted large-scale electronic structure calculations in supercomputing centres in Finland, Sweden and Germany. They found that the 102-atom gold cluster was a "superatom" with a core of 79 gold atoms arranged into a truncated decahedron. The results confirmed the "divide and protect" structure first predicted by team member Hannu Hakkinen, a professor at the University of Jyvaskyla.
"In 2006, we predicted that gold atoms in this bonding motif were divided in two groups--those that made the metal core and those that helped to protect it," explained Haikkinen. "Now there was evidence that this was true."
In the study reported in PNAS, the researchers found that the clusters were stable because the surface gold atoms in the core each had at least one surface-chemical bond and the gold core exhibited a strong electron shell closing. With the 102-atom gold cluster, each gold atom in the cluster donated one valence electron. Forty-four of those electrons were immobilized in bonds between gold atoms and thiolates, leaving 58 electrons to fill a shell around the "superatom." In this configuration, the cluster wouldn't benefit from adding of shedding electrons, which would destabilize its structure. This process is similar to what happens in noble gases, which are chemically inert because they have just the right number of electrons to fill a shell around each atom's nucleus.
Associated with the filled electron shell, the gold-thiolate compound also had a major energy gap to unoccupied states. The calculated energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital states for the 102-atom compound was significant--0.5 electron volts. Metals typically have a gap of zero, so this gap indicates an atypical electronic stability of the compound.
In addition to the 102-atom compound, the researchers also determined the electronic structures for 11-, 13- and 39-atom gold cluster compounds. They found that the 11- and 13-gold atom clusters form closed electronic shells with 8 electrons and the 39-atom gold clusters with 34.
"The theoretical concepts published in this paper provide a solid background for further understanding of the distinct electrical, optical and chemical properties of the stable mono-layer-protected gold nanoclusters," said Whetten. The study also shows that experimentally well-characterized, structure-resolved, thermodynamically stable species of thiolate-, phosphine-halide-, and phosphine-thiolate-protected gold nanoparticles share common factors underlying their stability.
Once this initial work was completed, the researchers started predicting the structures of other stable gold cluster compositions that are still awaiting a precise structure determination.
"We now have a unified model that provides a solid background for nanoengineering ligand-protected gold clusters for applications in catalysis, sensing, photonics, bio-labeling and molecular electronics," said Haikkinen.
Abby J. Vogel
Georgia Institute of Technology
Anne Campbell, MCIC, has a BSc in chemistry from the University of Guelph and an MA in chemistry from Brown University in Providence, RI. She tutors chemistry students of all ages and is the CIC career services and student affairs officer.
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|Publication:||Canadian Chemical News|
|Date:||Sep 1, 2008|
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