What is Gold, Anyway?
Gold is a pirate's booty and an ingredient in microcircuits. It's been used to make jewelry since at least 4000 B.C. and to treat cancer only in recent decades. It's in the pot at the end of the rainbow and in the coating on astronaut visors.
Gold, the 79th element on the Periodic Table of the Elements, is one of the more recognizable of the bunch.
- Atomic Number (number of protons in the nucleus): 79
- Atomic Symbol (on the Periodic Table of Elements): Au
- Atomic Weight (average mass of the atom): 196.9665
- Density: 19.3 grams per cubic centimeter
- Phase at Room Temperature: Solid
- Melting Point: 1,947.7 degrees Fahrenheit (1,064.18 degrees C)
- Boiling Point: 5,162 degrees F (2,850 degrees C)
- Number of isotopes (atoms of the same element with a different number of neutrons): Between 18 and 59, depending on where the line for an isotope is drawn. Many artificially created gold isotopes are stable for microseconds or milliseconds before decaying into other elements. One stable isotope.
- Most common isotopes: Au-197, which makes up 100 percent of naturally occurring gold.
Shiny star matter
Humans have been decorating themselves with gold since at least 4000 B.C., according to the National Mining Association. From Eastern Europe to the Middle East to the tombs of Egyptian Pharaohs, gold appears throughout the ancient world. A Stone Age woman found buried outside of London wore a strand of gold around her neck; Celts in the third century B.C. wore golden dental implants; a Chinese king who died in 128 B.C. was buried with gold-gilded chariots and thousands of other precious objects.
Gold is malleable and shiny, making it a good metalworking material. Chemically speaking, it's a transition metal. Transition metals are unique, because they can bond with other elements using not just their outermost shell of electrons (the negatively charged particles that whirl around the nucleus), but also the outermost two shells. This happens because the large number of electrons in transition metals interferes with the usual orderly sorting of electrons into shells around the nucleus.
All the gold that makes up earrings and cuff links and electronics components today originated in space: According to a 2011 paper in the journal Nature, a meteor bombardment nearly 4 billion years ago brought 20 billion billion tons of a gold-and-precious-metal-rich space rock to Earth. Tracing gold's origin back even further takes us into deep space. A 2013 study in The Astrophysical Journal Letters found that all of the gold in the universe was likely birthed during the collisions of dead stars known as neutron stars.
Veins of gold mined from the earth are the result of hot fluids flowing through gold-bearing rock, picking up gold and concentrating it in fractures, according to the American Museum of Natural History (AMNH).
What is a karat?
Most golden jewelry isn’t made of pure gold. The amount of this precious metal in a necklace or ring is measured on the karat scale. Pure gold is 24 karats. Golden bars kept in Fort Knox and elsewhere around the world are considered to be 99.95 percent pure, 24-karat gold.
As metals are added to it during jewelry making, the gold becomes less fine and the number of karats drops. For example, 12 karat gold contains 50 percent gold and 50 percent alloys by weight.
The word karat comes from the carob seed. In ancient Asian bazaars, the seeds were used to balance scales that measured the weight of gold.
The inferior mineral nicknamed fool’s gold only mimics gold in looks. Pyrite is more common, harder, and more brittle than gold. When crushed into powder, it looks greenish-black, whereas real gold powder is yellow.
Pyrite contains sulfur and iron. During World War II it was mined to produce sulfuric acid, an industrial chemical. Today, it is used in car batteries, appliances, jewelry, and machinery.
Although fool’s gold can be a disappointing find, it is often discovered near sources of copper and real gold. A miner who stops digging once they have a piece of pyrite in hand is the real fool.
It is still used in jewelry, of course, but this element has also gone high-tech. It's an excellent conductor of electricity and is very non-reactive with air, water and most other substances, meaning it won't corrode or tarnish.
It's also used in medicine. The radioactive gold isotope Au-198 can be injected directly into the site of a tumor, where its radiation can destroy tumor cells without much spillover to the rest of the body. In 2012, researchers reported in the journal Proceedings of the National Academy of Sciences that they could link nano-particles of Au-198 with a compound found in tea leaves to treat prostrate cancer. The tea compound is attracted to the tumor cells, keeping the nano-particles glued to the right spot for several weeks while the radiation treatment occurs. (The method has yet to be tested on humans.)
In some cases, gold nano-particles are the only way a drug can work. The anti-cancer drug TNF-alpha kills cancer very effectively. Unfortunately, it's also incredibly toxic to healthy cells. However, clinical trials now underway have found that linking TNF-alpha drugs to gold nano-particles can successfully treat tumors, because the drugs hit their targets directly, according to Benchmarks, an online publication of the National Cancer Institute.
There's just one problem with humanity's continued love affair with this element: Getting it out of the ground. About 83 percent of the 2,700 tons of gold mined each year is extracted using a process called cyanidation, said Zhichang Liu, a postdoctoral researcher in chemistry at Northwestern University in Illinois. This process uses cyanide to leach gold out of the rock that holds it. Unfortunately, cyanide is toxic, and the process is anything but environmentally friendly.
There could be hope for lovers of golden baubles (and electronic circuits and nano-medicine), however. In 2013 Liu and his colleagues reported in the journal Nature Communications that they'd stumbled upon a way to extract it from ore with benign starch rather than toxic cyanide.
"Actually, we found this method by accident," Liu told Live Science. While trying to fabricate a porous material, the researchers mixed a starch called alpha-Cyclodextrin with gold salts (charged molecules of gold). To their surprise, the precious metal precipitated out of the solution rapidly.
The team has patented the method, which easily extracts gold at more than 97 percent purity in one step, Liu said. They're now working with investors to scale up the process.
"Hopefully, we can find a nice, green way to replace the cyanidation process," Liu said.