Precious!

Not all metals oxidize, tarnish, or otherwise form coatings when exposed to air. With the exception of silver, the so-called precious metals - gold, platinum, iridium, palladium, rhodium, ruthenium, rhenium, and osmium - do not react with chemicals in the air, and remain shiny and beautiful. Because of this, they are often used to make jewelry.

And these metals really are precious! Here are some typical prices for one ounce of some precious metals as of July 2011: $40 for silver (not too bad!), $800 for palladium, $1600 for gold, $1800 for platinum, and a whopping $4500 for rhenium!

The funny thing is that one metal used to be precious, but isn't any more, and that's regular old aluminum. We now make disposable cans and food containers out of aluminum (although it's always good to recycle when possible), but at one time, aluminum was more expensive than gold! In fact, in the early 1800's, the French emperor Napoleon had special flatware (forks, spoons, and knives) made out of aluminum, which he used for only his most important dinner guests. Why was aluminum so expensive? Unlike many other metals, aluminum does not occur naturally in pure metallic form. Much of is was found as alum, a mineral from which the aluminum must be separated. In Napoleon's time, this was a very long and difficult process. But in the late 1800's, an American chemist named Charles Martin Hall discovered a process that used electricity to obtain aluminum from alum cheaply and easily. Its price fell from $600 per pound in 1850 to 18 cents per pound in 1914. Hall and some partners formed the Aluminum Company of America (now Alcoa). One other interesting note about Hall - he was very young when he discovered his aluminum process. In fact, he had just graduated from college a few years before!

Rust, Tarnish, and Verdigris

OK, so you certainly know what rust looks like, and may have seen tarnished silver, but what is that other thing? And what does it have to do with rust and tarnish?

Rust, tarnish, and verdigris (and the oxide coating on old pennies) all result from the chemical reaction of a bare metal surface with something in the air. That something may be oxygen, water, carbon dioxide, another chemical, or some combination. Rust forms when iron is exposed to air and moisture (water). If a piece of iron is left outside, it will eventually rust away to nothing. If you don't want an iron object to rust, you need to protect its surface by painting it or coating it with another metal, such as zinc (called galvanizing). You might think that silver tarnish also forms from a reaction with oxygen, but tarnish actually needs both oxygen and sulfur to form. Tarnished silver may look ugly, but the tarnish actually protects the silver underneath from further reaction!

Verdigris (VUR-di-gree) is the green coating that forms on copper or other copper-containing metals, such as bronze or brass. It forms from the reaction of copper with oxygen, carbon dioxide, and chloride (near seawater). Perhaps the most recognizable American example of verdigris is the Statue of Liberty. She is made of copper, and was originally brown! But over time, a verdigris coating has formed, giving her the familiar green color. You can sometimes see other examples of verdigris on copper roofs on old buildings, or on outdoor statues. Keep your eyes open, and you may spot some verdigris in your own neighborhood!

All About Alloys

There are some familiar metals that are actually mixtures of several different types of metals - they are called alloys (AL-oise). Here are a few you've probably heard of (and some that are likely new to you):

• Steel: mixture of iron and one or more other metals (there will be a separate post about steel later)
• Brass (copper and zinc): a shiny yellow metal that looks a little like gold
• Bronze (copper and tin): a dark gold to brown metal that gets darker in color as it ages
• Pewter (copper, tin, and lead): a dark gray metal used for bowls, flatware, jewelry and other decorative items
• Solder (lead and usually tin): an alloy that melts at a relatively low temperature; used to make electrical circuits
• Nitinol (nickel and titanium): this alloy remembers its original shape; used to make eyeglass frames
• Electrum (gold and silver): also known as white gold; used in jewelry
• Alnico (aluminum, nickel, and copper): used to make magnets
• Amalgam (silver, tin, copper, zinc, and mercury): a soft alloy that has been used to fill dental cavities
• Nichrome (nickel and chromium): an alloy that glows orange-red and gives off heat when an electrical current is passed through it; used in toasters and electric burners

Take a look around your house and see how many examples of alloys you can find!

Fun Facts 8

Rolling along...

• A little while ago, we discussed the Periodic Table. Dmitri Mendeleev, a Russian chemist, is the person who is most responsible for the modern Periodic Table, so an element was named in his honor: mendelevium (Md, #101)
• Welders and glassblowers, who work with very bright flames, wear special protective goggles made with didymium glass, which contains praseodymium (Pr, #59)
• Holmium (Ho, #67) oxide has the odd property of appearing light brown in natural light, but red under fluorescent light!
• Toothpaste often contains fluorine (F, #9) because it makes the surface of teeth stronger and more resistant to tooth decay.
• Regular incandescent light bulbs are filled with argon (Ar, #18) gas which helps the filament last longer.
• You might expect that the U.S. nickel (5-cent) coin is made out of nickel (Ni, #28). Actually, it's only 25% Ni (the rest is copper, Cu).
• Many antiperspirants contain a zirconium (Zr, #40) compound that absorbs moisture.
• You may have seen those glasses that change from clear indoors to dark gray outside. They're known as photochromic or photo-gray glasses. Did you know that the color change is due to the presence of silver (Ag, #47) in the lenses? Light causes silver chloride (AgCl), which is clear, to turn into silver metal (Ag), which looks dark.
• Rock layers near the K-T boundary, which corresponds to the disappearance of dinosaurs, is unusally rich in iridium (Ir, #77). Since Ir is more common in meteorites than on earth, scientists believe that the impact of a large meteorite is at least partially responsible for dinosaur extinction.
• Plutonium (Pu, #94) is a very dangerous radioactive element. However, it is used to supply power to spacecraft.