From silverware to skyscrapers, metals form the physical foundation of our society. We use them to create artificial joints, build computers, solve transportation needs, fill dental cavities, wire our cities, incarcerate our criminals and adorn our bodies. Yet how much do you really know about these all-purpose elements?
For example, did you realize that calcium was a metal? How about sodium? Or, for that matter, potassium or lithium? Then there are the metals that sound like science-fiction writers named them: praseodymium, gadolinium and ytterbium.
So what exactly is a metal?
Because metals make up about 75 percent of the periodic table of the elements, their characteristics are extremely varied. Yet a generally accepted definition is that a metal is an element (something that can't be broken down into other components) that exhibits a luster or shine, has a certain degree of ductility and malleability (which means it can be hammered or twisted into wires), and conducts heat and electricity [source: Moore].
But as we'll see, just like some notorious heavy metal guitarists, not all metals obey the rules.
You might think of sodium as table salt instead of as a wildly volatile metal. But what you have in a shaker next to the pepper is actually a compound known as sodium chloride. It consists of sodium metal bonded to another element -- the toxic gas, chlorine -- that makes it an innocuous (and tasty) seasoning. Sodium all by itself, however, is another story entirely.
Sodium belongs to a class of metals known as the alkali metals. Like the other members of this family, the thing that makes sodium so un-metal-like is the fact that it reacts violently with water. This is because the alkali metals only have one electron circling around their nuclei. That electron is eager for company, so it readily reacts with other substances (like water) in a sometimes-violent reaction that leads to a more stable compound.
Sodium is also an extremely light metal that will float on water -- that is, until it bursts into flame. Another quality of sodium that makes it seem less like a metal is that it's extremely soft and can be cut with a butter knife. Just make sure the knife is completely dry!
4: Mercury
Mercury is probably the most well-known strange metal. That's because it's a liquid at room temperature, which makes it look like a melted mirror. Although mercury is the most famous liquid metal, it's not the only one:
Francium melts at 80.6 degrees Fahrenheit (27.0 degrees Celsius)
Cesium melts at 83.2 degrees Fahrenheit (28.4 degrees Celsius)
Gallium melts at 85.6 degrees Fahrenheit (29.8 degrees Celsius)
All of these would be liquid in a warm room. Mercury, however, melts at -37.8 degrees Fahrenheit (-38.8 degrees Celsius), so it's pretty much a liquid wherever it's found on Earth -- except when bound to sulfide in the ore known as cinnabar, which is how it's most commonly mined.
Mercury is also fascinating because it's 13.5 times denser than water. This allows heavy objects such as bricks and cannonballs to float on its surface. Although mercury has traditionally been used in such common equipment as thermometers, barometers and mirrors, it's considered highly toxic. In fact, the term "mad as a hatter" is believed to come from the 19th-century workers who treated hats with mercury and suffered debilitating physical and mental symptoms as a result. Mercury was allegedly painted under the wings of German planes by the allies in World War II, as it dissolves aluminum and would have caused the planes to fall apart in midair. It's also believed that Mozart may have died from mercury poisoning; he would have taken copious amounts of the toxic metal to help treat his syphilis.
3: Radium
If mercury made hatters crazy in the 19th century, then radium worked its destruction on the women who painted the faces of watches with the material in the early 1910s. The metal was used to make a paint that coated the numerals and hands of the timepieces because of radium's most un-metal-like property: it glows.
Unfortunately for the "radium girls," as they've come to be known, radium is also highly radioactive. Because the girls were encouraged to lick the tips of the brushes to help the bristles stay together in a sharp point -- and because the girls also used the paint to cover their teeth and nails as a surprise for their friends -- many of them became seriously ill and even died. This eventually led to a lawsuit against U.S. Radium.
First isolated by Marie Curie and the French physicist André-Louis Debierne in 1911, radium has been used in cancer treatments, although it's gradually being replaced by safer technologies. It's considered a rare metal and is found in less than 1 part in 10 trillion in the Earth's crust. Only approximately 5 pounds of radium has been produced in the world since 1954 [source: Keller].
2: Iridium
Iridium is unique among metals because it resists corrosion. Leave a metal horseshoe outside, and it'll soon turn an orange-brown color as exposure to oxygen and rain causes it to rust. Leave a piece of iridium outside (if you could find one), and it would pretty much look the same for decades.
The process by which metals corrode in the presence of air is known as oxidation. When this occurs, atoms of one material are drawn to atoms of another. In the case of the iron horseshoe, atoms of the metal are lost to atoms of oxygen, and iron oxide -- rust -- forms.
Iridium isn't the only metal that resists rust. Anyone who's stumbled upon an old aluminum can on the side of the road knows that it doesn't rust, either -- but that doesn't mean it won't oxidize. In fact, the reason aluminum is so water-resistant is precisely because it does oxidize. It's just that when it's exposed to air, instead of rust, it produces a thin and super-strong protective layer that guards the aluminum underneath from corrosion.
Iridium, on the other hand, resists oxidation like Superman resists bullets, and that's why it makes our list of the top five metals that don't act like metals.
Another metal that, when oxidized, forms a coating that prevents reactions with air and water is beryllium. But that's not why it's included here as a black sheep of the metal family. Beryllium makes the top five for another non-metal attribute: it's virtually transparent to X-rays.
As anyone who's ever had dental X-ray knows, most metals show up as dark spots on film. Beryllium, on the other hand, doesn't share this quality. That's because beryllium atoms, with an atomic number of just four, are relatively small (as opposed to the calcium in our bones, which has an atomic number of 20). They don't absorb X-ray photons -- something that's necessary for them to appear on the film. For this reason, beryllium is used in the creation of X-ray tubes.
Beryllium is also frequently combined with copper to make spark-proof tools. Copper doesn't spark when it strikes iron, but it's too weak to be forged into a useful tool on its own. Bonded with beryllium, however, it gains the required strength.
Want to learn more about metals? Check out the next page for lots more information.
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