Who are you calling an "oid"?

Humor me, and pretend that today is Wednesday. There is a very complicated explanation for why I didn’t post on time yesterday, but let’s just condense it to “life got in the way”. So, pretending that it is Wednesday, that would make today yet another chemistry Wednesday! Hurray! Last week, we got a summer vacation from our march across the periodic table (sunblock chemistry is interesting, and topical in both senses of the word), but today, we’re back to business. Last time we looked at the transition metals. That covered most of the metals on the periodic table, but not all. Today, we’re going to look at the small but mighty group of metalloids.

Ignore the bit cut off at the bottom, we'll deal with it in a week or two.

Metalloid. Metalloid. What a cool name. So scientific. The “oid” suffix comes from the Greek word “oides”, meaning “resembling in form or appearance”. So, metalloids resemble metals in form or appearance, but are not quite metals. The metalloids are a not-quite-one-thing, not-quite-the-other category - they are similar to metals in many ways, but are not metals. Look at the property of conducting electricity. As we saw a few weeks ago, conducting electricity is one of the defining properties of whether or not something is a metal. Metalloids also conduct electricity, but they do not conduct it as well as a true metal. Individual metalloid elements vary in which metallic properties they exhibit - silicon, for example, has the luster one would expect of a metal, but not the ductility - silicon may be as shiny as copper, but it is as brittle as graphite.

Take another look at the arrangement of metalloids. Here, we deviate from the group arrangements of elements we’ve been looking at - the alkali earth metals are a single group, the transition metals are a collection of groups, etc. The metalloids are spread across five groups, but have no more than two elements per group. Additionally, the “metalness” of the metalloids increases as you follow the group down towards the right - boron, at the top left, is barely metallic, while polonium, at the bottom right, is practically a true metal. What is going on here?

Time for electron arrangements! Yes, electron arrangements are really the key to most of chemistry. So, the real root of metal behavior is a tendency to give up the outermost electrons in a chemical reaction. As you move across each period of the table, the number of electrons in the atom steadily increases, and the tendency to give up electrons decreases. The metalloids mark a boundary between elements which usually give up electrons, and elements which usually do not give up electrons. Metalloid elements can bond metallically, and even form alloys in much the same way as true metals, but not quite as well as true metals. In the case of the metalloids, larger atoms are more likely to give up the outermost electrons (the larger the electron cloud, the weaker a hold the nucleus has over the outermost electrons), so the largest atoms exhibit much stronger metallic bonding. Boron, the smallest metalloid, exhibits such weak metallic bonding that it is sometimes not even included in the metalloids.

Let’s take a closer look at some members of this family, starting with boron (B), the smallest. Boron tends to bond covalently, and forms a number of commercially important compounds - boric acid, borax, and one of the major ingredients in fiberglass, to name a few. One isotope of B, boron-10, has a tendency to absorb one of the major forms of radiation, and is used to insulate nuclear reactors.

 You don't hear too much about boron-10, because it's so busy protecting us all from nuclear armaggedon

 Silicon (Si) is probably the best known of the metalloids - something to do with being the seventh-most abundant element in the universe, and the second-most abundant in the Earth’s crust. Silicon is similar in many ways to carbon, and forms an array of strong, brittle compounds - silicon carbide (Si and carbon) is widely used as an industrial abrasive. Silicon compounds are also widely used in electronics manufacture - the combination of ability to form covalent bonds and ability to weakly conduct electricity and heat makes Si an essential component of computer chips. Germanium (Ge) is another element similar to Si, and has similar applications in the manufacture of computer chips. Arsenic (As), somewhat surprisingly, is also used in the manufacture of computer chips, in addition to its better known use - poison. Whether it be rat poison, insecticide, or an Agatha Christie murder mystery, As has some truly unpleasant uses. And some beautiful uses. Orpiment, an arsenic-sulfur compound, was widely used as a pigment in the ancient and not-so-ancient world, creating rich canary yellows. If you see a yellow paint in an Egyptian wall mural, still vibrant after all of these centuries, it’s probably orpiment. Arsenic was also combined with coper and used in an array of green pigments - Scheele’s Green, Paris Green and Emerald Green, to name a few. Emerald Green was both the most vibrant and the most dangerous - the pigment was highly unstable, and in sufficiently damp conditions, capable of releasing arsenic-laden fumes to the air. This was something of a problem, given the preference for Emerald Green wallpaper in notably damp Britain...

 

 Don't drink the paint and don't breathe the air

Another element with a pigmented past is antimony (Sb). While now mostly used in computer chip manufacture and lead alloys, Sb was once a popular eye makeup among the Egyptians. Moving along, we reach tellurium (Te), which is a fairly metallic metalloid, and (shocker) used in the manufacture of computer chips. It is also used in alloys with true metals. Finally, we have polonium (Po), another discovery by the freakishly accomplished Marie Curie. Curie named this element in honor of her native Poland - something of a daring statement, given that Poland was then divided between multiple countries, and that expressing that sentiment was enough to warrant arrest in some parts of the world. Like pretty much everything Curie was interested in, Po is strongly radioactive. It is not overly abundant, but has some niche uses in photography, and may some day be used as a power source in spacecraft. Finally, we have astatine (At), which was created in a laboratory in 1940. Astatine belongs to a class of elements I like to think of as "theoretically possible, but useless" - you can create them, but they are highly unstable, and have no practical uses. Still, their existence confirms the predictive powers of the periodic table, and in that, they do serve a purpose.

The metalloids aren’t as clearly defined a cluster of elements as, say Group I, and there is some debate as to exactly what elements are within the group and which aren’t - for simplicity’s sake, I’ve given the most widely accepted definition, but by no means the universally accepted one. The metalloids are a good reminder that, try as we might to categorize everything in science, some things are never going to quite fit in. It’s reminiscent of the recent kerfuffle over whether or not Pluto is a planet. Sometimes, it’s useful to remember that the natural world existed long before humans came along with their classification schemes. While a good classification schema can be an invaluable tool for understanding how the universe works, it’s easy to get carried away and assume that how we categorize something can actually alter its properties. Metalloids are quirky, but enjoy them for their quirkiness, and (needed) havoc they can introduce to a neat system of element identification.