Metalloids — Overview & Properties

Metalloids of the Periodic Table

One of the smaller classes of elements on the periodic table are the metalloids or semi-metals. These elements make a zig-zag on the right side of the periodic table. There are only seven elements of this class: boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (te), and astatine (At).

Properties of Metalloids

These elements are the go-betweens of metals and non-metals, so they share characteristics with each.

• Many are silvery.

• Most are solid at room temperature.

• They are brittle.

• They are poor conductors of heat and electricity, sometimes called semi-conductors.

Interesting Facts on Metalloids

Silicon is a well-known metalloid and is often used in computer hardware and other electronics. It is also the second most abundant element on the earth after oxygen. Tellurium is the least abundant element on earth. Arsenic is a toxic metalloid and has been known to be found in drinking water. The levels of arsenic in water can be dangerous and ways to remove arsenic from water have been researched by environmental chemists.

(Video) Christopher Rafalik

by Metals Metalloids Non Metals

In this video, Christopher Rafalik compares metals, metalloids, and nonmetals. He starts with their location on the periodic table. Then he discusses their chemical and physical properties. The metals are shiny, malleable, and solids. They also have high boiling points and conduct heat and electricity. Why? Because of their metallic bonding. Next, he moves to the nonmetals. They are often gases and insulators. Any nonmetals that are solids are brittle.

The metalloids' behavior is between metals and nonmetals. For example, many metalloids are semiconductors. What's that? A semiconductor's conduction changes with temperature. It's how your computer's processor sends and receives information. Do you want to learn more about the different classes of elements? This video is a great comparison!

Chemistry of the Metalloids

The metalloids are a unique class of elements. They span groups three to seven of the periodic table. So, their chemistry is quite diverse. In this lesson, we'll study each metalloid in a group. In this lesson, I'll be addressing advanced aspects of chemical reactions. So, if you're a beginning chemistry student, don't expect to understand everything. That's okay! I hope that you'll learn some and have more appreciation for chemistry. If you've been studying chemistry with us for a while, I hope you'll see how various chemistry topics apply.

Group 3: Boron

Boron is the only metalloid in group three. It has unbelievably unique chemistry! We know it only has three valence electrons. So, we may expect boron to form ionic bonds. It would make sense for it to lose three electrons. But, the ionization energies are massive! So, boron only bonds covalently. How many times can it bond? Only three. So, it's an exception to the octet rule. For a full octet, boron has to bind with a ligand. Ligands can donate a lone pair of electrons. So, boron doesn't have to contribute.

Boron's chemistry gets even more unique! Let's zoom in on its orbitals. Its electron configuration is $[He]2s^{2}2p^{1}$. But, when bonding, one of the 2s-orbital electrons gets promoted. So, the configuration becomes $[He]2s^{1}2p^{2}$. But why? So it can produce three hybrid orbitals. It has the options of $sp^{3}$ or $sp^{2}$ hybridization. So, it can form three single bonds, or a single and a double bond. The molecular geometry of the $sp^{3}$ hybridization creates a trigonal planar. But often, it binds to a ligand to create a tetrahedral.

So, what are some common boron compounds? Some of the most common are boron halides. For example, boron trifluoride is common in organic chemistry. Because they can accept an electron pair, they are lewis acids in acid-base reactions. But, some boron compounds are Arrhenius acids. For example, boric acid ($B(OH)_{3}$) is a weak triprotic acid.

Another fascinating class of compounds is the boranes. They form between boron and hydrogen. So, borane is $BH_{3}$. But, it's not very interesting. Diborane is $B_{2}H_{6}$. At first, you might assume there is a bond between the two boron atoms. But how could that happen? The central boron doesn't have any electrons to share. Instead, there are two sets of boron-hydrogen-boron bonds. The classical picture is an electron pair bonding three atoms! Today physical chemists argue there is a unique overlap of the molecular orbitals. Either way, it's super cool!

Group 4: Silicon and Germanium

We could write whole textbooks about silicon. We use it every day, almost 24/7! Why? It's the main component of electronic processors. The reason I can write this explanation on my computer is silicon. When I get bored and pick up my phone, it's using silicon. Silicon is a semiconductor. It conducts electricity based on its temperature. I'm not a computer expert. But, computers essentially send data through binary code. That's 0 or 1. Off or on. How do we control that? By controlling silicon's temperature. If the temperature is low, it can't conduct. If the temperature is high, it can.

Germanium has a unique history. It is an unbelievably rare element. So, it was challenging to find. Dimitri Mendeleev predicted its existence on his periodic table. But, it would take over 20 years before anyone confirmed its existence. What do we do with it? The most common application is as a germanium-silicon alloy. The alloy combines the semiconductor properties of both elements. But, the temperature range is more narrow. So, it's easier to switch between on and off. What's the result? It's a faster semiconductor than the pure elements.

Group 5: Arsenic and Antimony

I'm sure you've heard of arsenic. Why? It's toxic and carcinogenic! Technically, the $+3$ oxidation state is the problem. It's soluble. So, it can end up in drinking water. What does it do to us? It competes with the sulfur-containing proteins in the body. What does that mean? Let's consider Lipoic acid. It's a part of the enzyme pyruvate dehydrogenase. It contains two sulfur atoms that help it bind to the enzyme complex. But, arsenic is better at binding than sulfur. If arsenic binds, pyruvate dehydrogenase doesn't work. What's the problem? Pyruvate dehydrogenase starts the citric acid cycle. If it's not working, our cells can't create energy! Thankfully, arsenic toxicity requires moderately large concentrations. But, even at low concentrations, it's a carcinogen. Some studies suggest an increase in cancer risk above 100 parts per billion! So, how do we remove it from water? We need to use an oxidation-reduction reaction. Arsenic (V) has a low solubility. Moreso, we can affect the solubility by changing the pH. So, we can take advantage of selective precipitation.

What about antimony? It's not as exciting or worrisome as arsenic. It has some applications as an alloy, catalyst, and fire retardant.

Group 6: Tellurium

Tellurium isn't a super exciting element. Its main uses are semiconductors, alloys, and catalysts. Its most unique feature is its conductivity is altered by light. So, it has photoconductivity. How does that work? From the photoelectric effect, we know that when a photon hits an electron, it can eject it. But, the metals and metalloids have some degree of metallic bonding. The ejected electron doesn't fly away. Instead, it participates in the sea of electrons. So, it can move through the material. In the metalloids, the electrons are tightly bound. They can't move unless we add energy. Often, increasing the temperature gives them a capacity for conductivity. But, in tellurium's case, absorption of light causes conductivity.

Group 7: Astatine

There is debate if Astatine is a metalloid or a halogen. Why? It's unbelievably rare. So, it's challenging to study. All its isotopes are radioactive. The most stable has a half-life of about eight hours. That's not a lot of time to do experiments! Most of its physical and chemical properties are theoretical. What's our theoretical prediction based on? Its position on the periodic table.