What are the metalloids? Most elements fit relatively neatly into one of two categories: metals and nonmetals. Metalloids, however, can be classified as somewhere in-between. For this reason, they are sometimes, but less commonly, referred to as semimetals.
Elements categorized as metalloids are often highlighted into what can be called the “Metalloid Stair Step” because this group of elements somewhat resembles a staircase when colored differently than the neighboring elements. The metalloids separate the transition metals from the nonmetals on the periodic table. Let’s look at the properties and characteristics of metalloids, and a list of them.
Metalloids tend to have a metallic appearance, but behave more like nonmetals in most chemical reactions. All Metalloids are solid at room temperature. They are much more brittle than metals but are much poorer electrical conductors. The hybrid characteristics metalloids possess offer a broad range of real-world applications such as metal alloys, flame retardants, and semiconductors/electronics.
Characteristic Properties of Metalloids
- Metalloids are solids
- They have a metallic luster, and generally look like metals
- They are brittle, and easily shattered
- Metalloids can conduct electricity, but not as well as metals.
- Chemically, they act more like nonmetals, easily forming anions, having multiple oxidation states, and forming covalent bonds.
- Their ionization energies and electronegativities are in between the values of metals and nonmetals.
Metalloids are by far the smallest group of elements, as there are only six elements definitively classified as metalloids. They can have anywhere from three to six valence electrons in their outer energy shell. This is the driver of their reactivity/chemical behavior. Boron, which only has three valence electrons, behaves much like a metal during chemical reactions by giving up its electrons. The other metalloids, with four or more valence electrons, tend to behave more like nonmetals, gaining electrons during reactions. Let’s learn some facts about the individual metalloids, starting with boron.
A List of Metalloids
The element boron has a strong tendency to form covalent bonds, meaning that chemically, it is a nonmetal, like carbon or silicon. It has many allotropes (different forms of a pure element) and possesses high electronegativity because it is in the upper right hand corner of the periodic table.
Boron has two primary stable isotopes, 11_B and 10_B, which are found at about 80.4% and 19.6% abundance, respectively. Pure boron is often used for fuel ignition, but the element has numerous medical applications as well, such as antiseptics and eye drops, which utilize boron-based compounds. Some people consume boron as a dietary supplement because the trace mineral is important for hormonal regulation, recovery, and the uptake of some vitamins. Synthetic organic chemists also use boron-based reagents to perform hydroboration oxidation reactions, for example.
Silicon is a brittle, crystalline solid with a metallic appearance. It is a potent semiconductor, meaning it conducts electricity more efficiently at higher temperatures. Silicon compounds called silicates make up almost 90% of the earth’s crust, pure silicon is rare. It is, however, relatively common in asteroids, moons, and cosmic dust. Silicates are frequently used in the manufacturing of cement, porcelain, and ceramics.
In the 21st century, silicon has had a massive influence on the world economy through its importance in the development of semiconductor electronics. Pure silicon has been vital to the development of integrated circuit chips and transistors, both of which are crucial components of modern electronic devices, such as cell phones, televisions, and household appliances.
Germanium is a hard-brittle metalloid with a light gray appearance much like that of silicon. It is also a good semiconductor and is rarely found in the pure elemental form on earth. Germanium frequently crystallizes into a diamond structure. Germanium was predicted to exist by Dimitri Mendeleev years before it was actually discovered. He was also able to predict many of its properties using his understanding of periodic trends and knowledge of other metalloids and nearby elements.
Like silicon, germanium is also critical to modern technology, although it is primarily used in different applications than its metalloid cousin. Germanium is often used for infrared optics, solar energy, and numerous metal alloys.
Arsenic is an element with a shiny, greyish metallic appearance much like other metalloids. It readily forms covalent bonds with nonmetals. Arsenic has applications with regards to alloys, electronics, and pesticides/herbicides. However, arsenic is highly toxic to most mammals, including humans, and thus many of its uses in industry are slowly being phased out. It is classified as a Group-A carcinogen. Despite its toxicity, very small quantities of arsenic are required for human metabolism, but the mechanism for this is unknown.
Antimony is a lustrous, silvery-white metalloid with a brittle texture. It is found in nature at about ⅕ the abundance of Arsenic. Antimony has a similar atomic structure to arsenic as well, with three half-filled electron shells in the outermost shell. It typically forms covalent bonds and is highly reactive with halogens, such as sulfur, and produces a brilliant blue flame when burned.
Antimony has the unique ability to strengthen metal alloys when added, even in relatively small quantities. Antimony can also impart strength to pure metals (forming alloys), namely lead. Because of its strengthening properties, antimony is indispensable for many industries, such as car batteries, ammunition, cables, and plumbing equipment. As is consistent with other metalloids, highly purified antimony can be used in semiconductor technologies.
Tellurium is a metalloid that exhibits a similar description to antimony. It is typically found in a grey powder form. tellurium is highly reactive with sulfur and selenium and shows a green-blue flame when burned. Tellurium is industrially used as a steel additive and can be alloyed with aluminum, copper, lead, or tin.
Like antimony, tellurium can also strengthen other metals, but can also reduce corrosion when added to the aforementioned metals. Additionally, tellurium serves as a strong semiconductor, particularly when exposed to light. In nature, most tellurium is found in coal, though trace amounts are found in some plants.
Conclusion on Metalloids
It is worth it to point out, that sometimes selenium is also considered a semimetal – it depends on who is doing the classifying. Astatine and polonium are also probably semimetals, but they are highly radioactive and only handled under special conditions. Regardless of which elements you include, we can all agree that the metalloids are an interesting and fascinating group of elements!
Housecroft, Catherine E., and Alan G. Sharpe. Inorganic Chemistry. 5th Edition, Pearson, 2018.
Lejla peace – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15363940
Miessler, Gary L., and Donald A. Tarr. Inorganic Chemistry. 3rd Edition, Pearson, 2009.
Pauling, Linus. General Chemistry. 1st Edition, Dover, 1947.