A metalloid is an element that shares properties of both metals and nonmetals, sitting right at the boundary between the two on the periodic table. The most commonly recognized metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium. These elements look somewhat metallic but behave unpredictably: they can conduct electricity under some conditions but not others, and they tend to be brittle rather than bendable. That in-between nature makes them extraordinarily useful, especially in electronics.
Where Metalloids Sit on the Periodic Table
If you look at a periodic table, you’ll usually see a stair-step line running diagonally from boron down toward polonium. This line separates the metals on the left from the nonmetals on the right. The metalloids cluster along this dividing line, which is why they exhibit mixed characteristics.
The exact list of metalloids varies slightly depending on the source. Six elements appear on nearly every list: boron, silicon, germanium, arsenic, antimony, and tellurium. Some references add selenium or polonium, but a 2010 paper in the Journal of Chemical Education argued that polonium is unambiguously a metal and astatine is unambiguously a nonmetal, despite many textbooks shading them as metalloids. The confusion comes from assuming that every element touching the stair-step line must be a metalloid, which isn’t the case.
Physical Properties: Metallic Looks, Nonmetallic Behavior
All metalloids are solid at room temperature. Several of them, silicon being the best example, have a shiny, metallic luster that makes them look like metals at first glance. But pick one up and try to bend it, and it snaps. Metalloids are brittle, a trait they share with many nonmetals. They also conduct heat and electricity far less effectively than true metals like copper or aluminum.
That poor-but-not-zero conductivity is actually the defining physical trait of the group. Metals conduct electricity freely. Insulators barely conduct at all. Metalloids fall in between as semiconductors, meaning their ability to carry electrical current changes with temperature, light exposure, or the addition of tiny amounts of other elements. This property is the reason metalloids are the backbone of the modern electronics industry.
How Semiconductors Work
In any solid material, electrons occupy energy levels grouped into bands. In metals, electrons flow freely between these bands with no energy barrier, which is why metals conduct so well. In insulators like diamond, the gap between the filled band and the empty band above it is enormous (5.5 electron volts for diamond), so almost no electrons can jump across, and electricity doesn’t flow.
Metalloids have a small energy gap. Silicon’s gap is 1.12 electron volts, germanium’s is 0.67, and tellurium’s is just 0.35. These gaps are small enough that a modest push (heat, light, or an applied voltage) can kick electrons into the conducting band. This is what makes a semiconductor: a material whose conductivity you can control. By adding trace impurities to silicon or germanium, engineers tune exactly how much current flows through a chip, which is the foundation of every transistor, processor, and solar cell on the planet.
Chemical Behavior
Chemically, metalloids split the difference between metals and nonmetals too. They form covalent crystals the way nonmetals do, building rigid lattice structures by sharing electrons between atoms. But unlike typical nonmetals, they generally don’t form simple negatively charged ions in solution.
Some metalloids react with both acids and bases, a trait chemists call amphoteric behavior. Silicon, for instance, develops a thin protective oxide layer on its surface (similar to how aluminum resists corrosion). Strong bases can dissolve that oxide layer and then attack the silicon underneath, releasing hydrogen gas. Boron, meanwhile, forms boric acid, an extremely weak acid that works not by donating a proton the way most acids do but by accepting an electron pair from water molecules. These quirks make metalloid chemistry difficult to predict from simple rules, which is part of why they occupy their own category.
Silicon and Germanium in Electronics
Silicon dominates the semiconductor industry for practical reasons: it’s the second most abundant element in Earth’s crust, its oxide forms a natural insulating layer, and decades of manufacturing infrastructure are built around it. Every smartphone, laptop, and data center runs on silicon-based chips.
Germanium was actually used first. The earliest transistors in the 1950s were germanium devices. It fell out of favor because silicon’s oxide layer made mass production easier. But germanium is making a comeback. Researchers have recently developed germanium-silicon composite materials that achieved a record charge mobility of 7.15 million square centimeters per volt-second, meaning electrons move through the material far more easily than through pure silicon. That kind of performance could lead to faster, more energy-efficient quantum computing chips.
Boron in Glass and Industry
Boron’s most familiar application is borosilicate glass, the heat-resistant glass used in laboratory beakers, baking dishes, and high-end cookware. Adding boron oxide to the glass recipe lowers its thermal expansion, which means it doesn’t crack when you pour boiling water into it or move it from oven to countertop. Boron compounds also show up in detergents, fertilizers, and ceramics.
In nuclear energy, boron plays a safety role. It absorbs neutrons exceptionally well, so boron-containing materials are used in control rods and shielding to regulate nuclear reactions.
Arsenic and Antimony in Alloys
Arsenic and antimony are best known for their toxicity, but both have important industrial roles. Antimony is alloyed with lead to increase hardness, mechanical strength, and corrosion resistance. This combination has historically been used in lead-acid battery grids, small arms ammunition, cable sheathing, and lead pipe. One unusual property of antimony alloys is that they expand slightly when cooling, which makes them valuable for casting applications where a precise mold fit matters, such as in traditional metal type for printing.
The health risks are real, though. Workers in battery manufacturing plants can be exposed to antimony compounds at levels five times higher than workers in other industrial settings. Arsenic exposure, whether through contaminated groundwater or occupational contact, is a well-documented public health concern worldwide.
Metalloids as Trace Elements in Biology
At least one metalloid may be essential for human health. The World Health Organization lists silicon as a “probably essential” trace element. Silicon appears to play a role in bone and connective tissue formation, though the exact biological mechanisms are still being clarified. Boron also shows up in trace amounts in the body and has been linked to bone metabolism and immune function in nutritional research.
The flip side is toxicity. Arsenic, antimony, and tellurium are all harmful at elevated exposures. The same in-between chemistry that makes metalloids useful in industry also makes some of them biologically reactive in ways that pure metals or nonmetals are not.