Metalloids are elements that share properties of both metals and nonmetals, sitting in a unique middle ground on the periodic table. Six elements are widely recognized as metalloids: boron, silicon, germanium, arsenic, antimony, and tellurium. They line up along the diagonal “staircase” that separates metals on the left side of the periodic table from nonmetals on the right, and their in-between position explains nearly everything about how they look, conduct electricity, and react chemically.
Where Metalloids Sit on the Periodic Table
If you look at a periodic table, you’ll notice a zig-zag line running diagonally from boron down toward astatine. Metalloids cluster along both sides of this line. They aren’t found in the far-left columns (where the most reactive metals live), the far-right columns (home to nonmetals and noble gases), or the bottom two rows of lanthanides and actinides. Their diagonal placement reflects a gradual shift in properties: elements become more nonmetallic as you move from left to right across a period, and metalloids occupy the transitional zone where that shift happens.
Some classification lists expand beyond the core six to include polonium, gallium, or even astatine, which behaves more like a metalloid than a typical halogen due to quirks in its electronic structure. But boron, silicon, germanium, arsenic, antimony, and tellurium are the six you’ll see in virtually every chemistry textbook.
Appearance and Physical Form
All six common metalloids are solids at room temperature. Silicon, germanium, arsenic, antimony, and tellurium have a shiny, metallic luster that makes them look like metals at first glance. Boron is the odd one out, appearing darker and less reflective, but it still has a somewhat glassy sheen in its crystalline form.
Despite looking metallic, metalloids are brittle. Hit a chunk of silicon or germanium with a hammer and it will shatter rather than bend. Metals, by contrast, are malleable and ductile. You can flatten a piece of copper into a sheet or draw it into wire, but you can’t do the same with a metalloid. This brittleness is one of the clearest ways metalloids behave like nonmetals.
Electrical Conductivity and Semiconduction
The single most important property of metalloids is their electrical conductivity, which falls between that of metals and nonmetals. Metals conduct electricity very well. Nonmetals hardly conduct at all. Metalloids land in between, making several of them natural semiconductors.
What makes semiconductor behavior special is how it responds to temperature. In a metal, conductivity decreases as temperature rises because heat disrupts the orderly flow of electrons. In a semiconductor like silicon or germanium, the opposite happens: conductivity increases rapidly with temperature. That’s because heat gives electrons enough energy to jump across the “band gap,” a small energy barrier that normally keeps them from flowing. Silicon’s band gap measures about 1.11 electron-volts at room temperature, while germanium’s is smaller at roughly 0.66 electron-volts. The smaller the gap, the easier it is for electrons to start conducting.
This temperature-dependent conductivity is the foundation of modern electronics. Silicon, the most abundant metalloid in Earth’s crust, dominates the semiconductor industry precisely because its conductivity can be finely tuned by adding tiny amounts of other elements, a process called doping.
Chemical Reactivity and Bonding
Metalloids sit at intermediate electronegativity values, meaning they don’t pull electrons toward themselves as aggressively as nonmetals do, but they pull harder than metals. This middle-ground electronegativity has a direct consequence for bonding: metalloids rarely form ionic bonds. Instead, they almost always form covalent bonds, sharing electrons with other atoms rather than fully transferring them.
In reactions with metals, metalloids tend to accept electrons. In reactions with nonmetals, they tend to give electrons up. This dual behavior mirrors their position between the two groups. Boron, for example, forms covalent compounds with hydrogen and oxygen, while arsenic and antimony can bond with metals in alloy-like compounds where they modify the metal’s properties, such as hardness or melting point.
Amphoteric Oxides
When metalloids combine with oxygen, they form oxides that can behave as acids or bases depending on the situation. This dual nature is called amphoteric behavior, and it’s another hallmark of the metalloid group.
Pure metal oxides tend to be basic: dissolve them in water and the solution becomes alkaline. Pure nonmetal oxides tend to be acidic. Metalloid oxides split the difference. Place an amphoteric oxide in an acidic solution and it reacts like a base, neutralizing the acid. Place the same oxide in a basic solution and it reacts like an acid. This flexibility comes from the metalloid’s intermediate electronegativity, which allows the oxygen bonds to break in either direction depending on the chemical environment.
This trend also follows the periodic table’s left-to-right pattern. Within any row, oxides become progressively more acidic as you move toward the nonmetal side. Metalloid oxides sit right at the crossover point, which is why they can swing either way.
Thermal Conductivity and Density
Metalloids conduct heat better than most nonmetals but less efficiently than metals. Their densities also fall in an intermediate range. Silicon, for instance, is noticeably lighter than iron or copper but heavier than typical nonmetals like sulfur or phosphorus. This pattern holds for the group as a whole, though antimony and tellurium sit closer to the metallic end of the density spectrum.
Common Uses Tied to These Properties
Nearly every general property of metalloids points toward a practical application. Their semiconductor behavior makes silicon and germanium essential for computer chips, solar cells, and transistors. Boron’s hardness and high melting point make it useful in heat-resistant glass (borosilicate) and abrasives. Arsenic and antimony show up in specialized alloys where they increase hardness or lower melting points. Tellurium is used in thermoelectric devices that convert heat directly into electricity, taking advantage of its unusual thermal and electrical properties.
The semiconductor industry alone accounts for an enormous share of metalloid demand. Silicon wafers form the backbone of virtually every electronic device, from smartphones to satellites. Germanium, with its smaller band gap, is favored in infrared optics and fiber-optic systems where silicon’s properties aren’t quite right. In each case, it’s the metalloid’s in-between nature, not fully metal and not fully nonmetal, that makes it uniquely suited for the job.