Metalloids have properties that fall between metals and nonmetals, making them one of the more unusual groups on the periodic table. They can look shiny like metals but shatter like glass, and they conduct electricity only under certain conditions. The six elements most commonly classified as metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, with selenium, polonium, and astatine sometimes included depending on which criteria you use. They sit along a diagonal staircase line on the periodic table, right where the metals and nonmetals meet.
Physical Properties: Shiny but Brittle
Most metalloids have a metallic luster, meaning they look shiny and silvery at first glance. Pick one up, though, and the resemblance to metals ends quickly. Metalloids are brittle, meaning they crack or shatter under pressure rather than bending the way copper or aluminum would. This combination of looking metallic while behaving mechanically like a nonmetal is one of their defining traits.
Some metalloids can exist in multiple structural forms, called allotropes. Silicon is a good example: in one form it appears as a dull brown powder, while in another it takes on the shiny gray appearance familiar from computer chips. Boron is notably hard, scoring 9.3 on the Mohs hardness scale (diamond is 10), and has an extremely high melting point of 2,075°C. These are unusual numbers that set boron apart from most other elements in the group.
Electrical Conductivity: The Semiconductor Sweet Spot
The single most important property of metalloids, at least in terms of practical impact, is their ability to act as semiconductors. Metals conduct electricity freely. Nonmetals barely conduct at all. Metalloids sit in between: they conduct electricity, but only partially, and their conductivity can be fine-tuned by adding tiny amounts of other elements or by changing the temperature.
This behavior comes down to something called the band gap, which is essentially the energy barrier electrons need to overcome before they can flow through a material. Silicon has a band gap of about 1.11 electron volts at room temperature, while germanium’s is smaller at roughly 0.66 electron volts. These values are large enough to block current under normal conditions but small enough that a little energy input (heat, light, or voltage) gets electrons moving. Metals have no meaningful band gap, so current flows easily. Insulators like rubber have enormous band gaps, so current doesn’t flow at all. Metalloids land in the useful middle ground.
This is why silicon dominates the electronics industry. It powers the chips in your phone, your laptop, and the solar panels on rooftops. Roughly 80 to 90 percent of photovoltaic panels worldwide are built from silicon wafers. Germanium, with its smaller band gap, is used in specialized electronics and fiber optics.
Chemical Properties: More Nonmetal Than Metal
While metalloids physically resemble metals, their chemical behavior leans toward the nonmetal side. They tend to participate in reactions the way nonmetals do, sharing electrons rather than giving them up freely. Their electronegativity values cluster around 2.0 on the Pauling scale, which places them neatly between metals (generally below 1.8) and nonmetals (generally above 2.2). Electronegativity measures how strongly an atom pulls on electrons during a chemical bond, so metalloids sit at the tipping point between electron-givers and electron-takers.
Metalloids can take on a wide range of oxidation states, from +5 to -2, depending on which element you’re looking at and what it’s reacting with. This flexibility means they form a variety of compounds. Some metalloid oxides are amphoteric, meaning they can react as either an acid or a base depending on the situation. This dual chemical personality mirrors their position between the two major element categories.
Where Metalloids Sit on the Periodic Table
You’ll find metalloids along a zigzag line (sometimes drawn as a staircase) that cuts diagonally across the right side of the periodic table, roughly from boron in the upper left down to tellurium and polonium in the lower right. Elements to the left of this line are metals. Elements to the right are nonmetals. The metalloids occupy the boundary, which is why their properties blend characteristics of both groups.
Not every reference draws the line in exactly the same place. The core six (boron, silicon, germanium, arsenic, antimony, and tellurium) appear on virtually every list. Polonium is sometimes included, though some classification systems treat it as a metal because its physical properties lean more metallic. Astatine and selenium occasionally appear as well, though selenium is often grouped with the nonmetals.
Arsenic and Antimony: Toxic but Medically Useful
Two metalloids, arsenic and antimony, have significant effects on human health. Arsenic contamination in groundwater is a global problem estimated to affect up to 200 million people. In parts of Bangladesh, China, Thailand, Argentina, and Australia, naturally occurring arsenic in groundwater has been measured at concentrations of 5,000 parts per billion or higher, far exceeding the World Health Organization’s recommended limit of 10 parts per billion for drinking water. Chronic exposure can damage virtually every organ, with the highest concentrations accumulating in the liver and kidneys. Long-term arsenic exposure is linked to skin disorders, cardiovascular disease, diabetes, several types of cancer, and neurological problems.
Antimony poses similar concerns near mining and smelting sites, where water concentrations have reached 29,000 parts per billion in some areas of China. Chronic exposure can affect the skin, lungs, heart, and digestive system.
Despite their toxicity, both elements have a long history in medicine. Arsenic-based drugs are currently used to treat a form of blood cancer called acute promyelocytic leukemia, while antimony compounds are used against certain parasitic diseases. The same chemical reactivity that makes these metalloids dangerous also makes them effective at disrupting disease processes in targeted doses.
How Metalloids Compare to Metals and Nonmetals
- Appearance: Metalloids can be shiny like metals or dull like nonmetals, depending on their form. Metals are almost always lustrous. Nonmetals are typically dull.
- Brittleness: Metalloids shatter under stress, like nonmetals. Metals bend and deform without breaking.
- Electrical conductivity: Metals conduct freely. Nonmetals insulate. Metalloids conduct partially and conditionally, making them semiconductors.
- Chemical behavior: Metals tend to lose electrons in reactions. Nonmetals tend to gain them. Metalloids can go either way, with a general lean toward nonmetallic behavior.
- Electronegativity: Metals fall below roughly 1.8, nonmetals above roughly 2.2, and metalloids cluster around 2.0.
This in-between nature is what makes metalloids so useful in technology. Their electrical properties can be precisely controlled in ways that pure metals or pure nonmetals cannot match, which is why silicon became the foundation of the modern electronics industry and why the term “Silicon Valley” exists at all.