A metalloid is an element whose physical and chemical properties fall between those of metals and nonmetals. There is no single defining trait. Instead, metalloids share a cluster of in-between characteristics: they look shiny like metals, shatter like glass, conduct electricity only under certain conditions, and can react with both acids and bases. Six elements are commonly recognized as metalloids: boron, silicon, germanium, arsenic, antimony, and tellurium.
The Core Physical Properties
Metalloids look metallic. They have a noticeable luster, which is why they can be mistaken for metals at first glance. But pick one up and stress it, and it will crack or shatter rather than bend. Metals are malleable and ductile; metalloids are brittle, behaving mechanically more like nonmetals.
Their density, melting points, and boiling points all land in a middle range, higher than typical nonmetals but lower than most metals. This intermediate physical profile is one of the clearest signals that an element doesn’t fit neatly into either category.
Semiconductor Behavior
The property that matters most in modern life is electrical conductivity. Metals conduct electricity well, and their conductivity improves as they get colder. Nonmetals are either insulators or very poor conductors. Metalloids sit between: most of them are semiconductors, meaning they conduct electricity partially, and their conductivity increases as temperature rises.
This happens because of the energy gap between the electrons that are locked in bonds and the energy level where electrons can move freely and carry a current. Metals have no gap at all. Insulators have a huge gap. Semiconductors have a small, crossable gap. Among the recognized metalloids, boron has a band gap of about 1.5 electronvolts, silicon 1.12 eV, germanium 0.67 eV, and tellurium 0.35 eV. Arsenic and antimony behave slightly differently as semimetals, where the gap essentially closes but far fewer electrons are available to carry current compared to a true metal.
This semiconductor and semimetal behavior is what makes metalloids indispensable in electronics, solar panels, and computer chips. Silicon, the most commercially important metalloid, powers virtually all modern computing precisely because its conductivity can be fine-tuned by adding tiny amounts of other elements.
Chemical Behavior: Bonding and Reactivity
Metals tend to give up electrons and form ionic bonds. Nonmetals tend to grab electrons and form covalent bonds. Metalloids do both, depending on what they’re reacting with. This flexibility traces back to their electronegativity, a measure of how strongly an atom attracts electrons in a bond.
On the Pauling scale, highly reactive metals like sodium and potassium score below 1.0, while strong nonmetals like fluorine (3.98) and oxygen (3.44) score well above 3.0. The recognized metalloids cluster in a narrow middle band: boron at 2.04, silicon at 1.90, germanium at 2.01, arsenic at 2.18, antimony at 2.05, and tellurium at 2.10. That intermediate pull on electrons is what allows them to go either way in a chemical reaction.
Metalloids also tend to form amphoteric oxides. That means their oxides can react with both acids and bases to produce a salt and water. Metal oxides are typically basic (they neutralize acids), and nonmetal oxides are typically acidic (they neutralize bases). Metalloid oxides can do both, which is another sign of their dual nature.
Where They Sit on the Periodic Table
Metalloids occupy a diagonal staircase-shaped band on the periodic table, running roughly from boron in the upper right down to tellurium and polonium in the lower portion. This diagonal separates the metals on the left from the nonmetals on the right. The position isn’t a coincidence: moving left across a period increases metallic character, moving right increases nonmetallic character, and moving down a group increases metallic character. The diagonal is the boundary where those trends collide, producing elements with mixed identities.
Why the List Isn’t Fully Settled
There is no official, universally agreed-upon definition from IUPAC (the international body that standardizes chemistry terminology). The concept of “metalloid” has only been reasonably well accepted since around 1940 to 1960, and the boundaries remain fuzzy. A survey of chemistry textbooks found that six elements appear consistently: boron (in 86% of sources), silicon (95%), germanium (96%), arsenic (100%), antimony (88%), and tellurium (98%).
Beyond that core six, things get debatable. Polonium shows up in about 49% of sources and astatine in 40%. Both are highly radioactive and exist in such tiny quantities that studying their bulk properties is extremely difficult. Selenium appears in 23% of sources. It is a semiconductor with a band gap of 1.8 eV, which would seem to qualify it, but its overall chemical behavior leans more toward nonmetal. These borderline cases highlight that “metalloid” is more of a practical classification than a sharp dividing line.
What Actually Qualifies an Element
No single property makes an element a metalloid. The classification comes from a combination of traits that, taken together, place an element in a gray zone:
- Metallic luster paired with brittleness
- Semiconductor or semimetal electrical behavior
- Intermediate thermal conductivity, better than nonmetals but weaker than metals
- Electronegativity in the roughly 1.9 to 2.2 range on the Pauling scale
- Amphoteric oxides that react with both acids and bases
- Ability to form either ionic or covalent bonds depending on the reaction partner
An element does not need to check every single box. Arsenic, for instance, is universally classified as a metalloid despite being a semimetal rather than a true semiconductor. The classification is about the overall pattern of behavior, not a rigid checklist, which is exactly why the edges of the category remain open to interpretation.