The periodic table organizes all known elements into three broad categories: metals, nonmetals, and metalloids. Metalloids represent a chemical and physical boundary, possessing characteristics that fall between the two major categories. They act as an essential transition zone, preventing the abrupt shift in properties between highly conductive, malleable metals and brittle, insulating nonmetals.
Defining the Metalloid
A metalloid is defined by its intermediate properties, blending the physical look of metals with chemical behavior often closer to nonmetals. Physically, metalloids typically appear shiny, exhibiting a metallic luster, but they are often brittle and break easily, a characteristic of nonmetals. They are solid at room temperature and have intermediate values for properties like ionization energy and electronegativity. Chemically, metalloids tend to act more like nonmetals, primarily forming covalent bonds by sharing electrons rather than forming positive ions like true metals.
The Periodic Table’s Dividing Line
Metalloids occupy a specific, visually distinct region on the standard periodic table, following a diagonal or “staircase” line. This zigzag boundary separates the metals, which are found to the left and bottom, from the nonmetals, located to the upper right. Elements that sit directly along this dividing line exhibit the hybrid properties that define the metalloid class.
The commonly agreed-upon metalloids include six elements that form a diagonal bridge across the p-block of the table. Their placement is an exception to the general rule that elements with similar properties fall within the same vertical group. The six metalloids are:
- Boron (B)
- Silicon (Si)
- Germanium (Ge)
- Arsenic (As)
- Antimony (Sb)
- Tellurium (Te)
The Key Property: Semiconductivity
The most significant functional property of metalloids is their ability to act as semiconductors, distinguishing them from both metals and nonmetals. Metals are excellent conductors because their electrons move freely, while nonmetals are insulators because their electrons are tightly bound. Metalloids fall in the middle, conducting electricity better than nonmetals but much worse than true metals.
This intermediate conductivity is not fixed; it can be precisely controlled through a process called doping, which involves introducing tiny amounts of impurities. For example, adding Phosphorus (five valence electrons) to Silicon (four valence electrons) creates an excess of mobile electrons, forming an n-type semiconductor. Conversely, doping Silicon with Boron (three valence electrons) creates “holes” or electron vacancies, resulting in a p-type semiconductor. This controlled manipulation of charge carriers allows engineers to create tiny, reliable electronic switches, or transistors. The electrical conductivity of semiconductors also increases with temperature, which is the opposite behavior observed in metals.
Everyday Uses of Metalloids
The ability of metalloids to control electrical flow has made them the foundation of the modern digital world. Silicon is the most commercially important metalloid, serving as the base material for nearly all integrated circuits, microchips, and computer processors. Its semiconducting properties enable transistors to function as the on/off switches that power computing devices.
Other Key Metalloids
Germanium remains important for specialized applications like fiber optic communication and infrared technology. Boron is often used as a doping agent in silicon and to create borosilicate glass, which is highly resistant to temperature changes. Tellurium is used in high-efficiency thin-film solar panels and in thermoelectric devices that convert heat directly into electricity.