The periodic table categorizes all known elements, primarily sorting them into metals and nonmetals. Most elements fall neatly into one of these two large groups, exhibiting distinct properties, such as high electrical conductivity for metals or poor conductivity for nonmetals. A small group of elements, known as semimetals or metalloids, exists at the border between these two categories. Semimetals present a mix of characteristics, reflecting their intermediate nature.
Defining Semimetals
Semimetals are defined by their intermediate physical and chemical properties, bridging the gap between true metals and nonmetals. Physically, they often resemble metals, displaying a characteristic shiny or metallic luster. They are typically solid at room temperature and can form alloys when mixed with metals.
The chemical behavior of semimetals tends to lean toward that of nonmetals, meaning they can sometimes gain electrons or form covalent bonds. Unlike metals, which are malleable and ductile, semimetals are often brittle and can break easily. Their most distinctive property relates to electrical flow: they function as semiconductors, meaning their ability to conduct electricity can be precisely controlled.
The Stair-Step Boundary
The physical location of semimetals is along a diagonal, jagged line that effectively separates the metals from the nonmetals. This boundary is often referred to as the “stair-step” or “zig-zag” line, as it moves downward and to the right across the table.
To the left of this stair-step boundary are the metals, and to the right are the nonmetals. The elements classified as semimetals are those that directly touch or flank this dividing line. The line generally begins near Group 13 and extends through Group 16 of the main block elements. Aluminum, which borders the line, is considered a metal because all its properties align with metals.
The Core Elements
A core group of six elements is universally recognized as semimetals:
- Boron (B)
- Silicon (Si)
- Germanium (Ge)
- Arsenic (As)
- Antimony (Sb)
- Tellurium (Te)
These six elements consistently exhibit the intermediate characteristics that define the group. Silicon, in particular, is the most widely recognized due to its widespread use in technology.
The classification of two other elements, Polonium (Po) and Astatine (At), is often debated and remains ambiguous. Both elements sit on the lower portion of the boundary line but are highly radioactive, which makes studying their bulk properties extremely difficult. Polonium often shows more metallic behavior, and Astatine is generally predicted to behave more like a metal.
Practical Applications
The semiconducting nature of semimetals drives their modern technological relevance. Their conductivity is much better than nonmetals but substantially less than metals, and this ability can be manipulated by adding trace amounts of impurities in a process called doping. This precise control over electrical flow makes them indispensable in the electronics industry.
Silicon, the most commercially important semimetal, is the foundational material for integrated circuits and microchips. These components, which act as switches and amplifiers, are the building blocks of computers, smartphones, and every other digital device. Germanium is also used in high-speed applications and specific types of transistors. Semimetal-based technology is central to solar energy production, as silicon-based photovoltaic cells convert sunlight directly into electricity.