Metalloid
Elements with properties between metals and nonmetals.
Metalloids are a group of chemical elements with properties intermediate between those of metals and nonmetals. The term originates from Latin *metallum* and Greek *oeidḗs*, meaning 'resembling in form or appearance.' There is no standard definition or complete agreement on which elements are metalloids, yet the term remains in use in the literature. The six commonly recognized metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium, with five others—carbon, aluminium, selenium, polonium, and astatine—less frequently classified as such. They occupy a diagonal region of the p-block on the periodic table, from boron at upper left to astatine at lower right.
Historically, the word originally referred to nonmetals, but its modern meaning—as a category of elements with intermediate or hybrid properties—became widespread between 1940 and 1960. These elements are sometimes called semimetals, though this usage is discouraged because the term "semimetal" more commonly denotes a specific electronic band structure; by that physics-based definition, only arsenic and antimony qualify as semimetals, yet both are still widely recognized as metalloids.
Typical metalloids have a metallic appearance, are often brittle, and conduct electricity only moderately. They can form alloys with metals, and many of their physical and chemical properties fall between those of metals and nonmetals. Their applications include alloys, biological agents, catalysts, flame retardants, glasses, optical storage and optoelectronics, pyrotechnics, semiconductors, and electronics.
Classification is inherently judgment-based: a metalloid is an element whose properties are a mixture of metallic and nonmetallic traits, making it difficult to categorize as either. Most elements have a blend of such properties, but only those lacking a clear preponderance of one set are considered metalloids. The inclusion of specific elements varies by author; boron, silicon, germanium, arsenic, antimony, and tellurium are the most consistent, while selenium, polonium, and astatine are sometimes added, and boron or tellurium are occasionally excluded. Many other elements—including hydrogen, beryllium, nitrogen, phosphorus, sulfur, zinc, gallium, tin, iodine, lead, bismuth, and radon—have been occasionally classified as metalloids, though heavy synthetic elements like flerovium through oganesson ar
- Commonly recognized
- Boron, silicon, germanium, arsenic, antimony, tellurium
- Less frequently classified
- Carbon, aluminium, selenium, polonium, astatine
- Typical properties
- Metallic appearance, brittle, fair conductors of electricity
- Uses
- Alloys, semiconductors, glasses, flame retardants, catalysts, optoelectronics
- Alternative name
- Semimetals (discouraged due to distinct physics meaning)
Lore & Background
The term metalloid originally referred to nonmetals. Metalloids are sometimes called semimetals, but this practice is discouraged because 'semimetal' has a more common usage in physics referring to a specific electronic band structure; only arsenic and antimony are semimetals in that context, and they are commonly recognized as metalloids. Typical metalloids have a metallic appearance, may be brittle, and are only fair conductors of electricity. They can form alloys with metals, and many of their physical and chemical properties are intermediate between those of metallic and nonmetallic elements. Their compounds are used in alloys, biological agents, catalysts, flame retardants, glasses, optical storage and optoelectronics, pyrotechnics, semiconductors, and electronics. Classification of metalloids is judgment-based and varies by author. Some criteria include electronegativity values from 1.8 or 1.9 to 2.2, packing efficiency between 34% and 41%, or the Goldhammer–Herzfeld ratio around 0.85 to 1.1.
Reader's Guide
Metalloids hold significance as elements that defy simple classification into metals or nonmetals, illustrating the gradual nature of property changes across the periodic table. Their intermediate properties make them essential in modern technology, particularly as semiconductors and in optoelectronics, where their electrical conductivity can be finely tuned. The diagonal relationship they exhibit—where elements in different groups share similar properties—highlights an organizing principle of the periodic table, arising from competing horizontal and vertical trends in nuclear charge. The lack of a standard definition means that the number and identity of metalloids depend on the criteria used, with lists ranging from four to twelve elements. This ambiguity reflects the arbitrary nature of classification at the margins, as noted by Sharp. Despite this, the term persists because it captures a useful category for elements that are neither strongly metallic nor strongly nonmetallic. The dividing line between metals and nonmetals on some periodic tables helps locate metalloids, but alternative treatments may group them with metals, nonmetals, or treat them as a subcategory. Their legacy lies in challenging binary classification and enabling advances in electronics and materials science.
Did You Know?
- The word metalloid comes from Latin metallum ('metal') and Greek oeidḗs ('resembling in form or appearance').
- There is no standard definition of a metalloid and no complete agreement on which elements are metalloids.
- The six commonly recognized metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium.
- Metalloids are sometimes called semimetals, but this practice is discouraged because 'semimetal' has a more common usage in physics.
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