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A metalloid is a chemical element with properties that are in-between or a mixture of those of metals and nonmetals, and which is considered to be difficult to classify unambiguously as either a metal or a nonmetal. There is no standard definition of a metalloid nor is there agreement as to which elements are appropriately classified as such. Despite this lack of specificity the term continues to be used in the chemistry literature.

The six elements commonly recognized as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. They are metallic-looking brittle solids, with intermediate to relatively good electrical conductivities, and each having the electronic band structure of either a semiconductor or a semimetal. Chemically, they mostly behave as (weak) nonmetals, have intermediate ionization energy and electronegativity values, and form amphoteric or weakly acidic oxides. Being too brittle to have any structural uses, the metalloids and their compounds instead find common use in glasses, alloys and semiconductors. The electrical properties of silicon and germanium, in particular, enabled the establishment of the semiconductor industry in the 1950s and the development of solid state electronics from the early 60s onwards.[2][3]

Other elements less commonly recognized as metalloids include carbon, aluminium, selenium, polonium and astatine. On a standard periodic table these elements, as well as the elements commonly recognized as metalloids, occur in or near a diagonal region of the p-block, having its main axis anchored by boron at one end and astatine at the other. Some periodic tables include a dividing line between metals and nonmetals and it is generally the elements adjacent to this line or, less frequently, one or more of the elements adjacent to those elements, which are identified as metalloids.

The term metalloid was first popularly used to refer to nonmetals. Its more recent meaning as a category of elements with intermediate or hybrid properties did not become widespread until the period 1940–1960. Metalloids are sometimes called semimetals, a practice which has been discouraged. This is because the term semimetal has a different meaning in physics, one which more specifically refers to the electronic band structure of a substance rather than the overall classification of a chemical element.

Chemical properties

Metalloids generally behave chemically as (weak) nonmetals, have intermediate ionization energies and electronegativities, and have amphoteric or weakly acidic oxides. They can also form alloys with metals. Relevant properties—general, specific and descriptive—are set out in the following table:

Property Metals Metalloids Nonmetals
General behaviour metallic nonmetallic[422] nonmetallic
Ionization energy relatively low intermediate ionization energies,[423] usually falling between those of metals and nonmetals[424] relatively high
Electronegativity usually low have electronegativity values close to 2[425] (revised Pauling scale) or within the narrow range of 1.9–2.2 (Allen scale)[23][n 33] high
When mixed
with metals
give alloys can form alloys[428][429][430] ionic or interstitial compounds formed
Oxides lower oxides basic; higher oxides increasingly acidic amphoteric or weakly acidic[409][431] acidic