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.

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.

Read more about Metalloid:  Definitions, Elements Commonly Recognized As Metalloids, Elements Less Commonly Recognized As Metalloids, Categorization and Periodic Table Territory, Comparison of Properties With Those of Metals and Nonmetals, Quantitative Description

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Nonmetal - Metallic Allotropes
... Boron 13 ... metalloid Common allotropes of boron have bandgaps of approximately 2 eV, but a high-pressure superconducting phase occurs at 160 GPa and 250 GPa ... Silicon 14 ... metalloid Under increasing pressure silicon transforms from a cubic diamond structure to a β-tin (11–12 GPa), primitive hexagonal (13–16 GPa), hexagonal-close-packed ... Germanium 14 ... metalloid Germanium undergoes a semiconductor to metal transition at 11 GPa ...
Metalloid - Nomenclature and History - Origin and Usage
... The origin and usage of the term metalloid is convoluted ... (IUPAC) has previously recommended abandoning the term metalloid, and suggested using the term semimetal instead ... do not include any recommendations on the usage or non-usage of the terms metalloid or semimetal ...
List of The 118 Known Chemical Elements
... tables Actinide, alkali metal, alkaline earth metal, halogen, lanthanide, metal, metalloid, noble gas, non-metal, and transition metal ... Alkaline earth metal 5 Boron B 2 ... p Solid Primordial Metalloid 6 Carbon C 2 ... p Solid Primordial Non-metal 7 Nitrogen N 2 ... p Gas Primordial Non-metal 8 ...