Metalloid

Metalloid, in chemistry, an imprecise term used to describe a chemical element that forms a simple substance having properties intermediate between those of a typical metal and a typical nonmetal. The term is normally applied to a group of between six and nine elements (boron, silicon, germanium, arsenic, antimony, tellurium, and possibly bismuth, polonium, and astatine) found near the center of the p-block or main block of the periodic table. There is no single property that can be used to unambiguously identify an element as a metalloid. Since most metalloids tend to display semiconducting properties in at least one of their allotropic modifications, the class might reasonably be extended to also include the graphite form of carbon (which, unlike the diamond form, is a semimetal rather than an insulator).

Chemically, metalloids correspond to atoms having intermediate electronegativities (the ability to attract shared electrons in a chemical bond) and an ability to display a range of both positive and negative oxidation states in their compounds.

Properties

Metalloids, sometimes called semimetals, have physical properties that are closer to those of metals. They are solid at room temperature and generally have relatively high melting points compared with nonmetals. They are brittle and cannot be hammered or rolled into thin sheets or drawn into wires without breaking. Metalloids are usually lustrous solids and may appear in metallic, crystalline, or powdery forms. Their electrical conductivity is intermediate between that of metals and nonmetals; it increases with rising temperatures as electrons gain enough energy to cross the band gap from the valence band, where they are relatively restricted, into the conduction band, where they can move more freely and carry electric current. Conductivity can also be increased by adding small amounts of impurities to the metalloid in a process called doping.

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Chemically, metalloids are more similar to nonmetals. Their electronegativity values and ionization energies (the energy required to remove an electron from an isolated gaseous atom or molecule) are intermediate between those of electropositive metals and highly electronegative nonmetals. This allows metalloids to gain, lose, or share electrons, depending on the reacting species. They mostly form single covalent bonds, which play an important role in the crystalline structure of many of these elements. Metalloids generally form amphoteric oxides, meaning they can act as either acids or bases.

Uses

Metalloids have wide applications in industry and agriculture owing to their chemical versatility and controllable electrical conductivity, among other properties. Silicon, for example, plays a foundational role in the semiconductor industry. It is used in its elemental and compound forms to produce microchips, semiconductors, silicone sealants and lubricants, heat-resistant glass, aluminum-silicon alloys, solar cells, and more. Germanium is also used to manufacture solar cells, in addition to optical fiber and infrared optical components such as lenses and windows. Boron is used to create borosilicate glass—found in labware, cookware, and aerospace windshields—and is used in high-strength boron-fiber composites for military armor and aircraft. Boron-containing compounds are also used to make detergents, agricultural fertilizers, and nuclear reactor control rods for neutron absorption.

One of the most important applications of antimony compounds is as a flame retardant. Antimony forms alloys with elements such as lead, and these alloys are used in lead-acid batteries. Arsenic is toxic, and long-term exposure to this element can cause skin lesions and cancer, but certain arsenic compounds are used in anticancer treatments and have been investigated for antiviral and antimicrobial applications. Tellurium, a rare element, is used to make steel alloys, solar panels, and thermoelectric devices. Research suggests that it may play a causal role in cancer, autoimmune, and neurodegenerative diseases.