Semi-metals or metalloids are intermediate between metals and nonmetals in their properties. Their conductivity is intermediate between that of metals and nonmetals; unlike metals, they are not malleable. Their ionization energies and electronegativities are also intermediate between metals and nonmetals. There are seven metalloids among the elements, namely, boron, silicon, germanium, arsenic, antimony, tellurium and polonium. They form a stair-step pattern on the periodic table starting with boron in column 13. Some of them are very useful in industry.
Certain boron compounds like borane or diborane are very useful in organic chemistry. Chemists can use borane and diborane, for example, to add an -OH or alcohol group to a carbon chain that contains a double bond. Sodium borohydride or NaBH4 is an invaluable reducing agent that adds a hydrogen with two electrons to organic compounds. Ketones like acetone, for example, contain an oxygen atom double-bonded to a carbon atom in a chain. Adding sodium borohydride to a ketone will reduce it to an alcohol.
Boron is also important for glassware. Glass made with boron oxide is called borosilicate glass or Pyrex. Borosilicate is less likely than ordinary glass to crack when heated, so it's used to make lab glassware and some kinds of oven cookware as well.
Silicon is the second-most abundant element in the Earth's crust. Its most common compound is silicon dioxide, typically found as sand or quartz. When heated and combined with sodium and calcium oxides, silicon dioxide forms a hard transparent substance called glass that enjoys countless uses both in industry and around your home.
Although it's abundant, silicon isn't easy to refine and purify. Nonetheless, pure silicon has become essential to industry for the manufacture of devices the modern world considers essential: transistors and integrated circuits. The microchips in the computer you're using at this very moment contain silicon refined from quartz. Many solar panels are made from silicon as well.
Silicon and boron are the two most useful and abundant metalloids; the others have fewer uses. Germanium is often found as an impurity in zinc ore, so it's produced primarily from the flue dust of plants that process zinc. Like silicon, germanium is a semiconductor, so it's often used in transistors, diodes and other solid-state electronics. Other uses include catalysts for the manufacture of polymers in the chemicals industry and doping glass for use in fiber-optics cables.
Antimony is a toxic metalloid that was at one point in history had medicinal and cosmetic uses. Today, antimony is primarily used to harden lead for storage batteries, although it's also used to make solder for wire soldering. Arsenic was also once used in small doses to treat syphilis; today, it's used in some pesticides and to strengthen lead alloys in lead shot or other applications. Its uses are limited by its toxicity. Tellurium is a fairly rare element, but is used to make cadmium telluride thin-film solar cells. Polonium is a radioactive, unstable toxic element with no major uses at present. It achieved infamy in late 2006 when an assassin used it to poison ex-KGB agent Alexander Litvinenko.
- USGS Minerals Information: Germanium
- University of Guelph: Semi-Metals
- "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins, et al.; 2008
- Occupational Health and Safety Administration: Antimony and Compounds
- University of Denver, Physics Index: Arsenic
- USGS Minerals Information: Tellurium
- Photo Credit Ryan McVay/Photodisc/Getty Images
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