Semimetals or Metalloids

Semimetals or metalloids are chemical elements that have properties of both metals and nonmetals. Metalloids are important semiconductors, often used in computers and other electronic devices.

• Boron (B): Atomic number 5
• Silicon (Si): Atomic number 14
• Germanium (Ge): Atomic number 32
• Arsenic (As): Atomic number 33
• Antimony (Sb): Atomic number 51
• Tellurium (Te): Atomic number 52
• Polonium (Po): Atomic number 84
• Tennessine (Ts): Atomic number 117

Although oganesson (atomic number 118) is in the last periodic column of elements, scientists do not believe it is a noble gas. Element 118 will most likely be identified as a metalloid once its properties have been confirmed.

Semimetal or Metalloid Properties

Semimetals or metalloids are found in a zig-zag line on the periodic table, separating the basic metals from the nonmetals. However, the defining characteristic of metalloids is not so much their position on the periodic table as the extremely small overlap between the bottom of the conduction band and top of the valence band. A band gap separates a filled valence band from an empty conduction band. Semimetals do not have a band gap.

In general, metalloids have the physical properties of metals, but their chemical properties are closer to those of nonmetals:

• Semimetals tend to make excellent semiconductors, although most of the elements themselves are not technically semiconducting. Exceptions are silicon and germanium, which are true semiconductors, as they can conduct electricity under the right conditions.
• These elements have lower electrical and thermal conductivity than metals.
• Semimetals/metalloids have high lattice dielectric constants and high diamagnetic susceptibilities.
• Semimetals are typically malleable and ductile. One exception is silicon, which is brittle.
• Metalloids may either gain or lose electrons during chemical reactions. Oxidation numbers of elements in this group range from +3 to -2.
• As far as appearances go, metalloids range from dull to shiny.
• Metalloids are extremely important in electronics as semiconductors, although they are also used in optical fibers, alloys, glass, and enamels. Some are found in drugs, cleaners, and pesticides. The heavier elements tend to be toxic. Polonium, for example, is dangerous due to its toxicity and radioactivity.

Distinction Between Semimetals and Metalloids

Some texts use the terms semimetals and metalloids interchangeably, but more recently, the preferred term for the element group is "metalloids," so that "semimetals" may be applied to chemical compounds as well as elements that exhibit properties of both metals and nonmetals. An example of a semimetal compound is mercury telluride (HgTe). Some conductive polymers may also be considered semimetals.

Other scientists consider arsenic, antimony, bismuth, the alpha allotrope of tin (α-tin), and the graphite allotrope of carbon to be semimetals. These elements are also known as the "classic semimetals."

Other elements also behave like metalloids, so the usual grouping of elements isn't a hard-and-fast rule. For example, carbon, phosphorus, and selenium exhibit both metallic and nonmetallic character. To some extent, this depends on the form or allotrope of the element. An argument could even be made for calling hydrogen a metalloid; it normally acts as a nonmetallic gas but can form a metal under certain circumstances.

Sources

• Addison, C.C, and D.B Sowerby. "Main Group Elements - Groups v and Vi." Butterworths, 1972.
• Edwards, Peter P., and M. J. Sienko. “On the Occurrence of Metallic Character in the Periodic Table of the Elements.” Journal of Chemical Education, vol. 60, no. 9, 1983, p. 691.
• Vernon, René E. “Which Elements Are Metalloids?” Journal of Chemical Education, vol. 90, no. 12, 2013, pp. 1703–1707.