Metalloids or Semimetals: Definition, List of Elements, and Properties

Learn About the Metalloid Element Group

Silicon is a metalloid. Like other members of this element group, it may display a metallic luster.
Silicon is a metalloid. Like other members of this element group, it may display a metallic luster. Nara Won / Getty Images

Metalloid Definition

Between the metals and nonmetals is a group of elements known as either the semimetals or the metalloids, which are elements that have properties intermediate between those of the metals and nonmetals. Most metalloids have a shiny, metallic appearance, but are brittle, unexceptional electrical conductors, and display nonmetallic chemical properties. Metalloids are elements that have semiconductor properties and form amphoteric oxides.

Location on the Periodic Table

The metalloids or semimetals are located along the line between the metals and nonmetals in the periodic table. Because these elements have intermediate properties, it's sort of a judgement call as to whether a particular element is a metalloid or should be assigned to one of the other groups. You'll find different classification systems, depending on the scientist or author. There is no single "right" way to divide the elements.

List of Elements That Are Metalloids

The metalloids are generally considered to be:

  • boron
  • silicon
  • germanium
  • arsenic
  • antimony
  • tellurium
  • polonium (usually recognized, sometimes considered a metal)
  • astatine (sometimes recognized, otherwise seen as a halogen)

Element 117, tennessine, has not been produced in sufficient amounts to verify its properties, but is predicted to be a metalloid.

Some scientists consider neighboring elements on the periodic table to either be metalloids or to have metalloid characteristics. An example is carbon, which may be considered either a nonmetal or a metalloid, depending on its allotrope. The diamond form of carbon looks and behaves as a nonmetal, while the graphite allotrope has a metallic luster and acts as an electrical semiconductor, so is a metalloid. Phosphorus and oxygen are other elements that have both nonnmetallic and metalloid allotropes. Selenium is considered to be a metalloid in environmental chemistry. Other elements that may behave as metalloids under certain conditions are hydrogen, nitrogen, sulfur, tin, bismuth, zinc, gallium, iodine, lead, and radon.

Properties of the Semimetals or Metalloids

The electronegativities and ionization energies of the metalloids are between those of the metals and nonmetals, so the metalloids exhibit characteristics of both classes. Silicon, for example, possesses a metallic luster, yet it is an inefficient conductor and is brittle. The reactivity of the metalloids depends on the element with which they are reacting. For example, boron acts as a nonmetal when reacting with sodium yet as a metal when reacting with fluorine. The boiling points, melting points, and densities of the metalloids vary widely. The intermediate conductivity of metalloids means they tend to make good semiconductors.

Summary of Common Metalloid Properties

  • Electronegativities between those of metals and nonmetals
  • Ionization energies between those of metals and nonmetals
  • Possess some characteristics of metals/some of nonmetals
  • Reactivity depends on properties of other elements in reaction
  • Often make good semiconductors
  • Often have a metallic luster, although may have allotropes that appear nonmetallic
  • Usually behave as nonmetals in chemical reactions
  • Form alloys with metals
  • Usually brittle
  • Usually solids under ordinary conditions

Interesting Metalloid Facts

  • The most abundant metalloid in the Earth's crust is silicon, which is the second most abundant element overall (oxygen is most abundant).
  • The least abundant natural metalloid is tellurium.
  • Metalloids are valuable in the electronics industry. Silicon, for example, is used to make chips found in phones and computers.
  • Arsenic and polonium are highly toxic metalloids.
  • Antimony and tellurium are used primarily in metal alloys to add desirable properties.