A metalloid is a chemical element with properties in between, or that are a mixture of, those of metals and nonmetals. There is no standard definition of a metalloid, nor is there complete agreement as to which elements are appropriately classified as such. Despite this lack of specificity, the term remains in use in the literature of chemistry.
The six commonly recognised metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium. Elements less commonly recognised as metalloids include carbon, aluminium, selenium, polonium, and astatine. On a standard periodic table all of these elements may be found in a diagonal region of the p-block, extending from boron at one end, to astatine at the other. Some periodic tables include a dividing line between metals and nonmetals and the metalloids may be found close to this line.
Typical metalloids have a metallic appearance, but they are brittle and only fair conductors of electricity. Chemically, they mostly behave as nonmetals. They can form alloys with metals. Most of their other physical and chemical properties are intermediate in nature. Metalloids are usually too brittle to have any structural uses. They and their compounds are used in alloys, biological agents, catalysts, flame retardants, glasses, optical storage and optoelectronic, pyrotechnics, semiconductors, and electronics.