Recognition status, as metalloids, of some elements in the p-block of the standard form of periodic table. Other elements categorised as metalloids only on a passing or isolated basis, such as phosphorus and iodine, have not been highlighted. This is due to the relatively still more infrequent appearance of these elements in the metalloid literature. See also the categorisation and periodic table territory section of this article.
The grey staircase-shaped line, which passes between B-Al, Al-Si, Si-Ge, Ge-As, As-Sb, Sb-Te, Te-Po and Po-At, is a typical example of the arbitrary metal–nonmetal dividing line that can be found on some periodic tables. Germanium, if classified as a nonmetal, then appears to fall on the wrong side of the line. This is a result of the publicity this form of the line received in the late 1920s and early 30s. Germanium was also thought to be a poorly conducting metal, up to at least the late 1930s.
A metalloid is a chemical element that has properties that are in between or a mixture of those of metals and nonmetals and is consequently difficult to classify unambiguously as either a metal or a nonmetal. There is no standard definition of a metalloid, nor is there agreement as to which elements are appropriately classified as such. Despite this lack of specificity the term remains in use in chemistry literature.
The six elements commonly recognised as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. Other elements less commonly recognised as metalloids include carbon, aluminium, selenium, polonium and astatine. On a standard periodic table all of these elements can be found in or near a diagonal region of the p-block, having its main axis anchored by boron at one end and astatine at the other. Some periodic tables include a dividing line between metals and nonmetals and it is generally the elements adjacent to this line or, less frequently, one or more of the elements adjacent to those elements, which are identified as metalloids.
Physically, metalloids usually have a metallic appearance but they are brittle and only fair conductors of electricity; chemically, they mostly behave as (weak) nonmetals. They can, however, form alloys with metals. Ordinarily, most of the other physical and chemical properties of metalloids are intermediate in nature.