262The six elements commonly recognized as metalloids are boron, silicon, germanium,
arsenic, antimony and tellurium. They are metallic-looking brittle solids, with intermediate
to relatively good electrical conductivities, and each having the electronic band structure
of either a semiconductor or a semimetal.
Chemically, they mostly behave as (weak) nonmetals, have intermediate ionization energy
and electronegativity values, and form amphoteric or weakly acidic oxides. Being too brittle
to have any structural uses, the metalloids and their compounds instead find common use
in glasses, alloys and semiconductors. The electrical properties of silicon and germanium,
in particular, enabled the establishment of the semiconductor industry in the 1950s and
the development of solid state electronics from the early 60s onwards.
Other elements less commonly recognized as metalloids include carbon, aluminum,
selenium, polonium and astatine. On a standard periodic table these elements, as well as
the elements commonly recognized as metalloids, occur 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.
The term metalloid was first popularly used to refer to nonmetals. It’s more recent meaning
as a category of elements with intermediate or hybrid properties did not become
widespread until the period 1940–1960. Metalloids are sometimes called semimetals, a
practice which has been discouraged. This is because the term semimetal has a different
meaning in physics, one which more specifically refers to the electronic band structure of
a substance rather than the overall classification of a chemical element.
There is no universally agreed or rigorous definition of a metalloid. The feasibility of
establishing a specific definition has also been questioned, noting anomalies can be found
in several such attempted constructs. Classifying any particular element as a metalloid
has been described as 'arbitrary'.
The generic definition set out at the start of this article is based on metalloid attributes
consistently cited in the literature. Illustrative definitions and extracts include:
'In chemistry a metalloid is an element with properties intermediate between those
of metals and nonmetals.'
'Between the metals and nonmetals in the periodic table we find elements...[that]
share some of the characteristic properties of both the metals and nonmetals,
making it difficult to place them in either of these two main categories.'
'Chemists sometimes use the name metalloid...for these elements which are
difficult to classify one way or the other.'
'Because the traits distinguishing metals and nonmetals are qualitative in nature,
some elements do not fall unambiguously in either category. These elements...are
called metalloids...'.
More Broadly, Metalloids have also been referred to as:
'elements that...are somewhat of a cross between metals and nonmetals' or
'weird in-between elements.'