Metalloid is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, nearly every element in the periodic table can be termed either a metal or a nonmetal - however a few elements with intermediate properties are referred to as metalloids. (In Greek metallon = metal and eidos = sort)
There is no rigorous definition of the term, however the following properties are usually considered characteristic of metalloids:
- metalloids often form amphoteric oxides.
- metalloids often behave as semiconductors (B,Si,Ge) to semimetals (eg. Sb).
The concepts of metalloid and semiconductor should not be confused. Metalloid refers to the properties of certain elements in relation to the periodic table. Semiconductor refers to the physical properties of materials (including alloys, compounds) and there is only partial overlap between the two.
The following elements are generally considered metalloids:
Some allotropes of elements exhibit more pronounced metal, metalloid or non-metal behavior than others. For example, for the element carbon, its diamond allotrope is clearly non-metallic, however the graphite allotrope displays limited electric conductivity more characteristic of a metalloid. Phosphorus, tin, selenium and bismuth also have allotropes which display borderline behavior.
In the standard layout of the periodic table, metalloids occur along the diagonal line through the p block from boron to astatine. Elements to the upper right of this line display increasing nonmetallic behaviour; elements to the lower left display increasing metallic behaviour. This line is called the "stair-step" or "staircase." The poor metals are to the left and down and the nonmetals are to the right and up. In addition, the halogens are found at the right.
- ACS Periodic Table. 
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