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A metalloid has properties between that of a metal and that of a nonmetal.

In a condensed phase (liquid or solid), atoms are so close together that some of their outer electrons are attracted to other nearby nuclei. Instead of the few orbitals possible for an electron bound to a single atom, an immense number of orbitals associated with multiple nuclei become available. Instead of discrete levels like those in isolated atoms, electrons in a condensed material can take almost any energy within specific bands. The bands, remember, need not have any electrons in them. These are termed "conduction bands". They are always available in any condensed element - but do not necessarily have any electrons in them.

In metals, there are always electrons in conduction bands. In metalloids, conduction bands are empty at absolute zero, but the amount of energy needed to raise an electron to the conduction band is small. At temperatures above 0 K, there will be some electrons in a metalloid's conduction bands - but not many. Those few electrons, though, can conduct some current.

A second type of conduction is possible with metalloids. Electrons promoted into a conduction band leave vacant orbitals behind. Those vacancies, called "holes" act as though they were mobile positive charges. Current flow in a metalloid amounts to electrons moving in one direction and vacancies moving in the other.

Conduction in metalloids is a phenomenon which can be put to use. For further information, see "electronics".

Chemically, metalloids commonly act either like bases or like acids, depending on context. Simple metalloid ions can take either positive or negative oxidation states. (Positive oxidation states are common in nonmetals, but only in complex ions such as nitrate (NO3-)).

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All are solid at room temperature and are moderately good at conducting electricity examples include Silicon, Germanium and Boron.

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