Metalloids are elements with properties of both metals and non-metals. Their ionization energies and electronegativities are between those of metals and non-metals, and their properties in chemical reactions are dependent on the other reactants.
Not all scientific sources agree on which elements are metalloids. Boron, silicon, germanium, arsenic, antimony and tellurium are the commonly agreed-upon metalloids; polonium, selenium and astatine are also sometimes included as metalloids. Carbon and aluminum are very rarely included due to their positions on the periodic table.
Because their electrical conductivity falls between that of conducting metals and insulating non-metals, metalloids are useful for electronic applications. Silicon is widely used as a semi-conductor in computer chips due to its abundance and the ease of fabricating materials from it. Boron, germanium, arsenic and antimony are also used as semi-conductors, though less often than silicon.
Metalloids are also used for commercial glass production, optical storage, fireworks and medicine. Silicon and boron are both essential trace elements to humans; the other metalloids are either toxic to humans or non-toxic but nonessential. Arsenic is highly toxic, but it is used to manufacture drugs to fight cancer and parasitic infections. Antimony is used for anti-parasitic drugs, while tellurium compounds are effective anti-microbial drugs.