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Tellurium

Tellurium is a silvery-white shiny solid. It has a density of 6.24 g/cm, is brittle, and is the softest of the commonly recognised metalloids, being marginally harder than sulfur. Large pieces of tellurium are stable in air. The finely powdered form is oxidized by air in the presence of moisture. Tellurium reacts with boiling water, or when freshly precipitated even at 50 °C, to give the dioxide and hydrogen: Te + 2 H O → TeO + 2 H. It reacts (to varying degrees) with nitric, sulfuric, and hydrochloric acids to give compounds such as the sulfoxide TeSO or tellurous acid H TeO, the basic nitrate (Te O H) (NO), or the oxide sulfate Te O (SO). It dissolves in boiling alkalis, to give the tellurite and telluride: 3 Te + 6 KOH = K TeO + 2 K Te + 3 H O, a reaction that proceeds or is reversible with increasing or decreasing temperature.

At higher temperatures tellurium is sufficiently plastic to extrude. It melts at 449.51 °C. Crystalline tellurium has a structure consisting of parallel infinite spiral chains. The bonding between adjacent atoms in a chain is covalent, but there is evidence of a weak metallic interaction between the neighbouring atoms of different chains. Tellurium is a semiconductor with an electrical conductivity of around 1.0 Scm and a band gap of 0.32 to 0.38 eV. Liquid tellurium is a semiconductor, with an electrical conductivity, on melting, of around 1.9 × 10 Scm. Superheated liquid tellurium is a metallic conductor.

Most of the chemistry of tellurium is characteristic of a nonmetal. It shows some cationic behaviour. The dioxide dissolves in acid to yield the trihydroxotellurium(IV) Te(OH) ion; the red Te and yellow-orange Te ions form when tellurium is oxidized in fluorosulfuric acid (HSO F), or liquid sulfur dioxide (SO), respectively. It can form alloys with aluminium, silver, and tin. Tellurium shows fewer tendencies to anionic behaviour than ordinary nonmetals. Its solution chemistry is characterised by the formation of oxyanions. Tellurium generally forms compounds in which it has an oxidation state of −2, +4 or +6. The +4 state is the most stable. Tellurides of composition X Te are easily formed with most other elements and represent the most common tellurium minerals. Nonstoichiometry is pervasive, especially with transition metals. Many tellurides can be regarded as metallic alloys. The increase in metallic character evident in tellurium, as compared to the lighter chalcogens, is further reflected in the reported formation of various other oxyacid salts, such as a basic selenate 2TeO ·SeO and an analogous perchlorate and periodate 2TeO ·HXO. Tellurium forms a polymeric, amphoteric, glass-forming oxide TeO. It is a "conditional" glass-forming oxide – it forms a glass with a very small amount of additive. Tellurium has an extensive organometallic chemistry (see Organotellurium chemistry).