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Melting point: 340 °C (644 °F; 613 K) (sealed tube) Boiling point: 350 °C (662 °F; 623 K) subl. ... Selenium dioxide is the chemical compound with the formula SeO 2.
The minimum distance between chains is 343.6 pm. Gray selenium is formed by mild heating of other allotropes, by slow cooling of molten selenium, or by condensing selenium vapor just below the melting point. Whereas other selenium forms are insulators, gray selenium is a semiconductor showing appreciable photoconductivity.
The Gmelin rare earths handbook lists 1522 °C and 1550 °C as two melting points given in the literature, the most recent reference [Handbook on the chemistry and physics of rare earths, vol.12 (1989)] is given with 1529 °C.
Selenium forms two oxides: selenium dioxide (SeO 2) and selenium trioxide (SeO 3). Selenium dioxide is formed by the reaction of elemental selenium with oxygen: [5] + It is a polymeric solid that forms monomeric SeO 2 molecules in the gas phase. It dissolves in water to form selenous acid, H 2 SeO 3.
Selenium is toxic in high concentrations. As sodium selenite, the chronic toxic dose for human beings was described as about 2.4 to 3 milligrams of selenium per day. [7] In 2000, the US Institute of Medicine set the adult Tolerable upper intake levels (UL) for selenium from all sources - food, drinking water and dietary supplements - at 400 μg/day. [8]
Pentafluoroselenium hypofluorite can be produced from the reaction of selenium dioxide or selenium oxychloride with fluorine gas in the presence of silver difluoride catalyst. However, the reaction yield is low (14% [3]), and it is hard to purify due to large amount of SeF 6 byproduct. [4]
Selenous acid is easily formed upon the addition of selenium dioxide to water. As a crystalline solid, the compound can be seen as pyramidal molecules that are interconnected with hydrogen bonds. In solution it is a diprotic acid: [3] H 2 SeO 3 ⇌ H + + HSeO − 3 (pK a = 2.62) HSeO − 3 ⇌ H + + SeO 2− 3 (pK a = 8.32)
It is prepared by oxidising selenium compounds in lower oxidation states. One method involves the oxidation of selenium dioxide with hydrogen peroxide: SeO 2 + H 2 O 2 → H 2 SeO 4. Unlike the production sulfuric acid by hydration of sulfur trioxide, the hydration of selenium trioxide is an impractical method. [4]