Selenium

Selenium exists in several allotropic forms; red selenium is monoclinic and contains Se₈ rings. It has a melting point of 180°C and a relative density of 4.45. The grey (metallic) form of selenium is formed when other varieties are heated to 200°C; this has a melting point of 220°C and a relative density of 4.80. The metal burns in air and is unaffected by water. It is soluble in alkalis and concentrated nitric acid. The electrical properties of selenium are of particular interest as it is both photovoltaic (i.e. it becomes an electrical conductor when exposed to light or ultra violet radiation) and photoconductive (i.e. the electrical conductivity increases with increasing illumination). These properties mean that selenium has applications in the electronics industry (e.g. in the manufacture of photoelectric cells and solar cells). Selenium is used in the ceramics and glass industries (in the latter the element being used as a decolouriser and also in the manufacture of red glasses and enamels). Selenium is an important trace element for humans, the average body containing approximately 14 mg.

Symbol

Se

Discovered

1817 by J.J. Berzelius in Stockholm, Sweden.

Abundance

Selenium is widely distributed, mainly as the selenides of heavy metals, but only has an abundance of 0.05 ppm in the earth’s crust.

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Atomic number	34		Clip
Atomic / Molecular Weight	78.96	gmol-1	Clip
Density	4808	kgm-3	Clip
Lattice constant	436	fm	Clip
Melting Point	493	K	Clip
Boiling Point	958	K	Clip
Specific heat capacity	322	Jkg-1K-1	Clip
Bulk Modulus of Rigidity	8300000000	Nm-2	Clip

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See also: Berzelius, Jons Jakobs, Periodic Table.