|Type:Other Non-Metal, Chalcogen||Atomic weight:78.96|
|Density @ 293 K:4.79 g/cm3||Atomic volume:16.45 cm3/mol|
Selenium exists in only trace amounts around us. It was discovered by Jons J. Berzelius in 1817. Martin Klaproth, the discoverer of uranium and zirconium had concluded that a red colored byproduct of sulfuric acid production contained tellurium, an element whose compounds he had been studying. Berzelius analyzed the sample and decided it did not contain tellurium, but in fact contained a new element similar to tellurium. Since 'tellus' in Latin means earth goddess, Berzelius named the new element selenium from the Greek word 'selene', meaning moon goddess.
|State (s, l, g):solid|
|Melting point: 493 K (220 °C)||Boiling point:958 K (685 °C)|
|Specific heat capacity: 0.32 J g-1 K-1||Heat of atomization:227 kJ mol-1|
|Heat of fusion: 6.694 kJ mol-1||Heat of vaporization :26.32 kJ mol-1|
|1st ionization energy:940.9 kJ mol-1||2nd ionization energy:2044.5 kJ mol-1|
|3rd ionization energy: 2973.7 kJ mol-1||Electron affinity:194.97 kJ mol-1|
|Shells:2,8,18,6||Electron configuration: [Ar] 3d10 4s-2 4p4|
|Minimum oxidation number:-2||Maximum oxidation number:6|
|Min. common oxidation no.:-2||Max. common oxidation no.:6|
|Electronegativity (Pauling Scale):2.55||Polarizability volume:3.8 Å3|
|Structure:long, helical chains (crystalline hexagonal), Se8 rings (crystalline monoclinic)||Color:gray or red (crystalline), black or red (amorphous|
|Hardness: 2.0 mohs|
Selenium is considered to be toxic if taken in excess. It is carcinogenic and teratogenic. Selenates and selenites are very toxic, and hydrogen selenide (SeH2) is an extremely toxic, corrosive gas.
Selenium exists in several allotropic forms. The most stable, crystalline hexagonal selenium, is metallic gray. Crystalline monoclinic selenium is a deep red color. Amorphous selenium is red in powder form and is black in vitreous form. Gray crystalline 'metallic' selenium conducts electricity better in the light than in the dark (photoconductive) and it can convert light directly into electricity (photovoltaic).
Selenium is used in the glass industry to decolorize glass and to make red-colored glasses and enamels.
|Reaction with air:vigorous, w/ht ⇒ SeO2||Reaction with 6 M HCl:none|
|Reaction with 15 M HNO3:mild , ⇒ H2SeO3, NOx||Reaction with 6 M NaOH:|
|Atomic radius:119 pm||Ionic radius (1+ ion):pm|
|Ionic radius (2+ ion):pm||Ionic radius (3+ ion):pm|
|Ionic radius (2- ion):184 pm||Ionic radius (1- ion):pm|
|Thermal conductivity: 0.52 W m-1 K-1||Electrical conductivity:8 x 106 S m-1|
|Abundance earth's crust: 50 parts per billion by weight, 10 parts per billion by moles|
|Abundance solar system:parts per billion by weight, part per billion by moles|
|Cost, pure: $61 per 100g|
|Cost, bulk:$5.30 per 100g|
Selenium occasionally occurs free in nature, but more often occurs as selenides of iron, lead, silver, or copper. Commercially, selenium is obtained mainly from anode mud waste produced in the electrolytic refining of copper. Brazil nuts are the richest known dietary source of selenium.
Selenium has 24 isotopes whose half-lives are known, with mass numbers 67 to 91. Of these, five are stable: 74Se, 76Se, 77Se, 78Se and 80Se.