Feb 6 2003
Selenium (Se) was discovered in 1817 by J.J. Berzelius in Stockholm, Sweden.
Abundance and Occurrence
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.
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. The metal burns in air and is unaffected by water. Selenium is soluble in alkalis and concentrated nitric acid.
Applications
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.
Typical Properties
The typical properties of Selenium are tabulated below.
Table 1. Key properties
| Atomic Properties |
| Atomic number |
34 |
| Atomic radius - Goldschmidt ( nm ) |
0.116 |
| Atomic weight ( amu ) |
78.96 |
| Crystal structure |
Hexagonal |
| Electronic structure |
Ar 3d10 4s2 4p4 |
| Photo-electric work function ( eV ) |
5.9 |
| Thermal neutron absorption cross-section ( Barns ) |
12.2 |
| Valences shown |
-2, 4, 6 |
| Ionisation Potential |
No. |
eV |
| |
1 |
9.75 |
| |
2 |
21.2 |
| |
3 |
30.8 |
| |
4 |
42.9 |
| |
5 |
68.3 |
| |
6 |
81.7 |
| Natural Isotope Distribution |
Mass No. |
% |
| |
74 |
0.9 |
| |
76 |
9.0 |
| |
77 |
7.6 |
| |
78 |
23.5 |
| |
80 |
49.8 |
| |
82 |
9.2 |
| Electrical Properties |
| Electrical resistivity @ 20 °C ( µOhm.cm ) |
12 |
| Mechanical Properties |
| Material condition |
Polycrystalline |
| Bulk modulus ( GPa ) |
8.3 |
| Hardness – Mohs |
2.0 |
| Poisson's ratio |
0.447 |
| Tensile modulus ( GPa ) |
58 |
| Physical Properties |
| Boiling point (°C ) |
685 |
| Density @ 20 °C ( g.cm-3 ) |
4.79 |
| Melting point (°C ) |
217 |
| Thermal Properties |
| Coefficient of thermal expansion @ 0-100 °C ( x10-6 K-1 ) |
37 |
| Latent heat of evaporation ( J.g-1 ) |
333 |
| Latent heat of fusion ( J.g-1 ) |
69 |
| Specific heat @ 25 °C ( J.K-1.kg-1 ) |
321 |
| Thermal conductivity @ 0-100 °C ( W.m-1.K-1 ) |
0.5 |
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