Selenium, Se

Selenium is a chemical element with atomic number 34, with the chemical symbol Se. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurring in its elemental state in nature. It is toxic in large amounts, but trace amounts of it are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another). Selenium requirements in plants differ by species, with some plants apparently requiring none.

Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semimetal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see allotropic section below). Selenium also exists in many nonconductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter chemical cousin sulfur.

Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare.

Technical Data
Symbol Se Density (25°C)/gcm-3 (Hexagonal) 4.189
Atomic number 34 Density (25°C)/gcm-3 (Monoclinic) 4.389
No. of stable occurring isotopes 6 Density (25°C)/gcm-3 (Vitreous) 4.285
Atomic weight 78.96(+/-0.03) Melting point / °C 217
Electronic configuration [Ar]3dl04s24p4 Boiling point / °C 685
Atomic radius(12-coordinate)/pm 140* ΔHatomization/kJmol-1 206.7
Ionic radius(6-coordinate)/pm M2- 198 Ionization energy/kJmol-1 940.7
Ionic radius(6-coordinate)/pm M4+ 50 Band gap Eg/kJmol-1 178
Ionic radius(6-coordinate)/pm M6+ 42 Electrical resistivity (25°C)/μohm cm 1010**
Electronegativity χ 2.4  
Evaporation Techniques
Temperature (oC) @Vap. Pressure Techniques Remarks
10-8 Torr 10-6 Torr 10-4 Torr Electron Beam Crucible Coil Boat
89 125 170 Good Al2O3 & Vitreous Graphite Tungsten & Molybdenum Tungsten & Molybdenum Toxic. Poisons vacuum systems.

Evaporation Materials
S3-5000-M shot 3mm
S3-5006-M pieces 2-4mm
Powder
S3-10002-P -325 mesh