Selenium
34 Arsenic ← selenium → Bromine S
↑
Se
↓
Te General Name, Symbol, Number selenium, Se, 34 Chemical series nonmetals Group, Period, Block 16, 4, p Appearance gray, metallic luster
Atomic mass 78.96(3) g/mol Electron configuration [Ar] 3d10 4s2 4p4 Electrons per shell 2, 8, 18, 6 Physical properties Phase Solid Density (near r.t.) (gray) 4.81 g/cm³ Density (near r.t.) (alpha) 4.39 g/cm³ Density (near r.t.) (vitreous) 4.28 g/cm³ Liquid Density at m.p. 3.99 g/cm³ Melting point 494 K
(221 °C, 430 °F)Boiling point 958 K
(685 °C, 1265 °F)Heat of fusion (gray) 6.69 kJ/mol Heat of vaporization 95.48 kJ/mol Heat capacity (25 °C) 25.363 J/(mol·K) P/Pa 1 10 100 1 k 10 k 100 k at T/K 500 552 617 704 813 958 Atomic properties Crystal structure hexagonal Oxidation states ±2, 4, 6
(strongly acidic oxide)Electronegativity 2.55 (Pauling scale) Ionization energies
(more)1st: 941.0 kJ/mol 2nd: 2045 kJ/mol 3rd: 2973.7 kJ/mol Atomic radius 115 pm Atomic radius (calc.) 103 pm Covalent radius 116 pm Van der Waals radius 190 pm Miscellaneous Magnetic ordering no data Thermal conductivity (300 K) (amorphous)
0.519W/(m·K)Thermal expansion (25 °C) (amorphous)
37 µm/(m·K)Speed of sound (thin rod) (20 °C) 3350 m/s Young's modulus 10 GPa Shear modulus 3.7 GPa Bulk modulus 8.3 GPa Poisson ratio 0.33 Mohs hardness 2.0 Brinell hardness 736 MPa CAS registry number 7782-49-2 Notable isotopes iso NA Half-life DM DE (MeV) DP 72Se syn 8.4 d ε - 72As γ 0.046 - 74Se 0.87% Se is stable with 40 neutrons 75Se syn 119.779 d ε - 75As γ 0.264, 0.136,
0.279- 76Se 9.36% Se is stable with 42 neutrons 77Se 7.63% Se is stable with 43 neutrons 78Se 23.78% Se is stable with 44 neutrons 79Se syn 1.13×106 y β- 0.151 79Br 80Se 49.61% Se is stable with 46 neutrons 82Se 8.73% 1.08×1020 y β-β- 2.995 82Kr References
Selenium is a Chemical element in the Periodic table that has the symbol Se and Atomic number 34. It is a toxic Nonmetal that is chemically related to Sulfur and Tellurium. It occurs in several different forms but one of these is a stable gray Metallike form that conducts Electricity better in the light than in the dark and is used in photocells. This element is found in Sulfide ores such as Pyrite.
Applications
Selenium is an essential micronutrient in all known forms of life; it is a component of the unusual Amino acid Selenocysteine. Because of its photovoltaic and photoconductive properties, selenium is used extensively in electronics, such as photo cells and solar cells. Selenium is also extensively used in rectifiers.Selenium is used to remove color from glass, as it will counteract the green color that ferrous impurities impart. It also can be used to give a red color to glasses and enamels. Selenium is used to improve the abrasion resistance in vulcanized rubbers. It also finds application in photocopying.
Another use for selenium is the toning of photographs, and is sold by numerous photographic manufacturers including Kodak and Fotospeed. Its artistic use is to intensify and extend the tonal range of black and white photographic images, and it can also be used to increase the permanence of images.
History
Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jakob Berzelius who found the element associated with Tellurium.Growth in selenium consumption was driven by the development of new uses, including applications in rubber compounding, Steel alloying, and selenium rectifiers. By 1970, selenium in rectifiers had largely been replaced by Silicon, but its use as a photoconductor in plain paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced. In 1996, continuing research showed a positive correlation between selenium supplementation and Cancer prevention in humans, but widespread direct application of this important finding would not add significantly to demand owing to the small doses required. In the late 1990s, the use of selenium (usually with Bismuth) as an additive to Plumbing brasses to meet no-lead environmental standards became important.
Occurrence
Selenium occurs as Selenide in many Sulfide ores, such as those of Copper, Silver, or Lead. It is obtained as a byproduct of the processing of these ores, from the Anode mud of copper refineries and the mud from the lead chambers of Sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.Isotopes
Selenium has at least 29 isotopes, of which 5 are stable, and 6 are nuclear isomers.Precautions
While free selenium is nontoxic, many of its compounds are extremely toxic, and have modes of action similar to that of arsenic. Hydrogen selenide and other compounds are very toxic. Plants grown in selenium-rich soils, such as Locoweed, can cause effects on animals feeding on the plants.Selenium and health
Selenium is a trace element in humans. It is used in free radical elimination and other Antioxidant enzymes, and also plays a role in the functioning of the Thyroid gland. Dietary selenium comes from cereals, meat, fish, and eggs. Brazil nuts are a particularly rich source of selenium.Selenium deficiency in healthy people is relatively rare. It can occur in patients with severely compromised intestinal function, or those undergoing Total parenteral nutrition. Alternatively, people dependent on food that is sourced from selenium-deficient soil are also at risk. The Recommended Dietary Allowance for adults is 55 micrograms per day. More than 400 micrograms per day can lead to toxicity (selenosis).
In popular culture
Selenium from the shampoo Head & Shoulders was used to kill the aliens in the film Evolution.Compounds
- Hydrogen selenide (H2Se)
- Selenium dioxide (SeO2)
- Selenic acid (H2SeO4)
- Selenous acid (H2SeO3)
- Selenium sulfide
- Sodium selenite (Na2SeO3)