Allotropes of selenium

Figure 16.1 Structures of various allotropes of selenium and the structure of crystalline tellurium (a) the Seg unit in a- fi- and y-red selenium (b) the helical Se chain along the c-axis in hexagonal grey selenium (c) the similar helical chain in crystalline tellurium shown in perspective and (d) projection of the tellurium structure on a plane perpendicular to the c-axis.

The group 6A elements are oxygen, sulfur, selenium, tellurium, and polonium. As shown in Table 19.7, their properties exhibit the usual periodic trends. Both oxygen and sulfur are typical nonmetals. Selenium and tellurium are primarily non-metallic in character, though the most stable allotrope of selenium, gray selenium, is a lustrous semiconducting solid. Tellurium is also a semiconductor and is usually classified as a semimetal. Polonium, a radioactive element that occurs in trace amounts in uranium ores, is a silvery white metal. 

Mar. 27, 1824, Bonn, Germany - Nov. 28, 1914, Miin-ster, Germany) His early work at Berlin was carried out on the allotropes of selenium and phosphorus, but he is remembered for his work on ionic transport and cathode rays, in both areas making substantial contributions. In ionic transport theory, he was the first to formulate laws about the separate -> migration of oppositely charged ions to anode and cathode compartments and to derive a method for the measurement of... 

Selenium exists in a number of allotropic forms. Allotropes are forms of an element with different physical and chemical properties. One allotrope of selenium is an amorphous red powder. Amorphous means without crystalline shape. A lump of clay is an example of an amorphous material. A second allotrope of selenium has a bluish, metallic appearance. A number of other allotropes have properties somewhere between these two forms. 

Two thermodynamically unstable allotropes of selenium which are soluble in CS2 and contain cyclooctaselenium, Se8, can be obtained by evaporation... 

The allotropes of sulfur in the solid, liquid, and gaseous phases include a wide variety of structures dominated by S-S bonds. Cyclooctasulfur, intertwining superchains, helical chains, rings of various sizes, diatomic molecules, and free atoms characterize this element at various temperatures. Although not to the same extent as those of sulfur, the allotropes of selenium and tellurium have similar catenated chains and rings. Sulfur, selenium, and tellurium also exist in a variety of catenated polycations and polyanions. 

Arsenic and selenium, which fall directly below phosphorus and sulfur in the periodic table, are of interest for a variety of reasons. Arsenic is a true metalloid. A metallic form, called gray arsenic, has an electrical conductivity approaching that of lead. Another allotrope, yellow arsenic, is distinctly nonmetallic it has the molecular formula As4, analogous to white phosphorus, P4. Selenium is properly classified as a nonmetal, although one of its allotropes has a somewhat metallic appearance and is a semiconductor. Another form of selenium has the molecular formula Se8. analogous to sulfur. 

Basic physical properties of sulfur, selenium, and tellurium are indicated in Table 1.3. Downward the sulfur sub-group, the metallic character increases from sulfur to polonium, so that whereas there exist various non-metallic allotropic states of elementary sulfur, only one allotropic form of selenium is (semi)metallic, and the (semi)metallic form of tellurium is the most common for this element. Polonium is a typical metal. Physically, this trend is reflected in the electrical properties of the elements oxygen and sulfur are insulators, selenium and tellurium behave as semiconductors, and polonium is a typical metallic conductor. The temperature coefficient of resistivity for S, Se, and Te is negative, which is usually considered... 

Selenium chemistry is complex, and additional research is warranted on chemical and biochemical transformations among valence states, allotropic forms, and isomers of selenium. 

Fergusson et al. were the first to report the existence of binary compounds with a general formula Se Sg in these melts. They carried out an extensive investigation by X-ray powder diffraction and by absorption spectroscopy in the infrared, visible, and ultraviolet regions over the whole composition range of molten mixtures of sulfur and selenium cooled down to 20 °C. They also examined phases obtained by recrystallization of the cooled melts from carbon disulfide. All phases were isomorphic with one of the allotropes of Sg and SCg indicating that the structures also consist of cyclic eight-membered molecules ... 

Solid Allotropes.—Both amorphous and crystalline varieties of selenium occur. Amorphous selenium is best known as the vitreous and the finely divided brick-red forms, which are frequently described as two distinct allotropes they are, however, identical. The crystalline allotropes include several monoclinic varieties, red to brown in colour, as well as the so-called metallic selenium. 

Allotropes of both sulfur and selenium are known in which helical chains of great lengih. ire present. While the sulfur chains are unstable with respect to cyclic S , the chain form Tor selenium is most stable. Red phosphorus is polymeric and is thought to involve chains of pyramidal phosphorus atoms. 

Selenium forms several allotropes but tellurium forms only one. The thermodynamically stable form of selenium (a-selenium or gray selenium) and the crystalline form of tellurium are isostructural. In both Te and gray Se, the atoms form infinite, helical chains having three atoms in every turn, the axes of which lie parallel to each other in the crystal, as shown in Fig. 16.8.1. 

Elemental selenium and elemental sulfur (any allotrope of either one) react on heating (melting) to give a large number of cyclic molecules Se Sm. While the eight-membered rings are much preferred, HPLC analysis has shown that both six- and seven-membered species are also formed. For a review of these cyclic selenium sufides, see ref. (55). 

The increase in metallic character with increa se in molecular weight is striking. Sulfur is a non-conductor of electricity, as is the red allotropic form of selenium. The gray form of selenium has a small... 

The red phosphorus-P4Se3 system has been examined and the selenide shown to exist in three allotropic forms.607 Insertion of selenium into the P—P bond of (CF3)2PP(CF3)2 occurs on heating at 100°C but reactions of... 

Selenium is quite rare (9 X 10 % of the earth s crust). It occurs mainly as an impurity in sulfur, sulfide, and sulfate deposits. It is obtained from the flue dusts that result from roasting sulfide ores and from the anode mud formed in the electrolytic refining of copper. It is used as a red coloring in glass. The gray crystalline allotropic form of selenium has an electrical conductivity that is very light-sensitive, so it is used in photocopy machines and in solar cells. 

---