Selenium helical chain form

Trigonal selenium is variously called metallic gray or black selenium and occurs in lustrous hexagonal crystals, which melt at 220.5 °C. Its structure, which has no sulfur analogue, consists of infinite, unbranched helical chains. Its density, 4.82 g cm , is the highest of any form of the element. Trigonal selenium is a semiconductor (intrinsic p-type with a rather indirect transition at about 1.85 eV [5]), and its electronic and photoelectric properties are the basis for many industrial uses of this element. 

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. 

The stable crystalline forms of solid selenium are trigonal or hexagonal, both composed of helical chains. Trigonal selenium melts at 493.7 K. Two monoclinic... 

Sulfur exhibits allotropy and its structure in all phases is quite complex. The common crystalline modification, rhombic sulfur, is in equilibrium with a triclinic modification above 96°C. Both have structures based on Sg-rings but the crystals are quite different. If molten sulfur is poured into water a dark red plastic form is obtained in a semielastic form. The structure appears to be a helical chain of S atoms. Selenium and tellurium both have a gray metal-like modification but sulfur does not have this form. 

Selenium and tellurium do not form eight-membered rings in their elemental forms. (Section 7.8) The most stable allotropes of these elements are cr stalline substances containing helical chains of atoms ( FIGURE 22.18). Each atom is close to atoms in adjacent chains, and it appears that some sharing of electron pairs between these atoms occurs. 

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