Allotropes of selenium and tellurium

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 distance of two adjacent atoms within the chain are Se-Se 237 pm and Te-Te 283 pm. Each atom has four adjacent atoms from three different chains at an average distance of Se- Se 344 pm, Te- Te 350 pm. The interchain distance is significantly shorter than expected from the van der Waals separation (380 pm for Se and 412 pm for Te). 

Red monoclinic selenium exists in three forms, each containing Ses rings with the crown conformation of Sg (Fig. 16.4.1). Vitreous black selenium, the ordinary commercial form of the element, comprises an extremely complex and irregular structure of large polymeric rings. 

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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. 

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