Selenium allotropy

Sulphur, selenium and tellurium exhibit allotropy, and in certain of their crystalline forms the elements are isomorphous. As would be expected from the increased positive character of tellurium, the allotropy of this element is less well defined. In the liquid condition the elements arc miscible with one another and yield mixed crystals the ternary system, S— Sc—Tc, exhibits neither the formation of compounds nor ternary eutectics, but contains two zones of complete miscibility in which there exist mixed crystals of selenium and tellurium with sulphur, and of sulphur and tellurium with selenium.2... 

The analogy of selenium with sulphur is observable in the occurrence of allotropy, although no rigid relationship can be traced between the various forms of the two elements. 

The thermal conductivity of selenium depends upon the allotropie form, its age, and the temperature. As might be expected the value for the grey metallic form is greater than that for the vitreous modification j thus at 25° C. the thermal conductivity of the former varies between 0-00070 and 0-00188, whilst that of the latter lies between 0-000293 and 0-000328. The values generally increase with rise in the temperature of preparation of the sample, and with exposure of the sample to light on the other hand, the conductivity diminishes with age.7... 

Occurrence, Extraction and Purification—The Allotropy of Selenium—Colloidal Selenium—General Physical Properties of Selenium—Chemical Properties— Applications of Element—Physiological Action—Atomic Woighl—Valency— Constitution of Selenium Molecule—Detection and Estimation of Selenium. 

Liquid selenium appears to exhibit allotropy, hut tellurium does not. 

Solid selenium likewise exists m several allotropic forms. The i crystalline variety is labile, and may possibly occur in two modification both of which arc monoclmic.2 The grey crystalline form appears consist of two varieties, SeA and ScD, in dynamic equilibrium with ea other.1 Solid tellurium manifests allotropy, but to a much less pi nounccd degree. 

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. 

---