Compounds of Selenium, Tellurium and Polonium

All three elements combine readily with most metals and many non-metals to form binary chalcogenides. Indeed, selenides and tellurides are the most common mineral forms of these elements (p. 748). Nonstoichiometry abounds, particularly for compounds with the transition elements (where electronegativity differences are minimal and variable valency is favoured), and many of the chalcogenides can be considered 

Transition-element chalcogenides are also best prepared by direct reaction of the elements at 400-1000° C in the absence of air. They tend to be metallic nonstoichiometric alloys though intermetalhc compounds also occur, e.g. Ti...2Se, Ti-sSe, TiSeo.95, TiSei.os, Tio.gSe, Ti3Se4, Tio.vSe, TisSeg, TiSe2, TiSes, etc.  

Chizhikov and V. P. Shchastlivyi, Selenium and Selenides, Collet s, London, 1968, 403 pp. 

Fuller details of these many compounds are in the references cited. 

Most selenides and tellurides are decomposed by water or dilute acid to form H2Se or H2Te but the yields, particularly of the latter, are poor. 

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The elements and their compounds vary in their toxicity. It is interesting that oxygen and sulfur are essential to all life, while the compounds of selenium, tellurium, and polonium can be toxic. Selenium is an essential trace element even though at larger concentrations (above 5 ppm) it causes severe disorders. Tellurium compounds, while being toxic, have never been reported to cause a human fatality. Since polonium and its compounds are radioactive, they are considered extremely toxic in any concentration, see ALSO Oxygen Polonium Selenium Suleur Tellurium. 

We have already noted that the properties of the elements in Group VI show the characteristic trends that we have come to expect on descending a Group. The elements become more metallic in character oxygen is a covalently bonded gaseous diatomic molecule sulfur is a solid containing Sg molecules and is an insulator selenium (non-metal) and tellurium (semi-metal) are semiconductors with polymeric structures polonium is a metal. The compounds of selenium, tellurium and pollonium also illustrate the inert pair effect and a tendency to higher coordination numbers. 

Sulfur, Selenium, Tellurium and Polonium Table 1 Compounds of Group VI Elements and their Stereochemistries... 

Chalcogens - The Group VIA elements (oxygen, sulfur, selenium, tellurium, and polonium). Compounds of these elements are called chalcogenides. [7]... 

The group shows the normal property of a trend towards metallic character as it is descended. Selenium, tellurium and polonium have metallic allotropes and polonium has generally metalloid-type properties where they have been studied (Po is very rare). All the elements combine with a large number of other elements, both metallic and non-metallic, but in contrast to compounds of the halogens they are more generally insoluble in water, and even where soluble they do not ionize readily. 

The chemistry of sulfur is a broad area that includes such chemicals as sulfuric acid (the compound prepared in the largest quantity) as well as unusual compounds containing nitrogen, phosphorus, and halogens. Although there is an extensive chemistry of selenium and tellurium, much of it follows logically from the chemistry of sulfur if allowance is made for the more metallic character of the heavier elements. All isotopes of polonium are radioactive, and compounds of the element are not items of commerce or great use. Therefore, the chemistry of sulfur will be presented in more detail. 

Tellurium occurs in the same periodic classification as sulfur, selenium, and polonium, Tellurium, unlike sulfiu and selenium, has only two allotiopic forms. Due to its 5,r25p4 electron configuration, tellurium, like sulfur and selenium, forms many divalent compounds with covalent bonds and two lone pairs, and d-hybndization is quite common, lo form compounds with tellurium oxidation states of +4 and —6,... 

Polonium is a radioactive element that is difficult to study in the laboratory.) Oxygen has a tendency to accept two electrons to form the oxide ion (O ) in many ionic compounds. Sulfur, selenium, and tellurium also form dinegative anions (S, Se, and Te ). The elements in this group (especially oxygen) form a large number of molecular compounds with nonmetals. The important compounds of sulfur are SO2, SO3, and H2S. Sulfuric acid is formed when sulfur trioxide reacts with water ... 

Our discussion of organo-compounds of group 16 elements is confined to selenium and tellurium (polonium having been little studied, see Chapter 15). Of course, there are also vast numbers of organic compounds containing C—O or C—S bonds, and some relevant inorganic topics already covered are ... 

A chalcogen is an element in group 16 on the periodic table. This includes oxygen, sulfur, selenium, tellurium, polonium, and livermorium. This name comes from the Greek word meaning copper-former, and has its origins in the fact that some of these elements tend to coordinate to metals to form compounds with metals in ores. 

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