Elemental Selenium and Tellurium

A major use of selenium has been in photoelectric devices. Its conductivity increases with illumination, and this provides a way of measuring light intensity or operating electrical switches. However, newer types of photocells are available that are made of other materials (such as cadmium sulfide). A second and more important use of selenium is in rectifiers to convert alternating current to direct current. Also, some pigments contain selenium and tellurium compounds, and both elements have been used in vulcanization of rubber. Selenium compounds have been used in dandruff treatment shampoos, and low levels of selenium may be necessary for dietary balance. Some studies have shown that persons whose diets are deficient in selenium may have a higher incidence of heart attacks. 

Although sulfur shows a great tendency toward catenation, selenium and tellurium show much less tendency in this regard. Some structures containing chains and rings of Se and 

Tellurium (m.p. 450 °C, density 6.25 g cm-3) is more metallic in its appearance, but it is not a good electrical conductor as are most metals. Polonium, on the other hand, is typically metallic in its electrical properties. Selenium and tellurium are best regarded as semiconductors, and sulfur is nonmetallic in behavior (an insulator). Thus, the usual trend from nonmetallic to metallic behavior is shown in going down Group VIA of the periodic table. All of these elements differ substantially from oxygen in their chemical properties. 

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These closely resemble the corresponding sulphides. The alkali metal selenides and tellurides are colourless solids, and are powerful reducing agents in aqueous solution, being oxidised by air to the elements selenium and tellurium respeetively (cf. the reducing power of the hydrides). 

Although the sulfur-gold bond has been most investigated, the Group 16 elements selenium and tellurium have also attracted attention and are discussed in detail here (polonium has not received attention due to its radioactivity). 

Tetraalkylammonium borohydrides react with elemental selenium and tellurium to form the symmetrical dialkyl selenides and tellurides [44]. Diphenyl selenide yields the alkyl phenyl selenides. 

Reaction of PhLi TMEDA with Elemental Selenium and Tellurium... 

The first two elements in Group 16, oxygen and sulfur, have the electron configurations Hg ns np while the heavier elements, selenium and tellurium, have the electron configurations [Ng](n- All the Group 16 elements form thermally stable compounds... 

With concentrated nitric acid, selenium and tellurium form only their +4 oxoacids, H2Se03 and H2Te03 respectively, indicating a tendency for the higher oxidation states to become less stable as the atomic number of the element is increased (cf. Group V, Chapter 9). 

Sulphur is less reactive than oxygen but still quite a reactive element and when heated it combines directly with the non-metallic elements, oxygen, hydrogen, the halogens (except iodine), carbon and phosphorus, and also with many metals to give sulphides. Selenium and tellurium are less reactive than sulphur but when heated combine directly with many metals and non-metals. 

The elements, sulphur, selenium and tellurium form both di- and tri-oxides. The dioxides reflect the increasing metallic character of... 

A widely used procedure for determining trace amounts of selenium involves separating selenium from solution by reduction to elemental selenium using tellurium (as a carrier) and hypophosphorous acid as reductant. The precipitated selenium, together with the carrier, are collected by filtration and the filtered soflds examined directly in the wavelength-dispersive x-ray fluorescence spectrometer (70). Numerous spectrophotometric and other methods have been pubHshed for the deterruination of trace amounts of selenium (71—88). 

Nitrogen and sodium do not react at any temperature under ordinary circumstances, but are reported to form the nitride or azide under the influence of an electric discharge (14,35). Sodium siHcide, NaSi, has been synthesized from the elements (36,37). When heated together, sodium and phosphoms form sodium phosphide, but in the presence of air with ignition sodium phosphate is formed. Sulfur, selenium, and tellurium form the sulfide, selenide, and teUuride, respectively. In vapor phase, sodium forms haHdes with all halogens (14). At room temperature, chlorine and bromine react rapidly with thin films of sodium (38), whereas fluorine and sodium ignite. Molten sodium ignites in chlorine and bums to sodium chloride (see Sodium COMPOUNDS, SODIUM HALIDES). 

Since tire alkali and alkaline metals have such a high affinity for oxygen, sulphur aird selenium they are potentially useful for the removal of these iron-metallic elements from liquid metals with a lower affinity for these elements. Since the hairdling of these Group I and II elements is hazardous on the industrial scale, their production by molten salt electrolysis during metal rehning is an attractive alternative. Ward and Hoar (1961) obtained almost complete removal of sulphur, selenium and tellurium from liquid copper by the electrolysis of molten BaCla between tire metal which functioned as the cathode, and a graphite anode. 

Vol 12 PerfluorohaloorganiL Compounds of Main Group Elements Part 2 Compounds of Sulfur (Cuniinuanon) Selenium and Tellurium... 

A process for the gravimetric determination of mixtures of selenium and tellurium is also described. Selenium and tellurium occur in practice either as the impure elements or as selenides or tellurides. They may be brought into solution by mixing intimately with 2 parts of sodium carbonate and 1 part of potassium nitrate in a nickel crucible, covering with a layer of the mixture, and then heating gradually to fusion. The cold melt is extracted with water, and filtered. The elements are then determined in the filtrate. 

The realization that the better ordering criterion is atomic number rather than atomic weight invites us to consider triads of atomic numbers. This reveals a most remarkable fact, namely that —50% of all conceivable triads on a conventional periodic table are in fact exact. For example, the elements sulfur, selenium, and tellurium have atomic numbers of 16, 34, and 52, respectively, thus showing that the atomic number of the middle of these three... 

FIGURE 1.62 The Croup 16A/I elements. From left to right oxygen, sulfur, selenium, and tellurium. Note the trend from nonmetal to metalloid. 

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