Tellurium atomic properties

Table VIII lists some of the more important physical properties of TeF4. In the orthorhombic crystals, each tellurium atom is surrounded by three terminal and two bridging fluorine atoms, arranged at the apices of a distorted square pyramid. The square-pyramidal units are linked by cis-bridging atoms into endless chains with a bridge angle of 159°. The nearest intermolecular contacts to the tellurium atom are 2.94 and 3.10 A, so that there are no other significant interactions. This geometry is in accordance with the steric activity of the lone electron pair at the tellurium atom. Figure 6 shows the atomic arrangement (54).

The compound ZnTe has metallic properties, and so far as the valency is concerned it can be thought of as heteropolar. The electron cloud is more concentrated around the tellurium atoms, so that they assume a negative charge with respect to zinc and, consequently, the metallic bond acquires the characteristics of an ionic one. Decreasing ionization energy causes the homopolar to change over into the metallic, as for example in the series... 

A number of theoretical studies of six-membered heterocycles containing one selenium or tellurium atom were previously described in CHEC-II(1996) (section 5.11.2). All of these consisted of ab initio calculations. Several new reports have appeared since this time dealing with theoretical approaches to understanding the properties of these materials, their preparations, and their reactivity. 

As mentioned in CHEC-II(1996) (Section 5.11.9), there remains significant interest in the technical potential of compounds containing six-membered rings with one selenium or tellurium atom. The electronic properties of selenium-containing organic semiconductors such as 180 and 182 are particularly interesting <1993J(P2)1815>. As... 

The physical properties of tellurium are generally anistropic. This is so for compressibility, thermal expansion, reflectivity, infrared absorption, and electronic transport. Owing to its weak lateral atomic bonds, crystal imperfections readily occur in single crystals as dislocations and point defects. 

Tellurium Sulfide. In the hquid state, teUurium is completely miscible with sulfur. The Te—S phase diagram shows a eutectic at 105—110°C when the sulfur content is 98—99 atom % (94—98 wt %). TeUurium—sulfur aUoys have semiconductor properties (see Semiconductors). Bands attributed to teUurium sulfide [16608-21 -2] TeS, molecules have been observed. 

Table 16.2 Some atomic and physical properties of selenium, tellurium and polonium...

The silver white, shiny, metal-like semiconductor is considered a semimetal. The atomic weight is greater than that of the following neighbor (iodine), because tellurium isotopes are neutron-rich (compare Ar/K). Its main use is in alloys, as the addition of small amounts considerably improves properties such as hardness and corrosion resistance. New applications of tellurium include optoelectronics (lasers), electrical resistors, thermoelectric elements (a current gives rise to a temperature gradient), photocopier drums, infrared cameras, and solar cells. Tellurium accelerates the vulcanization of rubber. 

Sulfur vapor consists of a mixture of species that includes S8, S6, S4, and S2 (which like 02 is paramagnetic). Because the S8 molecule is nonpolar, it is soluble in liquids such as CS2 and C6H6. Selenium also consists of cyclic molecules that contain eight atoms, and tellurium is essentially metallic in character. In their vapors, several species are found that contain 2, 6, or 8 atoms. Both are useful as semiconductors, and selenium has been used in rectifiers. Because the electrical conductivity of selenium increases as the intensity of illumination increases, it has been used to operate electrical switches that open or close as a light beam is broken. Selenium was also used in light meters, but other types of meters are now available that are more sensitive. Table 15.1 gives a summary of the properties of the group VIA elements. 

The larger atomic radius and larger polarizability of selenium and tellurium together with the lesser electronegativity of these elements results in increased stability of the corresponding dications. Similarities in chemical properties and electronic structure allow for formation of mixed dichalcogen dications as well. 

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