Metals allotropes

Basic physical properties of sulfur, selenium, and tellurium are indicated in Table 1.3. Downward the sulfur sub-group, the metallic character increases from sulfur to polonium, so that whereas there exist various non-metallic allotropic states of elementary sulfur, only one allotropic form of selenium is (semi)metallic, and the (semi)metallic form of tellurium is the most common for this element. Polonium is a typical metal. Physically, this trend is reflected in the electrical properties of the elements oxygen and sulfur are insulators, selenium and tellurium behave as semiconductors, and polonium is a typical metallic conductor. The temperature coefficient of resistivity for S, Se, and Te is negative, which is usually considered... 

Which of the group 4A elements have allotropes with the diamond structure Which have metallic allotropes How does the variation in the structure of the group 4A elements illustrate how metallic character varies down a periodic group ... 

Selenium, a highly poisonous element, and tellurium also exist in a variety of al-lotropic forms, whereas polonium, a radioactive element, exists in two metallic allotropes. Selenium is a photoconductor, a poor conductor ordinarily, but a good conductor in the presence of light. It is used extensively in xerography, photoelectric cells, and semiconductor devices. 

The properties of these continua have not been explored to date. They may be metallic, and transitions in properties might occur as a function of temperature. They may be viewed as metal allotropes filled with electrons to a level far below their Fermi level. In addition, these "metals" would be very much effected by the electric fields of the zeolite framework. The mean charge on each non-framework cation in the structure can be varied by changing the composition of the zeolite framework, and by varying the extent of sorption. 

The crystal structures of the chemical elements provide a simple starting point for illustrating what can be learned from listed atomic coordinates. Tin has both a nonmetallic and a metallic allotrope (polymorph). 

Elements. Those elements that form extended covalent (as opposed to metallic) arrays are boron, all the Group IV elements except lead, also phosphorus, arsenic, selenium and tellurium. All other elements form either only metallic phases or only molecular ones. Some of the above elements, of course, have allotropes of metallic or molecular type in addition to the phase or phases that are extended covalent arrays. For example, tin has a metallic allotrope (white tin) in addition to that with the diamond structure (grey tin), and selenium forms two molecular allotropes containing Se8 rings, isostruc-... 

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. 

Finally, metallic allotropes with higher coordination numbers have been studied [39,51] and found to be well removed in enthalphy from four-fold coordinated structures below a pressure of 10 Mbar. Here, we report on the geometric optimization and doping properties of the recently proposed purely sp bonded carbon phases [15] and a propotype zeolite structure (sp bonded) called melanoph-logite using the density-functional based tight-binding method (DF-TB) [52,53]. 

Three allotrope modifications are known. The stable form is the grey metallic modification with a sublimation temperature of 613°C. Melting is possible in closed tubes at 817°C and a pressure of 28 bar. The metallic allotrope exists in a crystalline (specific weight 5.73 g/cm ) and an amorphous form (specific weight 4.7-5.1 g/cm. The yellow crystalline and the black amorphous modifications are metastable and transform into the grey arsenic under the influence of light or heat. Arsenic is nontoxic in its elementary form [1-5]. 

Tin is a silvery white metal, with great malleability, permitting it to be hammered into thin sheets, called tin foil. Ordinary white tin, which has metallic properties, slowly changes at temperatures below 18 C to a non-metallic allotropic modification, gray tin, which has the diamond structure. (The physical properties given in Table 18-3 pertain to white tin.) At very low temperatures, around —40°C, the speed of this conversion is sufficiently great that metallic tin objects sometimes fall into a powder of gray tin. This phenomenon has been called the tin pest. ... 

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