Rhombohedral selenium

This form of selenium can also be produced from amorphous selenium by heating with quinoline, pyridine, aniline or other basic organic solvent,7 and also by sublimation. In this latter case the sublimate also contains the amorphous form.8 Atmospheric oxidation of solutions of alkali selenides gives this variety of selenium as a granular deposit. When produced by these last three methods, leafy crystals may be obtained, and Muthmann (1890) discovered that crystals which he obtained by the sublimation method were of the trigonal (rhombohedral) system and isomorphous with those of tellurium. 

Class I. ELEMENTS. A. Metals. Cubic copper, silver, gold, iron, platinum, iridium. - Tetragonal tin. - Rhombohedral and Hexagonal arsenic, antimony, bismuth, tellurium, (Os, Ir). - B. Metalloids. Cubic diamond. - Hexagonal graphite. - Orthorhombic sulfur, iodine. - Monoclinic sulfur, selenium. - Class II. SULFIDES. - Class HI. HALIDES. -Class IV. OXIDES, divided into SIMPLE OXIDES and COMPLEX OXIDES, such as CARBONATES, PHOSPHATES, SILICATES, BORATES and SULFATES. 

Arsenic, antimony and bismuth are all isomorphous, and in the structure of these elements each atom is covalently bound, to three others to form an indefinitely extended puckered sheet of the type shown in fig. 7.05. Each such sheet may be regarded as a two-dimensional molecule of infinite extent analogous to the one-dimensional molecules in the structure of selenium, and the whole structure is formed by the superposition of these sheets to give a rhombohedral arrangement. 

The trend towards greater metallic character in the elements is complete at polonium. Whereas sulfur is a true insulator (specific resistivity in fiQ-cm = 2 x 1023), selenium (2X1011) and tellurium (2 x10s) are intermediate in their electrical conductivities, and the temperature coefficient of resistivity in all three cases is negative, which is usually considered characteristic of non-metals. Polonium in each of its two allotropes has a resistivity typical of true metals ( 43 juQ-cm) and a positive temperature coefficient. The low-temperature allotrope, which is stable up to about 100°, has a cubic structure, and the high temperature form is rhombohedral. In both forms the coordination number is six. 

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