Arsenic allotropes

Arsenic and antimony resemble phosphorus in having several allotropic modifications. Both have an unstable yellow allotrope. These allotropes can be obtained by rapid condensation of the vapours which presumably, like phosphorus vapour, contain AS4 and Sb4 molecules respectively. No such yellow allotrope is known for bismuth. The ordinary form of arsenic, stable at room temperature, is a grey metallic-looking brittle solid which has some power to conduct. Under ordinary conditions antimony and bismuth are silvery white and reddish white metallic elements respectively. 

Arsenic and selenium, which fall directly below phosphorus and sulfur in the periodic table, are of interest for a variety of reasons. Arsenic is a true metalloid. A metallic form, called gray arsenic, has an electrical conductivity approaching that of lead. Another allotrope, yellow arsenic, is distinctly nonmetallic it has the molecular formula As4, analogous to white phosphorus, P4. Selenium is properly classified as a nonmetal, although one of its allotropes has a somewhat metallic appearance and is a semiconductor. Another form of selenium has the molecular formula Se8. analogous to sulfur. 

Arsenic is classed as a semimetal, meaning that it is neither a metal like aluminum or lead, nor quite a nonmetal such as oxygen, sulfur, or chlorine. Arsenic s main allotrope is a silvery-gray, brittle, metal-like substance. Its other two isotopes are unstable crystalline substances. 

Steel-gray crystalline brittle metal hexagonal crystal system atomic volume 13.09 cc/g atom three allotropes are known namely, the a-metaUic form, a black amorphous vitreous solid known as P-arsenic, and also a yellow aUotrope. A few other allotropes may also exist but are not confirmed. Sublimes at 613°C when heated at normal atmospheric pressure melts at 817°C at 28 atm density 5.72 g/cc (P-metallic form) and 4.70 g/cm (p-amor-phous form) hardness 3.5 Mohs electrical resistivity (ohm-cm at 20°C) 33.3xlCh (B—metallic polycrystalline form) and 107 (p—amorphous form) insoluble in water. 

Arsenic exhibits allotropy, which is characteristic of non-metals the usual, more stable, metallic form resembles the typical metals in appearance and in being a fairly good conductor of electricity. Under atmospheric pressure it begins to volatilise at about 450° C. and passes into a vapour containing complex molecules, As4, which at higher temperatures dissociate to As2 this complexity is not unusual in non-metals. The yellow allotrope, which is stable at low temperatures, resembles white phosphorus in being soluble in carbon disulphide—a property which emphasises the non-metallic character of this variety. The reactivity of the allotropes, as in the case of phosphorus, differs considerably. 

That arsenic may exist in both crystalline and amorphous forms was observed by Berzelius,2 who designated them a- and /3-arsenic, respectively. Two crystalline allotropes, metallic arsenic (the a-form) and yellow arsenic, are now recognised, and three amorphous forms, vitreous arsenic (the /3-form), grey and brown amorphous arsenic, have been described. The majority of investigators, however, deny the existence of more than one amorphous form, and indeed, as will be seen, it is an open question whether any amorphous form is to be considered as a true allotrope. 

Engel (1883) and Linck 1 (1899) stated that amorphous arsenic is transformed at 360° C., irreversibly and with considerable development of heat, into metallic arsenic Erdmann and Reppert gave 303° C. as the transformation temperature, while Jolibois 2 and Gaubeau 3 determined the point of irreversible transformation both of the brown and grey varieties to be 270° to 280° C. Erdmann gave the transition point between the brown form and the grey form as 180° C., but such a critical point has not been substantiated. Jolibois asserted that his thermal observations admitted only two allotropes, the ordinary grey... 

Investigation by X-ray methods 3 of the structure of samples of arsenolamprite from two different localities showed only partial agreement with that of metallic rhombohedral arsenic the differences may be attributed to the presence of impurity in the minerals, but could also be explained by the presence of a second allotropic modification corresponding to black metallic phosphorus (see p. 35). 

As already described, other crystalline forms of arsenic besides the rhombohedral are known or suspected to exist. The crystals of the yellow allotrope belong to the cubic system, while native arsenolamprite (p. 8) contains crystals belonging to the rhombic, or possibly to the monoclinic, system. Yellow arsenic is soluble in carbon disulphide. 

Karttunen, A.J., Linnolahti, M. and Pakkanen, T.A. (2007) Icosahedral and ring-shaped allotropes of arsenic. ChemPhysChem, 8(16), 2373-78. 

Phosphorus has many allotropes. It is most commonly encountered as white phosphorous, which contains tetrahedral P molecules (1). Other forms, that are quite stable thermodynamically but kinetically harder to make, contain polymeric networks with three-coordinate P. White phosphorous is highly reactive and toxic. It will combine directly with most elements, glows in air at room temperature as a result of slow oxidation, and combusts spontaneously at a temperature above 35°C. Arsenic can also form As4... 

Yellow forms of arsenic and antimony (the latter very unstable) have been described. These are presumably the nonmetallic modifications of these elements, analogous to white phosphorus, and also consisting of discrete molecules (tetrahedral quartets) in the solid state. The grey or metallic forms of arsenic and antimony are the most stable. They are far denser than the yellow forms, are insoluble in organic solvents, and have appreciable electrical conductivities. Black amorphous forms of arsenic and antimony are also known, and an additional allotrope of antimony, explosive (but always impure), has been described. 

In a 1 1 complex between alcohols and primary amines, the usual zig-zag chains of H-bonds may be joined together alternately by N(H)0 interactions to generate sheets of trans-fused super-cyclohexane rings with chair-like conformations. From a topological point of view, these super-structures resemble the sheet structure of the stable allotrope of arsenic as super-arsenic sheets in which 0(H)0, N(H)N, and N(H)0 interactions are involved (Figure 4) [28],... 

Arsenic and Its Ores. Elementary arsenic exists in several forms. Ordinary gray arsenic is a semi-metallic substance, steel-gray in color, with density 5.73 and melting point (under pressure) 814. It sublimes rapidly at about 450"", forming gas molecules As similar in structure to P. An unstable yello v crystalline allotropic form containing AS4 molecules, and soluble in carbon disulfide, also exists. The gray form has a covalent layer structure (Fig. 11-8). 

Arsenic, antimony, and bismuth also exhibit a variety of allotropes. The most stable allotrope of arsenic is the gray (a) form, which is similar to the rhombohedral form of phosphorus. In the vapor phase, arsenic, like phosphorus, exists as tetrahedral AS4. Antimony and bismuth also have similar a forms. These three elements have a somewhat metallic appearance but are brittle and are only moderately good conductors. Arsenic, for example, is the best conductor in this group but has an electrical resistivity nearly 20 times as great as copper. 

Arsenic is a metallic element (symbol As atomic no. 33), which exists in several allotropic forms. Various ores contain crystalline forms of arsenic salts cobaltite contains cobalt arsenic sulfide mispickel (arsenopyrite) iron arsenic sulfide orpiment arsenic trisulfide proustite (ruby silver ore) silver arsenic sulfide realgar arsenic sulfide and tennantite copper arsenic sulfide. 

Arsenic occurs in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The more common form of arsenic is a shiny, gray, brittle, metallic-looking solid. The less common form is a yellow crystalline solid. It is produced when vapors of arsenic are cooled suddenly. 

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