Allotropes of arsenic

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

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, 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. 

Fig. 14.3 (a) The tetrahedral P4 molecule found in white phosphorus, (b) Part of one of the chain-like arrays of atoms present in the infinite lattice of Hittorf s phosphorus the repeat unit contains 21 atoms, and atoms P and P" are equivalent atoms in adjacent chains, with chains connected through P -P" bonds, (c) Part of one layer of puckered six-membered rings present in black phosphorus and in the rhombohedral allotropes of arsenic, antimony and bismuth. 

Elements from column V studied for their potential use in Li-ion batteries are essentially P, Sb and Bi. Data relative to As are relatively scarce in the literamre, probably because of its high toxicity and also its relatively high atomic mass, which limits the gravimetric capacity. The most common allotrope of arsenic is grey or metallic As, which crystallizes in the rhombohedric system. In a small paragraph of their paper on Li intercalation in ZnSb, Park and Sohn have reported a capacity of 330 mAh/g (1300 mAh/cm ) and 100 % capacity retention after 300 cycles for an As/C composite anode [106]. 

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. 

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. 

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. 

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. 

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. 

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 displays a remarkable number of allotropes of its members showing the trend from non-metallic forms through to metallic forms. Thus nitrogen has only the diatomic form phosphorus has a highly reactive form (brown), and a tetrahedral form P4 (white), forms based on broken tetrahedra P (red and violet), and a hexagonal layer-type lattice (black). Arsenic and antimony have the AS4 and Sb4 forms, which are less stable than the layer type in this case bismuth has the layer-lattice form only. 

Arsenic, antimony, and bismuth also exhibit allotropes. The most stable aUotrope of arsenic is the gray (a) form, which is similar to the rhombohedral form of phosphorus. 

On heating to higher temperatures, the tetraatomic vapours dissociate to diatomic species and eventually to single atoms. This dissociation takes place more readily with increasing pnictide atomic weight (arsenic is completely dissociated to As at 1700°C). The tetraatomic forms or the vapours from any of the allotropes of P, As or Sb are all extremely toxic either by inhalation or by ingestion. The red, black or metallic forms, unless vaporised, are, on the other hand, much less toxic. 

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