Metalloid: Difference between revisions

The chemistry of arsenic is predominately nonmetallic.<ref>[[#Bailar1965|Bailar, Moeller & Kleinberg 1965, p.&nbsp;477]]</ref> Whether or not arsenic forms a cation is unclear.{{refn|1=Sources mentioning cationic arsenic include: Gillespie & Robinson<ref>[[#Gillespie|Gillespie & Robinson 1963, p.&nbsp;450]]</ref> who find that, "in very dilute solutions in 100% sulphuric acid, arsenic (III) oxide forms arsonyl (III) hydrogen sulphate, AsO.HO₄, which is partly ionized to give the AsO⁺ cation. Both these species probably exist mainly in solvated forms, e.g., As(OH)(SO₄H)₂, and As(OH)(SO₄H)⁺ respectively"; Paul et al.<ref>[[#Pauletal|Paul et al. 1971]]; see also [[#Ahmeda|Ahmeda & Rucka 2011, pp.&nbsp;2893–94]]</ref> who reported spectroscopic evidence for the presence of As₄²⁺ and As₂²⁺ cations when arsenic was oxidized with [[peroxydisulfuryl difluoride]] S₂O₆F₂ in highly acidic media (Gillespie and Passmore<ref>[[#GillespieP|Gillespie & Passmore 1972, p.&nbsp;478]]</ref> noted the spectra of these species were very similar to S₄²⁺ and S₈²⁺ and concluded that, "at present" there was no reliable evidence for any homopolycations of arsenic); Van Muylder and Pourbaix,<ref>[[#Van Muylder|Van Muylder & Pourbaix 1974, p.&nbsp;521]]</ref> who write that, "As₂O₃ is an amphoteric oxide which dissolves in water and in solutions of pH between 1 and 8 with the formation of undissociated [[arsenious acid]] HAsO₂; the solubility…increases at pH’spH's below 1 with the formation of 'arsenyl' ions AsO⁺…"; Kolthoff and Elving<ref>[[#Kolthoff|Kolthoff & Elving 1978, p.&nbsp;210]]</ref> who write that, "the As³⁺ cation exists to some extent only in strongly acid solutions; under less acid conditions the tendency is toward [[hydrolysis]], so that the anionic form predominates"; Moody<ref>[[#Moody|Moody 1991, pp.&nbsp;248–49]]</ref> who observes that, "arsenic trioxide, As₄O₆, and arsenious acid, H₃AsO₃, are apparently amphoteric but no cations, As³⁺, As(OH)²⁺ or As(OH)₂⁺ are known"; and Cotton et al.<ref>[[#Cotton1999|Cotton & Wilkinson 1999, pp.&nbsp;396, 419]]</ref> who write that (in aqueous solution) the simple arsenic cation As³⁺ "may occur to some slight extent [along with the AsO⁺ cation]" and that, "Raman spectra show that in acid solutions of As₄O₆ the only detectable species is the pyramidal As(OH)₃".|group=n}} Its many metal alloys are mostly brittle.<ref>[[#Eagleson1994|Eagleson 1994, p.&nbsp;91]]</ref> It shows fewer tendencies to anionic behaviour than ordinary nonmetals.<ref name=Cox/> Its solution chemistry is characterised by the formation of oxyanions.<ref name=Hiller225/> Arsenic generally forms compounds in which it has an oxidation state of +3 or +5.<ref name="Massey267">[[#Massey2000|Massey 2000, p.&nbsp;267]]</ref> The halides, and the oxides and their derivatives are illustrative examples.<ref name="Bailar513">[[#Bailar1965|Bailar, Moeller & Kleinberg 1965, p.&nbsp;513]]</ref> In the trivalent state, arsenic shows some incipient metallic properties.<ref>[[#Timm1944|Timm 1944, p.&nbsp;454]]</ref> The halides are [[hydrolysed]] by water but these reactions, particularly those of the chloride, are reversible with the addition of a [[hydrohalic acid]].<ref>[[#Partington1944|Partington 1944, p.&nbsp;641]]; [[#Kleinberg1960|Kleinberg, Argersinger & Griswold 1960, p.&nbsp;419]]</ref> The oxide is acidic but, as noted below, (weakly) amphoteric. The higher, less stable, pentavalent state has strongly acidic (nonmetallic) properties.<ref>[[#Morgan1906|Morgan 1906, p.&nbsp;163]]; [[#Moeller1954|Moeller 1954, p.&nbsp;559]]</ref> Compared to phosphorus, the stronger metallic character of arsenic is indicated by the formation of oxoacid salts such as AsPO₄, As₂(SO₄)₃{{refn|1=The formulae of AsPO₄ and As₂(SO₄)₃ suggest straightforward ionic formulations, with As³⁺, but this is not the case. AsPO₄, "which is virtually a covalent oxide", has been referred to as a double oxide, of the form As₂O₃·P₂O₅. It consists of AsO₃ pyramids and PO₄ tetrahedra, joined together by all their corner atoms to form a continuous polymeric network.<ref>[[#Corbridge|Corbridge 2013, pp.&nbsp;122, 215]]</ref> As₂(SO₄)₃ has a structure in which each SO₄ tetrahedron is bridged by two AsO₃ trigonal pyramida.<ref>[[#Douglade|Douglade 1982]]</ref>|group=n}} and arsenic acetate As(CH₃COO)₃.<ref>[[#Zingaro1994|Zingaro 1994, p.&nbsp;197]]; [[#Emeléus1959|Emeléus & Sharpe 1959, p.&nbsp;418]]; [[#Addison1972|Addison & Sowerby 1972, p.&nbsp;209]]; [[#Mellor1964|Mellor 1964, p.&nbsp;337]]</ref> The oxide As₂O₃ is polymeric,<ref name=Pudd59/> amphoteric,<ref>[[#Pourbaix1974|Pourbaix 1974, p.&nbsp;521]]; [[#Eagleson1994|Eagleson 1994, p.&nbsp;92]]; [[#Greenwood2002|Greenwood & Earnshaw 2002, p.&nbsp;572]]</ref>{{refn|1=As₂O₃ is usually regarded as being amphoteric but a few sources say it is (weakly)<ref>[[#Wiberg2001|Wiberg 2001, pp.&nbsp;750, 975]]; [[#Silberberg2006|Silberberg 2006, p.&nbsp;314]]</ref> acidic. They describe its "basic" properties (its reaction with concentrated [[hydrochloric acid]] to form arsenic trichloride) as being alcoholic, in analogy with the formation of covalent alkyl chlorides by covalent alcohols (e.g., R-OH + HCl <big>→</big> RCl + H₂O)<ref>[[#Sidgwick1950|Sidgwick 1950, p.&nbsp;784]]; [[#Moody1991|Moody 1991, pp.&nbsp;248–9, 319]]</ref>|group=n}} and a glass former.<ref name=Rao22/> Arsenic has an extensive organometallic chemistry (see [[Organoarsenic chemistry]]).<ref>[[#Krannich2006|Krannich & Watkins 2006]]</ref>