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 Gray (alpha) Arsenic [ As4 up to 800°C, As2 above 1700°C ] |
| The most stable allotrope of arsenic, with arhombohedric structure. It has a layered microstructure, metallic glow, steel
grey colour, it is soft and very brittle. It conducts electricity.
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| At atmospheric pressure it sublimates at 613°C, and at pressure of 36.4 kPa it melts at 817°C. |
| Up to 800°C arsenic vapours are as As4, and above 1700°C it is As2. With humid air it reacts at room temperature, and if heated in air it burns a blue flame, creating thick fumes of As2O3 with a garlicky smell. At room temperature it directly bonds with fluorine and chlorine, and when heated it bonds with bromine, iodine and sulphur.
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| It does not bond with nitrogen, boron, silicon and carbon. Strong oxidative acids, as concentrated nitrogen acid, oxidize it to arsenic acid, dissolved nitrogen acid doesn't oxidize to arsenate acid. It forms alloys with most metals.
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| Yellow (beta) Arsenic [ As6 ] |
| An allotrope of arsenic with an hexagonal structure which forms by abrupt cooling of arsine gases (AsH3). It is soft as wax and doesn't conduct electricity. It is unstable at room temperature, when affected by light it quickly
and easily turns to grey arsenic.
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| Amorphous Black Arsenic [ ] |
| Amorphous black arsenic is produced by sublimation of arsenic without the presence of air. It also forms from the break up
of arsenic-hydrogen with heat where it is excreted as vapour by creating a mirror layer (so called "arsenic mirror"). This
reaction is used to prove small amounts of arsenic in case of suspicion of poisoning (Marsh test).
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| When heated at 360°C it turns to stable grey arsenic. |
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See Also