Gray arsenic

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. 

The structures of antimony and bismuth correspond to that of gray arsenic. With increasing atomic weight the distances between adjacent atoms within a layer and between layers become less different, i.e. the coordination polyhedra deviate less from a regular octahedron. This effect is enhanced under pressure (cf. next section). 

Section of a layer in gray arsenic and the position of two rings of the next layer. Two rings with the relative arrangement as in trans-decalin are emphasized... 

Under pressure black phosphorus transforms first to a modification that corresponds to gray arsenic. At an even higher pressure this is converted to the a-polonium structure. Then follows a hexagonal-primitive structure, which has also been observed for silicon under pressure (p. 122), but that hardly ever occurs otherwise. Above 262 GPa phosphorus is body-centered cubic this modification becomes superconducting below 22 K.. 

Synonyms and trade names arsenic black, arsenicals, arsenic-75, colloidal arsenic, gray arsenic, metallic arsenic... 

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). 

Figure 22. Arsenic in Appalachian Basin coal samples from the USGS Coalqual database. The Appalachian Basin is shown as light gray. Arsenic content is expressed by proportionally sized symbols. Note the occurrence of coal with elevated arsenic content (> 100 ppm).

As a metalloid, arsenic has both metallic and nonmetallic properties. Elemental arsenic exists at room temperature as metallic or gray arsenic and yellow arsenic. Gray arsenic represents the common stable form. Its density is 5.73 gcm , melting point 814 °C at 36.5 bar pressure, and vapor pressure 1 bar at 604°C (sublimating). Metallic arsenic is not soluble in common solvents. Yellow arsenic has a density of... 

Arsenic (atomic number 33, atomic mass 74.9216) is the 20th most abundant element in the earth s crust. It belongs to the elements of the P block of the Periodic System where it is placed below phosphorus and above antimony. The mass numbers of its isotopes range from 68 to 80 however, only the natural isotope 75 is stable. The gamma-emitting radioisotopes As (half-life 26.4 h), As (half-life 17.77 d), and As ( half-life 80.3 d) are commercially available and often used for method development and control (Krivan, 1987 Krivan and Arpadjan, 1989). Elemental arsenic exists at room temperature as metallic or gray arsenic, and yellow arsenic. As a center element of the P block it can be found both in metallic and covalent compounds. The oxidation states are -III, 0, -i-lll, and -I- V. Arsenic trihydride (arsine, AsHa) is a colourless, very poisonous, neutral gas with a characteristic garlic odour. 

Synonyms Arsenicals Arsenic black Arsenic, metallic Colloidal arsenic Gray arsenic Metallic arsenic... 

Gray acetate. See Calcium acetate Gray arsenic. See Arsenic Great burdock extract. See Burdock (Arctium lappa) extract... 

Arsenic is an element with the symbol As and the atomic number 33. It can occur as a pure element but is most often found in minerals containing sulfur and metals. Arsenic can exist in different structural forms (allotropes). However, gray arsenic is the most common. It is a metalloid that is brittle and a bit shiny. See Fig. 5 [25]. This form has metallic properties and has been used in industry to strengthen alloys of copper and lead. Arsenic is also a common n-type dopant in semiconductor electronic devices (example gallium arsenide is a semiconductor). Over the years arsenic and its compounds were used in the production of products like pesticides, insecticides, and treated wood items. However, because of its toxicity and harmful effects to humans, arsenic s applications have decreased. 

The Group VA elements also show the distinct transition from nonmetal (nitrogen, N, and phosphorus, P) to metalloid (arsenic. As, and antimony, Sb) to metal (bismuth, Bi). Nitrogen occurs as a colorless, odorless, relatively umeactive gas with N2 molecules white phosphorus is a white, waxy solid with P4 molecules. Gray arsenic is a brittle solid with metallic luster antimony is a brittle solid with a silvery, metallic luster. Bismuth is a hard, lustrous metal with a pinkish tinge. 

Arsenic is another commonly occurring pollutant and is included here. The element arsenic is often called a metalloid, because its common form, gray arsenic, has a steely gray color and some other properties similar to metals. However, the properties of its compounds are typically those of nonmetallic elanents. 

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