When heated in air, arsenic oxidizes to arsenic trioxide the fumes from this reaction have an odor resembling garlic. Though stable in dry air, arsenic forms a golden-bronze tarnish upon exposure to humidity which eventually becomes a black surface layer. Chemistry Īrsenic has a similar electronegativity and ionization energies to its lighter congener phosphorus and accordingly readily forms covalent molecules with most of the nonmetals. The most stable of arsenic's isomers is 68mAs with a half-life of 111 seconds. Isotopes that are lighter than the stable 75As tend to decay by β + decay, and those that are heavier tend to decay by β − decay, with some exceptions.Īt least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. The most stable of these is 73As with a half-life of 80.30 days.
As of 2003, at least 33 radioisotopes have also been synthesized, ranging in atomic mass from 60 to 92. Isotopes Īrsenic occurs in nature as one stable isotope, 75As, a monoisotopic element. As arsenic's triple point is at 3.628 MPa (35.81 atm), it does not have a melting point at standard pressure but instead sublimes from solid to vapor at 887 K (615 ☌ or 1137 ☏). īlack arsenic can also be formed by cooling vapor at around 100–220 ☌ and by crystallization of amorphous arsenic in the presence of mercury vapors. Black arsenic is similar in structure to black phosphorus. The yellow form has a density of 1.97 g/cm 3. It is rapidly transformed into gray arsenic by light. Solid yellow arsenic is produced by rapid cooling of arsenic vapor, AsĤ. This unstable allotrope, being molecular, is the most volatile, least dense, and most toxic. Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. Yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus ( PĤ). Gray arsenic is also the most stable form. Gray arsenic is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 eV if amorphized. This relatively close packing leads to a high density of 5.73 g/cm 3. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being slightly closer than the three atoms in the next. Because of weak bonding between the layers, gray arsenic is brittle and has a relatively low Mohs hardness of 3.5. 166) adopts a double-layered structure consisting of many interlocked, ruffled, six-membered rings.
The three most common arsenic allotropes are gray, yellow, and black arsenic, with gray being the most common. 8.5 Mapping of industrial releases in the USĬharacteristics Physical characteristics Ĭrystal structure common to Sb, AsSb and gray As.
8.3 Redox transformation of arsenic in natural waters.8.2.2 Hazard maps for contaminated groundwater.7 Essential trace element in higher animals.Arsenic is classified as a Group-A carcinogen. The United States' Agency for Toxic Substances and Disease Registry ranked arsenic as number 1 in its 2001 Priority List of Hazardous Substances at Superfund sites. The United States' Environmental Protection Agency states that all forms of arsenic are a serious risk to human health. Arsenic contamination of groundwater is a problem that affects millions of people across the world. However, arsenic poisoning occurs in multicellular life if quantities are larger than needed. Trace quantities of arsenic are an essential dietary element in rats, hamsters, goats, chickens, and presumably other species. Ī few species of bacteria are able to use arsenic compounds as respiratory metabolites. These applications are declining with the increasing recognition of the toxicity of arsenic and its compounds. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides, treated wood products, herbicides, and insecticides. It is also a component of the III-V compound semiconductor gallium arsenide. Arsenic is a common n-type dopant in semiconductor electronic devices. The primary use of arsenic is in alloys of lead (for example, in car batteries and ammunition). It has various allotropes, but only the gray form, which has a metallic appearance, is important to industry. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a chemical element with the symbol As and atomic number 33.
0 kommentar(er)
