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Arsenic

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                33             germanium ← arsenic → selenium
                 P
                ↑
                As
                ↓
                Sb

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                      Name, Symbol, Number arsenic, As, 33
                                                Chemical series metalloids
                                             Group, Period, Block 15, 4, p
                                                  Appearance metallic gray
                                            Atomic mass 74.92160 (2) g/mol
                               Electron configuration [Ar] 3d^10 4s^2 4p^3
                                           Electrons per shell 2, 8, 18, 5
                                                       Physical properties
                                                               Phase solid
                                      Density (near r.t.) 5.727 g·cm^−3
                                    Liquid density at m.p. 5.22 g·cm^−3
                                                     Melting point 1090  K
                                                     (817 ° C, 1503 ° F)
                                                 Boiling point subl. 887 K
                                                     (614 ° C, 1137 ° F)
                                               Critical temperature 1673 K
                                  Heat of fusion (gray) 24.44 kJ·mol^−1
                                Heat of vaporization  ? 34.76 kJ·mol^−1
                          Heat capacity (25 °C) 24.64 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                          P/Pa   1  10  100 1 k 10 k 100 k
                                         at T/K 553 596 646 706 781   874

                                                         Atomic properties
                                            Crystal structure rhombohedral
                                                   Oxidation states ±3, 5
                                                     (mildly acidic oxide)
                                    Electronegativity 2.18 (Pauling scale)
                                                       Ionization energies
                                           ( more) 1st: 947.0 kJ·mol^−1
                                                    2nd: 1798 kJ·mol^−1
                                                    3rd: 2735 kJ·mol^−1
                                                      Atomic radius 115 pm
                                              Atomic radius (calc.) 114 pm
                                                    Covalent radius 119 pm
                                               Van der Waals radius 185 pm
                                                             Miscellaneous
                                                 Magnetic ordering no data
                                Electrical resistivity (20 °C) 333 nΩ·m
                       Thermal conductivity (300 K) 50.2 W·m^−1·K^−1
                                                     Young's modulus 8 GPa
                                                       Bulk modulus 22 GPa
                                                         Mohs hardness 3.5
                                                 Brinell hardness 1440 MPa
                                             CAS registry number 7440-38-2
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of arsenic

                            iso   NA  half-life DM     DE ( MeV)      DP
                           ^73As syn  80.3 d    ε   -                ^73Ge
                                                γ   0.05 D, 0.01D, e -
                           ^74As syn  17.78 d   ε   -                ^74Ge
                                                β^+ 0.941            ^74Ge
                                                γ   0.595, 0.634     -
                                                β^- 1.35, 0.717      ^74Se
                           ^75As 100% As is stable with 42 neutrons

                                                                References

   Arsenic ( IPA: /ˈɑːsənɪk/, /ˈɑɹsənɪk/) is a chemical element in the
   periodic table that has the symbol As and atomic number 33. This is a
   notoriously poisonous metalloid that has many allotropic forms; yellow,
   black and gray are a few that are regularly seen. Arsenic and its
   compounds are used as pesticides, herbicides, insecticides and various
   alloys.

Notable characteristics

   Arsenic is very similar chemically to its predecessor phosphorus, so
   much so that it will partly substitute for phosphorus in biochemical
   reactions and is thus poisonous. When heated rapidly it oxidizes to
   arsenic trioxide; the fumes from this reaction have an odour resembling
   garlic. Arsenic and some arsenic compounds can also sublimate upon
   heating, converting directly to a gaseous form. Elemental arsenic is
   found in two solid forms: yellow and gray/metallic, with specific
   gravities of 1.97 and 5.73, respectively.

Applications

   Lead hydrogen arsenate has been used, well into the 20th century, as an
   insecticide on fruit trees (sometimes resulting in brain damage to
   those working the sprayers), and Scheele's Green has even been recorded
   in the 19th century as a coloring agent in sweets. In the last half
   century, monosodium methyl arsenate (MSMA), a less toxic organic form
   of arsenic, has replaced lead arsenate's role in agriculture.

   The application of most concern to the general public is probably that
   of wood which has been treated with chromated copper arsenate ("CCA",
   or " Tanalith", and the vast majority of older " pressure treated"
   wood). CCA timber is still in widespread use in many countries, and was
   heavily used during the latter half of the 20th century as a
   structural, and outdoor building material, where there was a risk of
   rot, or insect infestation in untreated timber. Although widespread
   bans followed the publication of studies which showed low-level
   leaching from in-situ timbers (such as children's playground equipment)
   into surrounding soil, the most serious risk is presented by the
   burning of CCA timber. Recent years have seen fatal animal poisonings,
   and serious human poisonings resulting from the ingestion - directly or
   indirectly - of wood ash from CCA timber (the lethal human dose is
   approximately 20 grams of ash). Scrap CCA construction timber continues
   to be widely burnt through ignorance, in both commercial, and domestic
   fires. Safe disposal of CCA timber remains patchy, and little
   practiced, there is concern in some quarters about the widespread
   landfill disposal of such timber.

   During the 18th, 19th, and 20th centuries, a number of arsenic
   compounds have been used as medicines, including arsphenamine (by Paul
   Ehrlich) and arsenic trioxide (by Thomas Fowler). Arsphenamine as well
   as Neosalvarsan was indicated for syphilis and trypanosomiasis, but has
   been superseded by modern antibiotics. Arsenic trioxide has been used
   in a variety of ways over the past 200 years, but most commonly in the
   treatment of cancer. The Food and Drug Administration in 2000 approved
   this compound for the treatment of patients with acute promyelocytic
   leukemia that is resistant to ATRA. It was also used as Fowler's
   solution in psoriasis.

   Copper acetoarsenite was used as a green pigment known under many
   different names, including Paris Green and Emerald Green. It caused
   numerous arsenic poisonings.

   Other uses;
     * Various agricultural insecticides and poisons.
     * Gallium arsenide is an important semiconductor material, used in
       integrated circuits. Circuits made using the compound are much
       faster (but also much more expensive) than those made in silicon.
       Unlike silicon it is direct bandgap, and so can be used in laser
       diodes and LEDs to directly convert electricity into light.
     * Also used in bronzing and pyrotechny.

History

   The word arsenic is borrowed from the Persian word زرنيخ Zarnikh
   meaning "yellow orpiment". Zarnikh was borrowed by Greek as arsenikon.
   Arsenic has been known and used in Persia and elsewhere since ancient
   times. As the symptoms of arsenic poisoning were somewhat ill-defined,
   it was frequently used for murder until the advent of the Marsh test, a
   sensitive chemical test for its presence. (Another less sensitive but
   more general test is the Reinsch test.) Due to its use by the ruling
   class to murder one another and its incredible potency and
   discreetness, arsenic has been called the Poison of Kings and the King
   of Poisons.

   During the Bronze Age, arsenic was often included in the bronze (mostly
   as an impurity), which made the alloy harder.

   Albertus Magnus(Albert the Great, 1193-1280) is believed to have been
   the first to isolate the element in 1250. In 1649 Johann Schroeder
   published two ways of preparing arsenic.

   The alchemical symbol for arsenic is shown below.
   Alchemical symbol for arsenic

   In the Victorian era, arsenic was mixed with vinegar and chalk and
   eaten by women to improve the complexion of their faces, making their
   skin more fair to show they did not work in the fields. Arsenic was
   also rubbed into the faces and arms of women to improve their
   complexion.

Arsenic in drinking water

   Arsenic contamination of groundwater has led to a massive epidemic of
   arsenic poisoning in Bangladesh and neighbouring countries. It is
   estimated that approximately 57 million people are drinking groundwater
   with arsenic concentrations elevated above the World Health
   Organization's standard of 10 parts per billion. The arsenic in the
   groundwater is of natural origin, and is released from the sediment
   into the groundwater due to the anoxic conditions of the subsurface.
   This groundwater began to be used after western NGOs instigated a
   massive tube well drinking-water program in the late twentieth century.
   This program was designed to prevent drinking of
   bacterially-contaminated surface waters, but unfortunately failed to
   test for arsenic in the groundwater.(2) Many other countries in South
   East Asia, such as Vietnam, Cambodia, and Tibet, are thought to have
   geological environments similarly conducive to generation of
   high-arsenic groundwaters.

   The northern United States, including parts of Michigan, Wisconsin,
   Minnesota and the Dakotas are known to have significant concentrations
   of arsenic in ground water.

   Arsenic can be removed from drinking water through co-precipitation of
   iron minerals by oxidation and filtering. When this treatment fails to
   produce acceptable results, adsorptive arsenic removal media may be
   utilized. Several adsorptive media systems have been approved for point
   of service use in a study funded by the United States Environmental
   Protection Agency (U.S.EPA) and the National Science Foundation (NSF).

   Magnetic separations of arsenic at very low magnetic field gradients
   have been demonstrated in point-of-use water purification with
   high–surface area and monodisperse magnetite (Fe3O4) nanocrystals.
   Using the high specific surface area of Fe3O4 nanocrystals the mass of
   waste associated with arsenic removal from water has been dramatically
   reduced.

Occurrence

   Massive native arsenic
   Enlarge
   Massive native arsenic

   Arsenopyrite also called mispickel (FeSAs) is the most common mineral
   from which, on heating, the arsenic sublimes leaving ferrous sulfide.
   Other arsenic minerals include realgar, mimetite, cobaltite and
   erythrite.

   The most important compounds of arsenic are white arsenic, orpiment,
   realgar, Paris Green, calcium arsenate, and lead hydrogen arsenate.
   Paris Green, calcium arsenate, and lead arsenate have been used as
   agricultural insecticides and poisons. Orpiment and realgar were
   formerly used as painting pigments, though they have somewhat fallen
   out of use due to their toxicity and reactivity. It is sometimes found
   native, but usually combined with silver, cobalt, nickel, iron,
   antimony, or sulfur.

   In addition to the inorganic forms mentioned above, arsenic also occurs
   in various organic forms in the environment. Inorganic arsenic and its
   compounds, upon entering the food chain, are progressively metabolised
   to a less toxic form of arsenic through a process of methylation.

Precautions

   Arsenic and many of its compounds are especially potent poisons.
   Arsenic disrupts ATP production through several mechanisms including
   allosteric inhibition of the metabolic enzyme lipothiamide
   pyrophosphatase during glycolysis. At the level of the citric acid
   cycle, arsenic inhibits succinate dehydrogenase and by competing with
   phosphate it uncouples oxidative phosphorylation, thus inhibiting
   energy-linked reduction of NAD+, mitochondrial respiration, and ATP
   synthesis. Hydrogen peroxide production is also increased, which might
   form reactive oxygen species and oxidative stress. Arsenic kills by
   enzyme inhibtion because enzymes are the best documented targets of
   metals; in this case, it causes toxicity but can also play a protective
   role. These metabolic interferences lead to death from multi-system
   organ failure (see arsenic poisoning) probably from necrotic cell
   death, not apoptosis. A post mortem reveals brick red colored mucosa,
   due to severe haemorrhage.

   Elemental arsenic and arsenic compounds are classified as " toxic" and
   "dangerous for the environment" in the European Union under directive
   67/548/EEC.

   The IARC recognizes arsenic and arsenic compounds as group 1
   carcinogens, and the EU lists arsenic trioxide, arsenic pentoxide and
   arsenate salts as category 1 carcinogens.

   Arsenic is known to cause arsenicosis due to its manifestation in
   drinking water, “the most common species being arsenate [HAsO[4]^2- ;
   As(V)] and arsenite [H[3]AsO[3] ; As(III)]”. The ability of arsenic to
   oxidized between As(III) and As(V) makes its availability in the
   environment possible. According to Croal, Gralnick, Malasarn, and
   Newman, “[the] understanding [of] what stimulates As(III) oxidation
   and/or limits As(V) reduction is relevant for bioremediation of
   contaminated sites (Croal). The study of chemolithoautotrophic As(III)
   oxidizers and the heterotrophic As(V) reducers can help the
   understanding of the oxidation and/or reduction of arsenic.

Compounds

     * Arsenic acid (H[3]AsO[4])
     * Arsenous acid (H[3]AsO[3])
     * Arsenic trioxide (As[2]O[3])
     * Arsine (Arsenic Trihydride AsH[3])
     * Cadmium arsenide (Cd[3]As[2])
     * Gallium arsenide (GaAs)
     * Lead hydrogen arsenate (PbHAsO[4])

Isotopes

   Arsenic has been proposed as a " salting" material for nuclear weapons
   (cobalt is another, better-known salting material). A jacket of As-75,
   irradiated by the intense high-energy neutron flux from an exploding
   thermonuclear weapon, would transmute into the radioactive isotope
   As-76 with a half-life of 1.0778 days and produce approximately 1.13
   MeV of gamma radiation, significantly increasing the radioactivity of
   the weapon's fallout for several hours. Such a weapon is not known to
   have ever been built, tested, or used.
   Retrieved from " http://en.wikipedia.org/wiki/Arsenic"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
