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germanium arsenic selenium
P
Ar
Sb  
 
 
Image:As-TableImage.png
General
Name, Symbol, Number arsenic, As, 33
Series metalloids
Group, Period, Block 15 (VA), 4, p
Density, Hardness 5727 kg/m3, 3.5
Appearance metallic grey
Atomic properties
Atomic weight 74.92160 amu
Atomic radius (calc.) 115 (114) pm
Covalent radius 119 pm
van der Waals radius 185 pm
Electron configuration [Ar]3d10 4s2 4p3
e- 's per energy level 2, 8, 18, 5
Oxidation states (Oxide) +-3,5 (mildly acidic)
Crystal structure rhombohedral
Physical properties
State of matter solid
Melting point 1090 K (817.2 C / 1503 F)
Boiling point 887 K (613.8 C / 1137 F)
Molar volume 12.95 10-6 m3/mol
Heat of vaporization 34.76 kJ/mol
Heat of fusion 369.9 kJ/mol
Vapor pressure __ Pa at __ K
Speed of sound __ m/s at __ K
Miscellaneous
Electronegativity 2.18 (Pauling scale)
Specific heat capacity 330 J/(kg*K)
Electrical conductivity 3.45 106/(mohm)
Thermal conductivity 50 W/(m*K)
1st ionization potential 947.0 kJ/mol
2nd ionization potential 1798 kJ/mol
3rd ionization potential 2735 kJ/mol
4th ionization potential 4837 kJ/mol
5th ionization potential 6043 kJ/mol
6th ionization potential 12310 kJ/mol
SI units & STP are used except where noted.

Arsenic is a chemical element in the periodic table that has the symbol As and atomic number 33. This is a notorious poisonous metalloid that has three allotropic forms; yellow, black and grey. Arsenic and its compounds are used as pesticides, herbicides and insecticides and various alloys

Notable characteristics

Arsenic is chemically very similar to its predecessor phosphorus, so much so that it will partly substitute for it in biochemical reactions and is thus poisonous. When heated it rapidly oxidizes to arsenous oxide, which has a garlic odor. Arsenic and some arsenic compounds can also sublime upon heating, converting to gaseous form directly. Elemental arsenic is found in two solid forms: yellow and gray/metallic, with specific gravities of 1.97 and 5.73, respectively.

Applications

 

Lead arsenate has been used, well into the 20th century, as a pesticide on fruit trees (resulting in neurological damage to those working the sprayers), and copper arsenate has even been recorded in the 19th century as a coloring agent in sweets.

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 later 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 childrens' 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 - roughly a tablespoon). 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.

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.
  • Arsenic trioxide has been used in hematology to treat patients with acute promyelocytic leukemia that are resistant to ATRA treatment
  • Arsenic trioxide is used in Australia for treating termite infestations in houses.
  • Also used in; bronzing, pyrotechny

History

The word Arsenic is borrowed from the Persian word 21FJ. Zarnik meaning "yellow orpiment". Zarnik was borrowed by Greek as arsenikon. Arsenic has been known and used in Persia and elsewhere since ancient times. It was frequently used for murder, the symptoms of arsenic poisoning being somewhat ill-defined, until the advent of the Marsh test, a sensitive chemical test for its presence.

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

Albertus Magnus is believed to have been the first to isolate the element in 1250. In 1649 Johann Schroeder published two ways of preparing arsenic.

Alchemical symbol for arsenic

The alchemical symbol for arsenic is shown opposite.

There is a massive epidemic of arsenic poisoning in Bangladesh, where it is estimated that approximately 57 million people are drinking groundwater with arsenic concentrations elevated above the World Health Organization's standard of 50 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. 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.

Occurrence

Arsenopyrite also called mispickel (FeSAs) is the most common mineral from which, on heating, the arsenic sublimes leaving ferrous sulfide.

The most important compounds of arsenic are white arsenic, its sulfide, Paris green, calcium arsenate, and lead arsenate. Paris green, calcium arsenate, and lead arsenate have been used as agricultural insecticides and poisons. 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 kills by massively disrupting the digestive system, leading to death from shock. See arsenic poisoning.

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References

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