Science Fair Project Encyclopedia
|Name, Symbol, Number||arsenic, As, 33|
|Group, Period, Block||15 (VA), 4, p|
|Density, Hardness||5727 kg/m3, 3.5|
|Appearance|| metallic grey|
|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)|
|State of matter||solid|
|Melting point||1090 K (817.2 ºC / 1503 °F)|
|Boiling point||887 K (613.8 ºC / 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|
|Electronegativity||2.18 (Pauling scale)|
|Specific heat capacity||330 J/(kg·K)|
|Electrical conductivity||3.45 106/(m·ohm)|
|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|
|Most stable isotopes|
|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, insecticides and various alloys.
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 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.
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 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 - 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.
During the 18th, 19th, and 20th centuries, arsenic compounds have been used as medicines. Among these are Paul Erlich's arsphenamine and Thomas Fowler's arsenic trioxide. Arsphenamine was indicated for syphilis and trypanosomiasis, but has been superceded 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 FDA in 2000 approved this compound for the treatment of patients with acute promyelocytic leukemia that is resistant to ATRA.
- 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 is used in Australia for treating termite infestations in houses.
- Also used in: bronzing , pyrotechny
The word Arsenic is borrowed from the Persian word زرنيخ Zarnik meaning "yellow orpiment". Zarnik 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.)
During the Bronze Age, arsenic was often included in the bronze (mostly as an impurity), which made the alloy harder.
In Victorian times, arsenic was mixed with vinegar and chalk and eaten by women to improve the complexion of their faces.
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.
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, 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.
- Antman, Karen H. (2001). The History of Arsenic Trioxide in Cancer Therapy. Introduction to a supplement to The Oncologist. 6 (Suppl 2), 1-2. PMID 11331433.
- WebElements.com – Arsenic
- EnvironmentalChemistry.com – Arsenic
- origen.net – CCA wood and arsenic: toxicological effects of arsenic
- Contaminant Focus: Arsenic by the EPA.
- Environmental Health Criteria for Arsenic and Arsenic Compounds, 2001 by the IARC .
- A summary of the previous report by the industry lobbying group GreenFacts.
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