Arsenic (As)

Science / Periodic Table of Elements / Arsenic (As): Atomic number: 33, Atomic mass: 74.9216 g.mol -1, Electronegativity: 2.0, Density: 5.7 at 14°C, Melting point: 814 °C (36 atm), Boiling point: 615 °C (sublimation), Vanderwaals radius: 0.139 nm, Ionic radius: 0.222 nm (-2) 0,047 nm (+5) 0,058 (+3), Isotopes: 8, Electronic shell: [ Ar ] 3d10 4s2 4p3, Energy of first ionisation: 947 kJ.mol -1, Energy of second ionisation: 1798 kJ.mol -1, Energy of third ionisation: 2736 kJ.mol -1, Standard potential: - 0.3 V (As3+/ As ), Discovered by: The ancients. Arsenic appears in three allotropic forms: yellow, black and grey; the stable form is a silver-gray, brittle crystalline solid. It tarnishes rapidly in air, and at high temperatures burns forming a white cloud of arsenic trioxide. Arsenic is a member of group Va of the periodic table, which combines readily with many elements. The metallic form is brittle, tharnishes and when heated it rapidly oxidizes to arsenic trioxide, which has a garlic odor. The non metallic form is less reactive but will dissolve when heated with strong oxidizing acids and alkalis. Applications: Arsenic compounds are used in making special types of glass, as a wood preservative and, lately, in the semiconductor gallium arsenade, which has the ability to convert electric current to laser light. Arsine gas AsH3, has become an important dopant gas in the microchip industry, although it requires strict guidelines regarding its use because it is extremely toxic. During the 18th, 19th, and 20th centuries, a number of arsenic compounds have been used as medicines; copper acetoarsenite was used as a green pigment known under many different names. Arsenic in the environment: Arsenic can be found naturally on earth in small concentrations. It occurs in soil and minerals and it may enter air, water and land through wind-blown dust and water run-off. Arsenic in the atmosphere comes from various sources: vulcanoes release about 3000 tonnes per year and microorganisms release volatile methylarsines to the extent of 20.000 tonnes per year, but human activity is responsible for much more: 80.000 tonnes of arsenic per year are released by the burning of fossil fuels. Despite its notoriety as a deadly poison, arsenic is an essential trace element for some animals, and maybe even for humans, although the necessary intake may be as low as 0.01 mg/day. Arsenic is a component that is extremely hard to convert to water-soluble or volatile products. The fact that arsenic is naturally a fairly a mobile component, basically means that large concentrations are not likely to appear on one specific site. This is a good thing, but the negative site to it is that arsenic pollution becomes a wider issue because it easily spreads. Arsenic cannot be mobilized easily when it is immobile. Due to human activities, mainly through mining and melting, naturally immobile arsenics have also mobilized and can now be found on many more places than where they existed naturally. A little uncombined arsenic occurs naturally as microcrystalline masses, found in Siberia, Germany, France, Italy, Romania and in the USA. Most arsenic is found in conjuction with sulfur in minerals such as arsenopyrite (AsFeS), realgar, orpiment and enargite. Non is mined as such because it is produced as a by-product of refining the ores of other metals, such as copper and lead. World production of arsenic, in the form of its oxide, is around 50.000 tonnes per year, far in excess of that required by industry. China is the chief exporting country, followed by Chile and Mexico. World resources of arsenic in copper and lead ores exceed 10 million tonnes.