33 As 74.9216

As - Arsenic

See metal norms for Arsenic

Chemical Element Arsenic Melting Point °C Arsenic does not melt, it sublimes at 616°C
Chemical Symbol As Boiling Point °C Arsenic does not boil as it has no liquid form
Atomic Number 33 Density g/cm3 5.8 or 2.0
Atomic Weight 74.9216 Oxide As2O3 and As2O5

Properties

Arsenic is a metalloid element and a member of Group 15 of the Periodic Table. It has 2 forms: Grey/∝-arsenic, which is metallic with a density of 5.8 g/cm3; or β-arsenic, which is a non metal with density 2.0 g/cm3. The metallic form is brittle, tarnishes, and burns in oxygen, while the non metal form is less reactive, but will dissolve when heated with strong oxidising acids and alkalis.

Arsenic is the 53rd most abundant element in the Earth’s crust, but only a small amount of arsenic occurs in its free form in nature. Arsenic is interesting, as it does not have a liquid form at pressures witnessed on Earth, and therefore does not melt or boil, but simply goes straight from a solid to a gas at the temperature of 616°C. This process is called sublimation. Arsenic is highly poisonous, and as little as 200mg is enough to kill a fully-grown adult. Known to many as the ‘Poison of Kings’ and the ‘King of Poisons’, it is no surprise that arsenic’s most notable feature is its toxicity to all living organisms, and its subsequent popular use as a poison.

History

Contact with arsenic goes back more than 5,000 years: on examination, an iceman, found preserved in a glacier in the Italian Alps, was found to have high levels of arsenic in his hair. His exposure to so much arsenic is thought to indicate that he may have been a copper smith by trade, as the ores from which copper is obtained, often contain arsenic also. Both Ancient Egyptian and Chinese cultures were aware of arsenic and its toxic properties, with the Chinese also using it as a pesticide in rice fields. Despite this history, the actual discovery of arsenic is credited to Albertus Magnus (1193-1280). Albertus Magnus, or Albert the Great, first documented arsenic in 1250 after he successfully isolated it by heating soap with arsenic trisulfide.

Sources

The major producing nations of arsenic trioxide are China, with an annual production of 25,000 mts, Chile, the second largest producer at 11,500mts, Morocco (7,000mts), Peru (4,000mts) and other nations such as Belgium, Kazakhstan, Russia and Mexico each producing about 1,000mts in 2009.  World reserves of arsenic are thought to be in the region of 20 times the annual world production, which is approximately 53,500mts (2009).  Recycling of arsenic, particularly in e-waste, is important, in order to reduce toxicity in the environment, which in many parts of the world is a significant problem, such as in Bangladesh.  It is found in many electronic items, such as circuit boards and the scrap produced by semiconductor manufacturing.

Uses

The USA is the world’s leading consumer of arsenic in the form of CCA (chromated copper arsenate) used in wood preservatives.  Arsenic was once mainly used in the strengthening of the grids in lead-acid storage batteries and small-arms ammunition, however these applications are now somewhat outdated.  Today, as a compound with gallium (Ga) and indium (In), it is used in the semiconductors found in solar cells, as well as in other electronic equipment such as mobile phones, and in various forms of herbicides, pesticides and insecticides.

As previously mentioned, arsenic is extremely toxic, which has become a major environmental concern.  There have been cases where arsenic contamination can still be found in soil after decades, and it is has been responsible for polluting water systems causing carcinogens to be consumed by people.  Molycorp, among other companies, has pioneered a way of filtering drinking water, which can greatly reduce the potential risk to communities living in regions affected by arsenic contamination, such as those living by the Ganges and Mekong Rivers in South East Asia.

The future outlook for an element like arsenic looks promising in the technology boom of the 21st century.  It has great potential, already utilised today, in semiconductors for electrical equipment and solar cells, and this market is only likely to grow.

References
  • Emsley, John. Nature’s Building Blocks, An A-Z Guide to the Elements, New Edition, Oxford University Press, 2011
  • Gray, Theodore. The Elements, A Visual Exploration of Every Known Atom in the Universe, Black Dog & Leventhal Publishers, Inc, NY, 2009
  • Stwertka, Albert. A Guide to the Elements, 3rd Edition, Oxford University Press, 2012

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