An Overview about the Chemical Element Arsenic

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Symbol: As

Atomic Number: 33

Atomic Mass: 74.9216 amu (atomic mass units)

Melting Point @ 28 times standard pressure: 817.0 C (1090.15 K, 1502.6 F)

Sublimation Point: 613.0 C (886.15 K, 1135.4 F)

Number of Protons: 33

Number of Electrons: 33

Number of Neutrons: 42

Classification: Metalloid (Semi-metal)

Group Name: Pnictogen

Crystal Structure: Rhombohedral

Density @ 293 K: 5.72 grams per cubic centimeter

Color: Gray

The element arsenic was known in antiquity. Compounds of arsenic were mined by the Ancient Greeks and the Romans who knew it as "arsenikon" and "arsenicum" respectively. The alchemist Albertus Magnus is thought to have identified arsenic in 1250.

When heated at standard pressure arsenic does not melt but sublimes changing directly from the solid state to the gaseous state. Liquid arsenic can only be formed at a higher than standard pressure.

The element can be found in a free state in nature but it is more commonly found in mineral ores such as arsenopyrite, realgar and orpiment. Commercially most arsenic is extracted from the ore arsenopyrite.

Arsenic has only one naturally occurring isotope, arsenic-75. Arsenic-75 is stable but there have been a number of unstable isotopes made with mass numbers which range from 60 to 92. Arsenic-88, arsenic-90 and arsenic-91 have the shortest half-lives at around 150 nanoseconds. They all decay by beta decay. Arsenic-73 has the longest half life at 80.3 days, it decays by electron capture.

Historically arsenic has had a number of uses. These include: the treatment of syphilis prior to the discovery of antibiotics; being use at a face powder; in paint pigments such as emerald green which was popular with impressionist painters and as a constituent of health tonics or patent medicines. In addition it has been used as a rat poison and a pesticide. These uses could and indeed frequently did lead to accidental cases of arsenic poisoning.

Today arsenic still has some uses.

* It is used as a wood preservative.

* Small quantities of arsenic are added to the element germanium in the manufacture of transistors.

* The compound gallium arsenide can produce laser light directly from electricity.

Arsenic and all of its compounds are poisonous. It acts as an allosteric inhibitor to some metabolic enzymes which can lead to multi-organ failure. The symptoms of acute arsenic poisoning are similar to cholera. Where poisoning takes place over a period of time with small but repeated doses of poison then it can resemble a number of chronic debilitating illnesses. Before scientific methods were available to differentiate these conditions arsenic was a popular poison for disposing of an unwanted relative or rival. It was nicknamed "inheritance powder" because of its use in killing elderly relatives in order to gain access to their estates.

There are still cases of accidental arsenic poisoning today. Some of this can be from occupational exposure with industries such as wood preserving and the manufacturing of semiconductors. Arsenic is often found in the emissions from the coke ovens used in the smelting industry. Cases of acute poisoning by arsenic may be treated using chelating agents such as Dimercaprol or Succimer.

In some parts of the world, such as Bangladesh, there is a high incidence of groundwater contamination with arsenic so people may be poisoned by their drinking water. The World Health Organization recommends a limit of no more than 0.01 milligrams of arsenic per liter of water (10ppb). This limit was set by the limitations of the technology used for measuring arsenic in water samples rather than a specific study measuring its affect on health. It is possible that even amounts as low as 0.00017 milligrams per liter (0.17ppb) may cause symptoms of arsenic poisoning.

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