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arsenic

Semimetallic element (a metalloid), atomic number 33, relative atomic mass 74.92. Like phosphorus, the element can appear in different modifications, of which the grey, ‘metallic’ arsenic is the most stable. Arsenic occurs in many ores and occasionally in its elemental state, and is widely distributed, being present in minute quantities in the soil, sea, and the human body. In larger quantities, it is poisonous. The chief source of arsenic compounds is as a by-product from metallurgical processes. It is used in making semiconductors, alloys, and solders.

Arsenic poisoning

As it is a cumulative poison, its presence in food and drugs is very dangerous. The symptoms of arsenic poisoning are vomiting, diarrhoea, tingling and possibly numbness in the limbs, and collapse. It featured in some drugs, including Salvarsan, the first specific treatment for syphilis. Its name derives from the Latin arsenicum. The maximum safe level for arsenic in drinking water, as recommended by the World Health Organization (WHO), is 10 micrograms per litre. Drinking water contaminated with arsenic remains a major concern in several countries, such as India and Bangladesh.

The acceptable US levels of arsenic in drinking water used to be 50 parts per billion, higher than any other developed nation. In January 2006 the US Environmental Protection Agency introduced a new standard of 10 parts per billion.

Metallic arsenic The metallic arsenic of commerce is chiefly prepared by strongly heating arsenopyrite in earthenware retorts from which the air is excluded, when most of the arsenic sublimes, together with a little sulphur. The crude metal is sometimes used as a poison for flies, and as a constituent of lead shot, a small quantity of arsenic hardening the lead and facilitating the formation of the spherical globules. The pure metal may be obtained from the white oxide of arsenic by heating it with carbon in the absence of air, when the arsenic sublimes as a dark grey powder. Metallic arsenic has a relative density of about 5.7 g/cm3 and a specific heat of 0.33 J/gK. It crystallizes in rhombohedra, and vaporizes at 616°C/1,140°F without melting.

Chief compounds of arsenic The chief compounds of arsenic are the hydride AsH3, the arsenious oxide As4O6, arsenic oxide As4O10, the disulphide As2S2, the trisulphide As2S3, the trichloride tribromide, tri-iodide, and trifluoride (AsCl3, AsBr3, and so on).

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metalloid