Arsenic – facts and properties
- Symbol
- As
- Atomic number
- 33
- Atomic mass
- 74.922 u
- Period
- 4
- Group
- 15
- Category
- metalloid
- Electron configuration
- [Ar] 3d¹⁰ 4s² 4p³
- Electronegativity
- 2.18
- Atomic radius
- 114 pm
- Valence electrons
- 5
- State at standard conditions
- solid
- Melting point
- 817 °C
- Boiling point
- 614 °C
- Density
- 5.776 g/cm³
- Discovered
- known since antiquity
Arsenic is a grey, brittle and shiny metalloid with atomic number 33. It has been known since antiquity and has a long history as a poison, from murder cases to green wallpaper that made people ill. At the same time it is a raw material for electronics, because gallium arsenide is found in phone chips and solar cells. As a natural contaminant of groundwater it is also a health problem for millions of people in some regions.
Position in the periodic table
Occurrence and extraction
Arsenic occurs in many minerals in the Earth's crust, usually combined with sulfur and metals. The most common is arsenopyrite, an iron arsenic sulfide with the formula FeAsS.
Arsenic compounds have been known since antiquity. According to tradition, the scholar Albertus Magnus prepared the element itself from arsenic sulfide in the thirteenth century. There is no exact year of discovery.
Today arsenic is obtained almost only as a by-product when copper, lead and gold ores are smelted. Roasting the ores produces arsenic trioxide, As₂O₃, a white powder that collects in the flue dust. The metalloid itself is made by reducing the oxide with carbon.
Arsenic also occurs naturally in the environment. It dissolves out of arsenic-bearing rock into groundwater, especially in Bangladesh, West Bengal and parts of South America, where many people have drunk contaminated well water for decades. The World Health Organization recommends no more than ten micrograms per litre of drinking water, and this is also the legal limit in the European Union and the United States. Germany is lowering its limit to four micrograms by 2036.
Properties and reactions
Arsenic is in period 4 and group 15, the nitrogen group below phosphorus. Its electron configuration is [Ar] 3d¹⁰ 4s² 4p³, with 5 valence electrons. Its electronegativity is 2.18. The usual grey form has a density of 5.776 g/cm³ and conducts electricity only moderately. A yellow form made of As₄ molecules also exists, but it turns into the grey form quickly in light.
Under normal pressure arsenic does not melt when heated. It sublimes, passing straight from solid to gas at 614 °C. Arsenic becomes liquid only under high pressure, and the melting point is then 817 °C. In air, arsenic burns with a bluish flame to give arsenic trioxide.
Arsenic is usually in oxidation state plus three or plus five, and minus three in arsenides such as gallium arsenide and in arsine. The trivalent compounds are usually more toxic than the pentavalent ones. Arsenic binds to sulfur groups in proteins and thereby blocks enzymes that cells need for their energy metabolism, which is the cause of its toxicity. Long-term exposure can lead to skin changes and cancer, and inorganic arsenic compounds are classed as carcinogenic to humans.
Arsine, AsH₃, is a colourless, extremely toxic gas formed when arsenic compounds react with freshly produced hydrogen.
Uses in everyday life and technology
The most important technical use is in semiconductors. Gallium arsenide is faster than silicon and gives off light when current flows. It is used in high-frequency components, infrared and laser diodes and solar cells for spacecraft.
Small amounts of arsenic harden lead in battery plates and lead shot. Pesticides and wood preservatives containing arsenic are now largely banned. Copper arsenite, known as Scheele's green, dyed wallpaper and clothes in the nineteenth century, and in damp rooms it could give off toxic arsenic compounds.
Arsenic has also done good. In 1909 Paul Ehrlich and Sahachirō Hata found Salvarsan, one of the first synthetic drugs, which helped against syphilis. Today arsenic trioxide treats a rare form of blood cancer in strictly controlled doses.
Unlike selenium, arsenic is not an essential trace element for humans. People who eat rice or certain seaweeds take in small amounts, because rice plants and some seaweeds accumulate arsenic. Health authorities advise keeping the intake as low as possible: babies and young children should not be given mainly rice products, and the seaweed hijiki is best avoided. Arsenic and its inorganic compounds are very toxic and carcinogenic, so they do not belong in school experiments or in private hands.
Molecules containing this element
- Sodium arsenite NaAsO2
- Potassium dihydrogen arsenate KH2AsO4
- Monosodium methanearsonate CH3AsO3HNa
Quick question
What is the atomic number of arsenic (As)?
FAQ
Is arsenic poisonous?
Yes, most arsenic compounds are poisonous, especially arsenic trioxide, formerly known as white arsenic. Arsenic blocks enzymes in the metabolism. Small amounts over a long time can damage skin, nerves and organs and cause cancer. The pure grey metalloid is less dangerous than its soluble compounds, but all handling requires protection and expertise.
Where does arsenic in drinking water come from?
Mostly from the ground. Arsenic-bearing minerals dissolve in groundwater, especially where the soil contains little oxygen. This happens naturally, without pollution. Water suppliers test against the legal limit, ten micrograms per litre in the EU and the United States, and where needed remove arsenic with filters containing iron compounds.
What is arsenic used for?
Mainly for semiconductors: gallium arsenide is in mobile phone chips, laser diodes and satellite solar cells. Arsenic also hardens lead alloys, and in medicine arsenic trioxide treats certain leukaemias. As a pesticide and wood preservative, arsenic is largely banned.