What Is Arsenic?
QUICK ANSWER
Arsenic is a chemical element with atomic number 33 and the symbol As. It's a metalloid with three main allotropes (gray, yellow, and black). Arsenic is best known historically as a poison but has industrial uses in semiconductors, pesticides, and certain medicines. Arsenic contamination of drinking water is a major global health concern.
Arsenic has one of the worst reputations of any element. From Victorian poisonings to modern groundwater contamination, arsenic has earned its association with toxicity. But the same element that's killed kings and caused mass illness also makes semiconductors work and was used as a medicine for centuries. Today, arsenic exposure from contaminated drinking water is a major global health problem affecting tens of millions of people.
Where is arsenic on the periodic table?
Arsenic has atomic number 33, the symbol As, and sits in group 15 of the periodic table alongside nitrogen, phosphorus, antimony, and bismuth. It's classified as a metalloid because it has properties of both metals and non-metals. Only one stable isotope exists naturally, As-75. The atomic mass is about 74.9. Arsenic has been known since ancient times, with arsenic sulfide minerals like realgar and orpiment used as pigments. Pure arsenic was first isolated in the 13th century, possibly by Albertus Magnus, by heating arsenic sulfide with soap.
What are the properties of arsenic?
Arsenic exists in several allotropes. Gray arsenic is the most stable form, a brittle, semi-metallic solid with metallic luster. Yellow arsenic is unstable and reverts to gray on warming or exposure to light. Black arsenic resembles glass. Arsenic sublimes (goes directly from solid to gas) at 615°C at normal pressure rather than melting first. It's relatively unreactive in dry air but tarnishes in moist air. Density is 5.73 g/cm³. Arsenic forms compounds with most elements, including the highly toxic arsenic trioxide (As₂O₃), historically the form most often used as poison.
Why is arsenic so toxic?
Arsenic disrupts cellular energy production by interfering with key enzymes and replacing phosphorus in ATP molecules, the cell's energy currency. Acute arsenic poisoning causes severe gastrointestinal symptoms and can be fatal within hours. Chronic exposure, often from contaminated drinking water, causes skin lesions, cardiovascular disease, neurological problems, and increased risk of several cancers including skin, lung, and bladder. The EPA limits arsenic in drinking water to 10 parts per billion, but tens of millions of people worldwide drink water with much higher levels, particularly in Bangladesh, India, and parts of South America.
What is arsenic used for legally?
Despite its toxicity, arsenic has important industrial uses. Gallium arsenide (GaAs) is a semiconductor material used in LEDs, solar cells, and high-frequency electronics where it outperforms silicon in some applications. Wood preservatives historically used chromated copper arsenate (CCA) to protect lumber from rot, though this was largely phased out for residential use in 2003. Animal feed once contained arsenic compounds to promote growth in poultry, though most have been banned. Arsenic trioxide is now used as a chemotherapy treatment for certain leukemias.
Arsenic earned its dark reputation, but it's more than just a poison. It's a semiconductor, a former medicine, and the cause of one of the world's worst ongoing public health crises through contaminated groundwater. The element that killed Napoleon may also have killed the cancer in some modern patients.
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