Arsenic

Arsenic is the chemical element with atomic number 33 and the symbol As. It is a metalloid in group 15 of the periodic table, meaning it has properties of both metals and nonmetals. Arsenic is best known as a poison, yet its compounds have also served as medicines, pigments and electronic materials.

History of arsenic

Arsenic has been known since ancient times. The Royal Society of Chemistry reports that the ancient Egyptians knew of it, and that one papyrus mentions it as a way of gilding metals. The Greek philosopher Theophrastus knew of two arsenic sulfide minerals, orpiment and realgar. The name is thought to come from arsenikon, the Greek name for the yellow pigment orpiment.

The Chinese also knew about arsenic. A great work on the natural world, the Pen Ts’ao Kan-Mu, was compiled in China in the 1500s, during the Ming dynasty, and it noted the toxicity of arsenic compounds and their use as pesticides in rice fields. A more dangerous form called white arsenic, the trioxide As2O3, was also long known, and it was a by-product of copper refining. When it was mixed with olive oil and heated it yielded the metal itself.

The discovery of the element is attributed to Albertus Magnus in the 1200s, and the Royal Society of Chemistry dates it to around 1250. Arsenic compounds were in use long before that date.

Properties of arsenic

Arsenic has a standard atomic weight of about 74.922, and NIST lists it as 74.921595 with an uncertainty of 6 in the last digit. An arsenic atom has 33 protons and an electron configuration of [Ar] 4s2 3d10 4p3, meaning the arrangement of argon plus two electrons in the 4s level, ten in the 3d level and three in the 4p level.

In its metallic form arsenic is bright, silver-grey and brittle, with a density of 5.776 grams per cubic centimeter. It exists in several allotropes, which are different structural forms of the same element: yellow, grey and black. Arsenic does not melt in the usual way at normal pressure. Instead it sublimes, which means it passes directly from solid to gas, and PubChem lists the transition at 887 kelvins, about 614 degrees Celsius.

PubChem lists arsenic’s common oxidation states as plus 5, plus 3 and minus 3, where an oxidation state is a number that tracks how many electrons an atom has lost, gained or shared. It also records an electronegativity of 2.18 on the Pauling scale, a first ionization energy of 9.815 electronvolts, an electron affinity of 0.81 electronvolts and an atomic radius of 185 picometers.

Occurrence and production of arsenic

A small amount of arsenic is found in its native state, but it is mainly found in minerals. The most common arsenic-containing mineral is arsenopyrite. Others include realgar, orpiment and enargite.

Most arsenic is produced as a by-product of the refining of copper and lead. It can also be obtained from arsenopyrite by heating, which causes the arsenic to sublime and leaves behind iron(II) sulfide. Arsenic is a natural component of the Earth’s crust and is widely distributed in air, water and land, according to the World Health Organization.

Uses of arsenic

Arsenic is a well-known poison. Its compounds are sometimes used as rat poisons and insecticides, but their use is strictly controlled. Arsenic can also have medicinal applications. In Victorian times a solution of potassium arsenate in water, known as Dr. Fowler’s Solution, was a popular tonic that even the author Charles Dickens used. Today organoarsenic compounds, which combine arsenic with carbon-based groups, are added to poultry feed to prevent disease and improve weight gain.

In electronics, arsenic is used as a doping agent in semiconductors such as gallium arsenide for solid-state devices. Doping means adding a small amount of another element to change how a material conducts electricity. Arsenic is also used in bronzing, in fireworks and in hardening shot. Its compounds can be used to make special glass and to preserve wood.

Isotopes of arsenic

NIST lists a single stable isotope of arsenic, arsenic-75, which makes up all of the natural element. An isotope is a form of an element with a particular number of neutrons, and the 75 is the combined count of protons and neutrons. Elements with one natural isotope are called monoisotopic, so the standard atomic weight of arsenic is the mass of a single arsenic-75 atom.

Health and environmental effects of arsenic

The World Health Organization states that arsenic is highly toxic in its inorganic form. The greatest threat to public health comes from contaminated groundwater. Inorganic arsenic is naturally present at high levels in the groundwater of a number of countries, including Argentina, Bangladesh, Cambodia, Chile, China, India, Mexico, Pakistan, the United States and Viet Nam. Drinking water, crops irrigated with contaminated water and food prepared with it are the main sources of exposure.

The World Health Organization estimates that about 140 million people in at least 70 countries have been drinking water that contains arsenic above its provisional guideline value of 10 micrograms per liter. Long-term exposure to inorganic arsenic through drinking water and food can cause cancer and skin lesions, and it has also been associated with cardiovascular disease and diabetes. In seafood arsenic is mainly present in a less toxic organic form.

The Royal Society of Chemistry adds that some scientists think arsenic may be essential in the diet in very small doses, while at higher doses it is toxic and a suspected carcinogen. The most important action in affected communities, according to the World Health Organization, is preventing further exposure by providing a safe water supply. The information here is general background rather than health advice.

This entry explains a medical term. It is not medical advice. If you are concerned about your health, speak with a qualified health professional.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue