Arsenic (As)

Arsenic (symbol As) is the chemical element with atomic number 33: a metalloid, solid at 0 °C, with an atomic mass of 74.922 u. Discovered in 1250 (Albertus Magnus).

Arsenic is a metalloid infamous for its highly toxic compounds, long used as untraceable poisons. Yet it also treated syphilis in the early 20th century and today treats a form of leukemia. Its biggest modern scourge is natural: contaminated well water in several parts of the world.

Key facts

Atomic number: 33
Symbol: As
French name: Arsenic
Atomic mass: 74.922 u
Molar mass: 74.922 g/mol
Family: Metalloids
Position: period 4, group 15, p-block
State at 0 °C: Solid
Melting point: 816.9 °C
Boiling point: 613.9 °C
Density: 5.73 g/cm³
Electronegativity (Pauling): 2.18
First ionization energy: 947 kJ/mol
Atomic radius: 115 pm
Electron configuration: [Ar] 3d¹⁰ 4s² 4p³
Abundance in Earth's crust: 1.8 ppm (54th most abundant element)
Indicative price: 1 $/kg (order of magnitude)
Radioactive: no
Discovery: 1250, Albertus Magnus

Origin of the name

From the Greek arsenikon, the name of yellow orpiment.

What does arsenic look like?

Its most common form is a brittle, metallic-gray solid that tarnishes in air. At ordinary pressure it does not melt: heated to 615 °C, it turns straight into a garlic-scented vapor.

Where is arsenic found?

Arsenic makes up about 2 ppm of Earth's crust. It often accompanies copper, lead, gold and cobalt ores, and dissolves naturally into some groundwater.

Natural sources

Arsenopyrite
Realgar
Orpiment
Löllingite
Enargite

How is arsenic produced?

It is mostly recovered as arsenic trioxide (As₂O₃) from the dust given off when copper and gold ores are roasted. World output is on the order of 30,000 to 50,000 tonnes of oxide a year, led by China and Peru.

Top producers

China
Peru
Morocco
Russia

What is arsenic used for?

Gallium arsenide semiconductors (high-speed electronics, LEDs).
Hardening lead alloys (batteries, ammunition).
Treating acute promyelocytic leukemia (arsenic trioxide).
Wood preservation (now heavily restricted).

In everyday life

Smartphone radio chips
Lead-acid car battery
Old greenish treated timber in gardens
Rice (natural traces)

Role in living things

Arsenic is not recognized as essential for humans. Rice, grown in flooded fields, accumulates traces of it. In Bangladesh, tens of millions of people drank well water contaminated with natural arsenic, which the WHO called the largest mass poisoning in history.

Hazards of arsenic

Inorganic arsenic compounds are highly toxic and carcinogenic: chronic exposure causes skin lesions and cancers. A dose of around 100 to 300 milligrams of the trioxide can kill an adult.

Cosmic origin

Supernovae: Forged in the cores of massive stars, then scattered when they explode.

History of arsenic

Its yellow and red sulfides have been used as pigments since antiquity; its name comes from the Greek arsenikon, meaning orpiment. Its isolation is often credited to Albertus Magnus around 1250. Nicknamed “inheritance powder,” it was the weapon of countless poisoners until the Marsh test (1836) made it detectable in bodies.

Did you know?

Salvarsan, developed by Paul Ehrlich in 1910 and the first synthetic antimicrobial, was an arsenic compound for syphilis.
“Scheele's green,” an arsenic pigment, colored wallpapers and dresses in the 19th century and poisoned many people.
In 2010, NASA announced a bacterium that could swap phosphorus for arsenic in its DNA; the claim was refuted by 2012.

Isotopes of arsenic

A single stable isotope, arsenic-75: all natural arsenic is therefore identical.

arsenic-73: radioactive, half-life 80.3 days
arsenic-74: radioactive, half-life 17.8 days. Used as early as the 1950s in the first trials of positron imaging.
arsenic-75: 100% of natural arsenic, stable

Common compounds

Gallium arsenide (GaAs)

Related elements

Same family (metalloids): Boron (B), Silicon (Si), Germanium (Ge), Antimony (Sb), Tellurium (Te).

Same column (group 15): Nitrogen (N), Phosphorus (P), Antimony (Sb), Bismuth (Bi), Moscovium (Mc).