Arsenic
Atomic Data
| Atomic Number | 33 |
| Symbol | As |
| Atomic Weight | 74.922 u |
| Density (STP) | 5.727 g/cm³ |
| Melting Point | 816.85 °C (1090 K) |
| Boiling Point | 613.85 °C (887 K) |
| Electronegativity | 2.18 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3 |
| Oxidation States | −3, +3, +5 |
| Phase at STP | Solid |
| Category | Metalloid |
| Period / Group | 4 / 15 |
| CAS Number | 7440-38-2 |
Electron Configuration
[Ar] 3d10 4s2 4p3
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 3 | 33 |
| Total | 33 | 33 | ||
Isotopes of Arsenic
Arsenic is monoisotopic: ⁷⁵As is its only naturally occurring stable isotope, accounting for 100% of all natural Arsenic.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Arsenic-75 | ⁷⁵As | 33 | 42 | 100 | Stable |
Abundance & Occurrence
Arsenic is present in Earth's crust at approximately 1.8 ppm by mass and at approximately 3 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Acute toxicity: Arsenic trioxide and other inorganic arsenic compounds are acutely toxic; the lethal dose for humans is approximately 1–3 mg/kg body weight, and arsenic poisoning has a long history of criminal misuse.
- Carcinogenicity: Inorganic arsenic is a Group 1 human carcinogen (IARC); chronic exposure via drinking water, occupational dust, or soil causes cancers of the skin, bladder, lung, and kidney.
- Arsine gas (AsH3): Arsine is one of the most acutely toxic gases known; exposure at just 3 ppm causes haemolysis, acute kidney failure, and cardiovascular collapse: it requires continuous gas monitoring and immediate evacuation protocols.
- Skin and mucous membranes: Arsenic dust and solutions cause contact dermatitis, hyperkeratosis, and Mees' lines (white bands on nails) with chronic exposure.
- Environmental persistence: Arsenic is highly persistent in soils and sediments; contaminated sites require careful remediation and ongoing monitoring of groundwater arsenic levels.
Arsenic in the Real World
Real-World Uses
- Semiconductor compounds: Gallium arsenide (GaAs) and indium arsenide (InAs): both arsenic-containing III-V semiconductors: are used in high-speed transistors, laser diodes, and multijunction solar cells for demanding electronic and photonic applications.
- Wood preservatives: Chromated copper arsenate (CCA) was widely used to pressure-treat timber for outdoor structures, though its use on residential lumber has been largely replaced by less toxic alternatives in many countries due to arsenic leaching concerns.
- Glass decolourant: Arsenic trioxide (As₂O₃) is added to high-quality glass melts to remove fine bubbles (as a fining agent) and to reduce the green-yellow tint caused by iron contamination, producing clearer optical glass.
- Cancer treatment: Arsenic trioxide (Trisenox) is an approved drug for treating acute promyelocytic leukaemia (APL), inducing apoptosis and differentiation of malignant promyelocytes.
- Pesticides and herbicides: Lead arsenate and calcium arsenate were historical crop pesticides; organoarsenical herbicides (MSMA) are still used in cotton agriculture in some regions despite environmental restrictions.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is arsenic used for?
Arsenic's main modern use is in compound semiconductors: gallium arsenide (GaAs) is used in solar cells, LEDs, and high-speed transistors. Wood preservatives containing arsenic compounds (CCA: chromated copper arsenate) were once widely used but have been phased out in many countries. Arsenic compounds are also used in some pesticides and herbicides, and historically in pigments and medicines.
Is arsenic poisonous?
Yes, arsenic is a well-known poison. Inorganic arsenic compounds, particularly arsenic trioxide, are highly toxic and were historically used in homicides and as rat poison. Chronic exposure through contaminated drinking water: a serious problem in Bangladesh, India, and other regions: causes arsenicosis, with symptoms including skin lesions, cancers, and neurological damage. Organic arsenic forms in seafood are far less toxic.
How was arsenic discovered?
Arsenic has been known since antiquity in the form of its sulphide minerals orpiment and realgar. The element itself was first described by the alchemist Albertus Magnus around 1250 CE, who heated orpiment with soap to produce metallic arsenic. The name derives from the Greek 'arsenikon', used for orpiment, which in turn may come from a Persian word for yellow.
What is the Arsenic Act of 1851?
The Arsenic Act 1851 was a British law passed in response to a wave of arsenic poisonings, since arsenic powder was widely available as a rat poison and was virtually undetectable in food at the time. The act required arsenic to be sold mixed with soot or indigo to make it coloured and identifiable, and restricted sales to adults known to the seller. It was one of the first drug-regulation laws in history.