As · Element 33 · Metalloid
The classic poison of history — tasteless, undetectable for centuries, and still a groundwater crisis for millions.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where arsenic sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All arsenic atoms have the same number of protons, 33, and differ only in their number of neutrons: those variants are its isotopes. The most common, As-75, has 42 neutrons and makes up 100% of natural arsenic.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| As-75 most common | 42 | 100% | Stable | — |
Here the classroom shortcut, rounded atomic mass (74.922) minus atomic number, gives the same answer: 42.
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 119 pm — that is 0.119 nm, so about 4.2 million of them side by side would span a millimetre.
Thermal range
At ordinary pressure arsenic has no liquid phase at all. It sublimes at 887 K, passing straight from solid to vapour. The 1090 K melting point is real, but only above about 3.63 MPa.
The story
Arsenic (As) is element 33, a metalloid in group 15, below phosphorus. Its most stable form, gray arsenic, is a brittle, metallic-looking solid with a density of 5.727 g/cm³. At normal pressure it does not melt but turns directly into vapor (sublimes) at 614 °C (887 K); it melts at about 817 °C (1,090 K) only under a pressure of about 36 atmospheres.
Arsenic is best known as a poison. Inorganic arsenic compounds are toxic and cause cancer, and arsenic that dissolves naturally into groundwater affects the drinking water of millions of people.
Arsenic has 33 electrons in four shells, arranged 2, 8, 18, 5, with the configuration [Ar] 3d¹⁰ 4s² 4p³. It forms compounds in the −3, +3 and +5 oxidation states, such as arsine (AsH₃), arsenic trioxide and the arsenates.
Natural arsenic is a single stable isotope, arsenic-75.
The arsenic sulfides orpiment and realgar were used as pigments and medicines in ancient Egypt, Greece, China and India. The name comes from the Greek arsenikon, the word for yellow orpiment, which probably derives from a Persian word for gold-colored.
The German scholar Albertus Magnus is credited with first isolating the element, around 1250, by heating orpiment with soap. In 1649 Johann Schröder published two methods for preparing it.
Arsenic trioxide is tasteless and looks like sugar or flour, which made it a notorious poison from the Middle Ages to the 19th century. In 1836 the British chemist James Marsh developed a sensitive test for arsenic in body tissues, and reliable detection helped end its use for murder.
Antoine Lavoisier included arsenic in his 1789 list of chemical elements.
Applications
Very pure arsenic is combined with gallium to make gallium arsenide (GaAs), a semiconductor used in LEDs, laser diodes, radio-frequency chips in mobile phones and high-efficiency solar cells on satellites. Arsenic is also used in small amounts as a dopant in silicon chips.
Arsenic trioxide is a standard treatment for acute promyelocytic leukemia, approved in the United States in 2000. Earlier, Salvarsan, an arsenic compound introduced by Paul Ehrlich in 1910, was the first effective treatment for syphilis.
Arsenic was once widely used in pesticides and herbicides, in pigments such as Scheele's green and Paris green, and in glassmaking. Most of these uses have ended because of its toxicity.
Where it comes from
1.8 mg/kg of Earth's crust · more abundant than 56% of elements
Arsenic makes up about 1.8 parts per million of Earth's crust. It is found in hundreds of minerals and occasionally as the native element.
Commercial arsenic trioxide is recovered from the dust and residues of copper, gold and lead smelting; Peru, China and Morocco, in that order, are the largest producers (USGS). Arsenic also dissolves naturally from rocks into groundwater. According to the WHO, an estimated 140 million people in at least 70 countries have been drinking water with arsenic above the WHO provisional guideline of 10 micrograms per liter, including large populations in Bangladesh and India.
Handling
Inorganic arsenic compounds are highly toxic. A large single dose causes vomiting, abdominal pain and diarrhea and can be fatal. Long-term exposure, mainly through drinking water, causes skin lesions and cancers of the skin, lungs and bladder, and is linked to heart disease and diabetes. The International Agency for Research on Cancer classifies arsenic as carcinogenic to humans.
Arsine (AsH₃), a gas used in the semiconductor industry, is one of the most toxic arsenic compounds: it destroys red blood cells. In the United States the OSHA limit for inorganic arsenic in workplace air is 10 micrograms per cubic meter.
Quick answers
Arsenic (symbol As) is element 33 on the periodic table, a metalloid in period 4, group 15. The classic poison of history — tasteless, undetectable for centuries, and still a groundwater crisis for millions. At room temperature it is a solid.
[Ar] 3d¹⁰ 4s² 4p³, giving 4 occupied shells with the distribution 2, 8, 18, 5. Its outer shell holds 5 electrons, which is what sets its bonding behaviour.At ordinary pressure arsenic does not melt at all — it sublimes, passing straight from solid to vapour at about 887 K (613.9 °C). A liquid phase only exists above roughly 3.63 MPa, where it melts near 1090 K. That is why a single "melting point" for arsenic is misleading without a stated pressure.
The standard atomic weight of arsenic is 74.922 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common arsenic atom, As-75, has 42 neutrons: its mass number, 75, minus its atomic number, 33. It makes up 100% of natural arsenic. The classroom shortcut, rounded atomic mass (74.922) minus atomic number, gives the same 42.
Arsenic has a density of 5.727 g/cm³. Water is 1.0 g/cm³, so a block of arsenic is about 5.7× heavier.
Arsenic has a Pauling electronegativity of 2.18. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Arsenic has been known since antiquity, long before anyone recorded its discovery, so no single person can be credited with it. Origin of the name: Greek arsenikon, yellow orpiment.
Arsenic makes up about 1.8 mg/kg of the Earth's crust — uncommon, but not rare.