Atomic number 33

Arsenic

As
+ phase
− phase

Shells Orbitals 3d 4s 4p

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33 electrons in 4 shells: 2, 8, 18, 5.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Metalloid
Period: 4
Group: 15
Block: p
Standard atomic weight: 74.922(1) Da
Electron configuration: [Ar] 3d¹⁰ 4s² 4p³
Phase at 25 °C: solid
Density: 5.776 g/cm³
Electronegativity (Pauling): 2.18
Discovered: About 1250; its sulfides were known in prehistoric times, Albertus Magnus (attributed)

Neighbours in the table

Ge Se

Arsenic warmed at ordinary atmospheric pressure never turns into a liquid. At 614 °C it passes straight from solid to vapour, a change called sublimation. Jefferson Lab’s note in PubChem’s record adds that at 28 times standard atmospheric pressure it melts, at about 817 °C. The two figures were measured at different pressures. OpenStax prints 613 °C for the lower figure.

Written out, arsenic’s 33 electrons read [Ar] 3d¹⁰ 4s² 4p³, with 2, 8, 18 and 5 in the shells. OpenStax counts a completely filled d subshell as core, which leaves five valence electrons, three short of a full shell of eight. So in the uncombined element every atom bonds three times. In the most stable of its solid forms, OpenStax says, each arsenic atom bonds to three neighbours inside puckered sheets, much like graphite’s layers but buckled.

Los Alamos describes a yellow form as well, far less dense than the steel-grey, brittle one. As a metalloid it looks metallic yet conducts as a semiconductor, since a metalloid’s nuclei bind its electrons more firmly than a metal’s do. OpenStax puts the in-between character partly down to electronegativity, the pull an atom exerts on the electrons of a shared bond; arsenic’s is 2.18 on the Pauling scale.

Where it is found

Jefferson Lab puts arsenic at 1.8 milligrams per kilogram of Earth’s crust. Some occurs free, but most sits in sulfides such as orpiment, As₂S₃, realgar, AsS, and arsenopyrite, FeAsS. Arsenopyrite, also called mispickel, is the commonest arsenic mineral, according to Los Alamos and the US Geological Survey. Morocco’s Bou Azzer mine yields a cobalt-arsenide ore.

Heating arsenopyrite drives the arsenic off as vapour and leaves ferrous sulfide behind, and most commercial arsenic is still won that way. More comes from the flue dust of copper, gold and lead smelters. Other refiners meet it as an impurity: it is distilled out of crude zinc because it boils lower, and roasting tin ore carries it away as a volatile oxide.

The trade counts arsenic as its trioxide. The US Geological Survey estimates world output at 58,000 tonnes of that oxide in 2024, and Peru, China and Morocco, in that order, made about 95 per cent of it. The United States has made neither the trioxide nor the primary metal since 1985, so every tonne it uses is imported, chiefly from China and Morocco in 2020–23.

Why it matters

In the United States most arsenic ends up protecting timber or killing pests. The US Geological Survey’s 2024 breakdown gives herbicides, insecticides and wood preservatives 84 per cent, semiconductors 5 and metallurgy 3. Arsenic trioxide becomes arsenic acid, a key ingredient of chromated copper arsenate, or CCA. Lumber pressure-treated with it becomes light poles, marine structures and retaining walls.

Semiconductors take less, and Jefferson Lab records arsenic added to germanium for transistors. An arsenic atom carries five valence electrons where silicon carries four, so one electron is left over when arsenic replaces silicon in a crystal. The valence band, the band of electron energies already full, has no room for it, so it goes into the conduction band above. There, at room temperature, it carries current, and arsenic counts as a donor impurity.

Metal of 99.9999 per cent purity goes into gallium arsenide, GaAs, for solar cells, space research and telecommunications. Los Alamos adds that gallium arsenide works as a laser material, turning electricity directly into coherent light. Revenues from GaAs devices rose in 2024 as fifth-generation networks and consumer devices spread, and the survey finds no effective substitute in defence uses. As a metal additive, arsenic hardens lead shot and wheel weights and stiffens the grids of lead-acid batteries.

Properties

Properties of arsenic Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	33
Standard atomic weight	74.922(1) Da	Da	IUPAC/CIAAW 2024
Melting point	817 °C	°C	PubChem: 1090 K
Boiling point	614 °C	°C	PubChem: 887 K
Density	5.776 g/cm³	g/cm³
Electronegativity (Pauling)	2.18
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p³
Phase at 25 °C	solid

At standard pressure arsenic does not melt: the boiling figure, 887 K, is where it sublimes, and the melting figure, 1090 K, was measured at 28 times standard atmospheric pressure (Jefferson Lab, in PubChem's element record).

Safety

Arsenic and its compounds are poisonous, and the element itself is classed as toxic if swallowed or inhaled. The European Union’s classification on PubChem gives it the signal word Danger and calls it very toxic to aquatic life, with lasting effects. Of the company reports to the European Chemicals Agency (ECHA), 45.8% add that it may cause cancer, and the International Chemical Safety Card calls it carcinogenic to humans. NIOSH, the US workplace-safety institute, treats the metal as a potential occupational carcinogen, and its card and the PubChem Laboratory Chemical Safety Summary below are the reference sheets.

Questions

Is arsenic a metal or a nonmetal?

Neither: it is a metalloid. Arsenic has a metallic look, yet OpenStax classes it with the semiconductors, and in its most stable form its atoms are held in puckered sheets by covalent bonds. Its compounds mostly show the +3 or +5 oxidation state. OpenStax says metalloids generally form no simple negative ions, but it also notes that arsenic forms arsenides at −3. Antimony, the next element down group 15, is the more metallic of the pair.

Why is it called arsenic?

Los Alamos traces the name to the Latin arsenicum and the Greek arsenikon. The note from CIAAW, IUPAC's Commission on Isotopic Abundances and Atomic Weights, in PubChem's element record links it to arsenikos, meaning male. The ancients thought metals came in different sexes. Arsenic sulfides were known in prehistoric times as poisons, and early Chinese, Greek and Egyptian civilisations mined its compounds. Albertus Magnus, a German alchemist, is believed to have first obtained the element itself around 1250.

How many isotopes does arsenic have?

Only one is stable: arsenic-75, which makes up every atom of natural arsenic. Its mass is 74.92159457 u in the figure from NIST, the US National Institute of Standards and Technology. Radioactive isotopes have uses of their own. Arsenic-72 and arsenic-74 emit positrons and can be bound to monoclonal antibodies, which gather in tumours, for positron emission tomography (PET) imaging. Arsenic-73 and arsenic-76 serve as radioactive tracers in environmental and biomedical work.

How are people exposed to arsenic?

For most people, the US Environmental Protection Agency (EPA) says in PubChem's safety record, arsenic arrives through the diet, including its inorganic compounds. In animals and plants the element combines with carbon and hydrogen into organic arsenic compounds. Chromated copper arsenate is no longer used for residential timber in the United States, though industry still uses it. Haz-Map, an occupational-disease database quoted in PubChem's safety record, links chronic exposure to cancers of the lung, liver and skin.