Atomic number 51

Antimony

Sb
+ phase
− phase

Shells Orbitals 4d 5s 5p

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51 electrons in 5 shells: 2, 8, 18, 18, 5.

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

Category: Metalloid
Period: 5
Group: 15
Block: p
Standard atomic weight: 121.76(1) Da
Electron configuration: [Kr] 4d¹⁰ 5s² 5p³
Phase at 25 °C: solid
Melting point: 630.63 °C
Boiling point: 1635 °C
Density: 6.685 g/cm³
Electronegativity (Pauling): 2.05
Discovered: Recognised in compounds in ancient times; known as a metal by the early 17th century

Neighbours in the table

Sn Te

Antimony’s symbol records an ancient cosmetic. CIAAW, IUPAC’s atomic-weights commission, derives Sb from stibium and the Greek stibi, “mark”. It explains that the substance blackened eyebrows and eyelashes. Jefferson Lab adds that ancient Egyptians made black eye make-up from stibnite. The name is read as anti plus monos, “not alone”, which CIAAW explains by the element turning up in so many compounds.

A lump is silver-white, lustrous, hard and brittle. Jefferson Lab calls it a poor conductor of heat and electricity. OpenStax gives it a layered crystal like arsenic’s, with sheets puckered where graphite’s lie flat.

Five valence electrons in the fifth shell complete antimony’s set of 2, 8, 18, 18 and 5, 51 in all, [Kr] 4d¹⁰ 5s² 5p³. OpenStax finds +3 and +5 its usual oxidation states, bookkeeping charges assuming fully ionic bonds. Below arsenic in group 15, antimony is the more metallic, and OpenStax ties that trend to outer electrons lying farther from the nucleus.

Metalloids, the six elements dividing metals from nonmetals, conduct worse than metals because they hold their electrons more tightly. Their chemistry is intermediate too: they build covalent crystals like nonmetals yet seldom form single-atom negative ions. OpenStax puts that partly down to their intermediate electronegativity, how strongly an atom draws shared bonding electrons; antimony’s is 2.05 on the Pauling scale.

Where it is found

Jefferson Lab’s figures give 0.2 milligrams per kilogram of Earth’s crust and 2.4 × 10⁻⁴ milligrams per litre of seawater. Los Alamos counts it in more than 100 mineral species and finds it occasionally native, but mostly as stibnite, Sb₂S₃. Jefferson Lab adds the oxide ore valentinite, Sb₂O₃. Its two stable isotopes, antimony-121 and antimony-123, make up about 57 and 43 per cent in CIAAW’s figures.

The US Geological Survey estimates world mine output in 2024 at about 100,000 metric tons, with China at 60,000, Tajikistan 17,000 and Russia 13,000. The United States mined no marketable antimony that year, though it holds resources in Alaska, Idaho, Montana and Nevada. Net import reliance stood at 85 per cent.

That dependence showed in 2024. In August, China’s government told exporters of antimony ore, metal and oxides to apply for licences. The survey records the metal price nearly doubling afterwards, from $8.91 a pound in July to $17.50 in November. In December China banned antimony exports to the United States. Earlier, in February, an Idaho mining company had won further conditional Department of Defense funding to restore a domestic source.

Why it matters

Pure antimony is seldom used by itself, because, as the hazards summary in PubChem’s safety record puts it, the metal breaks easily. Los Alamos says it greatly raises the hardness and mechanical strength of lead. Lead storage batteries, solder, bearings, cable sheathing, pewter and type metal all use antimony alloys, as do small-arms ammunition and tracer bullets. In tin, a little antimony or bismuth prevents the change of white tin to grey.

Much of the rest goes out as compounds. Textiles and plastics take antimony oxide to keep them from catching fire. Los Alamos lists oxides, sulfides, sodium antimonate and antimony trichloride in paints, ceramic enamels, glass and pottery. For American use in 2024, the US Geological Survey gives metal products such as antimonial lead and ammunition 40 per cent, flame retardants 39 and nonmetal products 21. Los Alamos, counting all production, puts about half into metal goods.

Antimony is not the only route to fire resistance: the survey names certain organic compounds and hydrated aluminium oxide as substitute flame retardants. In electronics, Los Alamos notes antimony in infrared detectors, diodes and Hall-effect devices. The survey’s average price for the metal rose from $2.67 a pound in 2020 to $9.50 in 2024.

Properties

Properties of antimony 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	51
Standard atomic weight	121.76(1) Da	Da	IUPAC/CIAAW 2024
Melting point	630.63 °C	°C	Los Alamos: 630.63 °C
Boiling point	1635 °C	°C	Los Alamos: 1635 °C
Density	6.685 g/cm³	g/cm³
Electronegativity (Pauling)	2.05
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p³
Phase at 25 °C	solid

Safety

Antimony dust and fumes are a workplace hazard, and Los Alamos notes that the element and many of its compounds are toxic. The main aggregate of company reports to the European Chemicals Agency (ECHA) gives elemental antimony the signal word Danger, though 18.7% of reports find no hazard. Of the reports that name a hazard, 31.8% suspect it of causing cancer and 26.9% call it harmful if inhaled. The reference sheets are the PubChem Laboratory Chemical Safety Summary and the card on the metal from NIOSH, the US National Institute for Occupational Safety and Health.

Questions

Is antimony a metal or a nonmetal?

Neither, strictly: PubChem files it as a metalloid. It looks metallic, yet OpenStax groups it with the semiconductors. With fluorine, chlorine, bromine or iodine it forms trihalides, and an excess of fluorine or chlorine takes it on to SbF₅ and SbCl₅. Heated with sulfur it gives Sb₂S₃ or Sb₂S₅. OpenStax adds that it barely tarnishes in dry air but oxidises readily once warmed.

Is antimony toxic?

Los Alamos calls antimony and many of its compounds toxic. The US Environmental Protection Agency (EPA), quoted in PubChem's safety record, names lung effects such as bronchitis as the main result of breathing it over years. Another excerpt there links long exposure to its dust with pneumoconiosis and a pustular skin rash. Studies of cancer in people are inconclusive, and EPA has not classified antimony as a carcinogen.

Does antimony have radioactive isotopes?

Natural antimony has none: both of its isotopes, antimony-121 and antimony-123, are stable. Radioactive ones are made. IUPAC's note in PubChem's record says antimony-124 and antimony-125, whose half-lives are 60.20 days and 2.7576 years in the Atomic Mass Data Center's table, were sold as tracers for oil pipelines in the 1950s. Stable antimony-123 is also the starting point for iodine-124, which serves radioimmunotherapy and positron emission tomography (PET) scans.

How is antimony recycled?

Most recycled antimony comes back by way of lead. The US Geological Survey says it is recovered as antimonial lead at secondary lead smelters, largely from spent lead-acid batteries, and returns to the battery industry. In 2024 recycling covered about 15 per cent of the antimony the United States used, with imports supplying the rest, and the secondary metal was worth about $73 million.