Atomic number 78

Platinum

Pt
+ phase
− phase

Shells Orbitals 4f 5d 6s

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78 electrons in 6 shells: 2, 8, 18, 32, 17, 1.

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

Category: Transition metal
Period: 6
Group: 10
Block: d
Standard atomic weight: 195.08(2) Da
Electron configuration: [Xe] 4f¹⁴ 5d⁹ 6s¹
Phase at 25 °C: solid
Melting point: 1768.3 °C
Boiling point: 3825 °C
Density: 21.46 g/cm³
Electronegativity (Pauling): 2.28
Discovered: 1735, Antonio de Ulloa

Neighbours in the table

Ir Au

The filling order a first course teaches would give platinum two electrons in 6s; the atom keeps one, with nine in 5d. The result is [Xe] 4f¹⁴ 5d⁹ 6s¹, with the shells holding 2, 8, 18, 32, 17 and 1. OpenStax says such exceptions involve subshells lying very close in energy, and that no simple method predicts which atoms show them.

A transition element forming a positive ion gives up its outer s electron before touching the d electrons, OpenStax explains. Jefferson Lab’s note in PubChem’s record lists +2 and +4 as platinum’s usual oxidation states, a bookkeeping charge that treats every bond as ionic. Los Alamos lists far more, from 6 down to −3. In the complex ion [PtCl₆]²⁻, six chloride ions bonded around one platinum, the metal stands at +4, OpenStax shows. OpenStax gives the reason heavier transition metals favour higher states: their ions are larger, and electrons farther from the nucleus are easier to remove.

Los Alamos describes the pure metal as silvery white, malleable and ductile, with an odd appetite for hydrogen. It absorbs large volumes of the gas, keeps it at ordinary temperatures and releases it when heated. Mercury, which forms an amalgam, or alloy, with most metals, leaves platinum alone; OpenStax names it as one of the main exceptions, beside iron, tungsten and tantalum.

Where it is found

Pre-Columbian peoples of South America used the metal before western scientists noticed it in 1735, Jefferson Lab notes. In PubChem’s record, IUPAC’s atomic-weights commission (CIAAW) credits the Spanish astronomer Antonio de Ulloa, who found it in Peru in 1735. It also credits the English metallurgist Charles Wood, who had it from Colombia in 1741. OpenStax places the find with Spanish explorers in Central America who mistook platinum nuggets for silver. The name comes from the Spanish platina, for silver, and the English physician William Brownrigg purified the metal in 1750.

Platinum occurs native, Los Alamos says, with a little iridium, osmium, palladium, ruthenium and rhodium alongside, in the river gravels of the Urals, Colombia and the American West. Around Sudbury, Ontario, it comes as the mineral sperrylite, PtAs₂, a byproduct of nickel mining. The platinum metals form about one part in two million of such ore.

The US Geological Survey estimates 2024 world mine output at 170,000 kilograms. South Africa supplied 120,000, and its Bushveld Complex holds the largest reserves. The Survey’s world gold output, 3,300 metric tons, is about nineteen times as much.

Why it matters

The US Geological Survey names catalytic converters, which cut harmful car exhaust, as the leading use of the platinum-group metals in the United States. Their first job, Jefferson Lab notes, is oxidation: carbon monoxide and unburned fuel combine with oxygen to give carbon dioxide and water vapour. OpenStax adds the second job in three-way converters, whose platinum-rhodium surface also splits nitric oxide into nitrogen and oxygen. Such a catalyst offers a surface on which the reaction runs, and converters are preheated electrically for full activity.

Palladium has taken platinum’s place in most petrol-engine converters because it has historically been cheaper, the Survey notes. In diesel converters it can routinely replace about 25 per cent of the platinum, up to 50 in some. American recyclers recovered 8,500 kilograms of platinum from spent converters in 2024. In hydrogen fuel cells, OpenStax says, platinum-based catalysts on graphite electrodes speed up both half-cell reactions.

Methanol vapour touching platinum turns into formaldehyde, and the metal glows red hot as it works, Jefferson Lab notes. Los Alamos adds that the effect has been sold in hand warmers and cigarette lighters. The metal expands on heating almost exactly as soda-lime-silica glass does, so it is used for electrodes sealed into glass apparatus. Los Alamos also lists thermocouples, laboratory vessels, dentistry and the contact process for sulfuric acid.

Properties

Properties of platinum 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	78
Standard atomic weight	195.08(2) Da	Da	IUPAC/CIAAW 2024
Melting point	1768.3 °C	°C	Los Alamos: 1768.3 °C
Boiling point	3825 °C	°C	Los Alamos: 3825 °C
Density	21.46 g/cm³	g/cm³
Electronegativity (Pauling)	2.28
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹ 4f¹⁴ 5d⁹
Phase at 25 °C	solid

Safety

Platinum as a ring, a coin or a bar is essentially harmless: PubChem’s safety record calls the metal nontoxic, except possibly as a fine powder. In 410 of 504 company reports (81.3%) it meets no hazard criteria. Some reports class the finely divided powder as a flammable solid that irritates the eyes and airways. A Japanese government classification adds skin irritation and possible allergy or asthma symptoms if inhaled. The PubChem Laboratory Chemical Safety Summary and the NIOSH card for platinum metal carry the detail.

Questions

Why doesn't platinum tarnish?

Oxygen leaves it alone. Los Alamos says the metal does not oxidise in air at any temperature, and OpenStax lists platinum with palladium, silver and gold as the transition metals that oxygen fails to attack even when heated. Oxides of those four exist, made by other routes, yet decompose once warmed. That resistance, the textbook adds, suits platinum to circuits and jewellery. Halogens, cyanides, sulfur and caustic alkalis are another matter: they corrode it.

What dissolves platinum?

Hydrochloric acid alone does not, and nor does nitric acid, Los Alamos reports; as aqua regia, the pair take platinum into solution as chloroplatinic acid. OpenStax describes the mixture as the one used for metals harder to oxidise than hydrogen, gold and platinum among them. PubChem's safety record adds that fluorine and chlorine attack the metal at red heat. Cyanides, sulfur and caustic alkalis also corrode it, Los Alamos notes, though a solid lump is generally inert.

Is platinum radioactive?

Only very slightly. Natural platinum is five stable isotopes plus platinum-190, about 0.012 per cent of the metal, which alpha-decays to osmium-186. Its half-life is 4.83 × 10¹¹ years in the nuclide table of PubChem's record. That slow clock has been used to date rocks and iron meteorites. The commonest isotope, platinum-195, makes up about 34 per cent.

How is platinum used to treat cancer?

Through cisplatin, a square planar platinum complex. In 1965 researchers at Michigan State University found that it stopped cell division in certain microorganisms, a hint that it might act against cancer. The US Food and Drug Administration approved it for some cancers in 1978. OpenStax notes that of its two isomers, forms with the same atoms arranged differently, only the cis form works. Carboplatin, oxaliplatin and satraplatin came later.