Atomic number 46

Palladium

Pd
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Shells Orbitals 4d

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46 electrons in 4 shells: 2, 8, 18, 18.

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

Category: Transition metal
Period: 5
Group: 10
Block: d
Standard atomic weight: 106.42(1) Da
Electron configuration: [Kr] 4d¹⁰
Phase at 25 °C: solid
Melting point: 1554.9 °C
Boiling point: 2963 °C
Density: 12.0 g/cm³
Electronegativity (Pauling): 2.2
Discovered: 1803, William Hyde Wollaston

Neighbours in the table

Rh Ag

Palladium belongs to period 5, yet its ground-state (lowest-energy) atom has an empty fifth shell. Outside the krypton core, all ten remaining electrons go into 4d, so the configuration is [Kr] 4d¹⁰ and the shells hold 2, 8, 18 and 18. A first course’s filling order would put two in 5s. OpenStax traces such departures to subshells of nearly equal energy, which no simple rule predicts. OpenStax gives palladium(II) as [Kr] 4d⁸, both lost electrons taken from 4d.

The English chemist William Hyde Wollaston found the element in 1803, analysing platinum ore brought from South America, Jefferson Lab notes in PubChem’s record. IUPAC’s atomic-weights commission (CIAAW) adds that he first announced it anonymously, claiming priority while keeping his preparation secret. The name honours the asteroid Pallas, spotted a year earlier by the German astronomer Wilhelm Olbers and named for Pallas Athene, Greek goddess of wisdom.

Los Alamos ranks palladium lowest of the platinum-group metals in density and melting point, which is about 1555 °C. Annealing (heat, then slow cooling) softens it; cold-working, shaping without heat, hardens and strengthens it. It keeps its lustre in air, though nitric and sulfuric acids attack it. OpenStax says heated oxygen leaves palladium untouched; PubChem’s safety record says red heat turns the metal into its monoxide.

Where it is found

Jefferson Lab estimates palladium at about 1.5 × 10⁻² milligrams per kilogram of Earth’s crust. The metal tends to turn up beside platinum, nickel, copper, silver and gold, and it is recovered as a byproduct of mining them. Los Alamos lists deposits shared with the other platinum-group metals in Australia, Ethiopia, Russia and both Americas. It also names the nickel-copper ores of South Africa and Ontario.

For 2024 the US Geological Survey estimates world mine output at 190,000 kilograms, down from 208,000 the year before. Russia, which the Survey ranks as the leading producer of mined palladium, supplied 75,000 of them. South Africa followed with 72,000, and Canada and Zimbabwe gave about 15,000 each. South Africa’s Bushveld Complex holds the largest platinum-group resources and reserves.

American mines produced 8,000 kilograms, chiefly from one company working in Montana. Recyclers in the United States recovered about 45,000 kilograms from spent car catalytic converters in that year, more than five times the domestic mine figure. Imports still covered the rest, and the Survey puts net import reliance at 36 per cent of apparent consumption.

Why it matters

Cutting harmful car exhaust is the leading American use of the platinum-group metals, says the US Geological Survey. The Survey reports that palladium has replaced platinum in most petrol-engine converters, a switch made on its historically lower price. In diesel converters about a quarter of the platinum can routinely give way to palladium, and as much as half in some designs.

Los Alamos reports that palladium can absorb hydrogen amounting to as much as 900 times the metal’s own volume. The product may be Pd₂H, though whether that is a true compound is still unsettled. Hydrogen also diffuses readily through the heated metal, and that passage serves to purify the gas. Finely divided palladium catalyses hydrogenation and dehydrogenation, adding or removing hydrogen in other molecules. OpenStax describes solid catalysts of this sort as furnishing an active surface on which a reaction can run.

Jefferson Lab’s list of uses runs from watch springs and surgical instruments to electrical contacts, dental crowns and fillings. The Survey adds multilayer ceramic capacitors and catalysts for petroleum refining, naming them for the platinum-group metals as a set. Between 2020 and 2023, Russia and South Africa each supplied 32 per cent of the palladium the United States imported.

Properties

Properties of palladium 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	46
Standard atomic weight	106.42(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1554.9 °C	°C	Los Alamos: 1554.9 °C
Boiling point	2963 °C	°C	Los Alamos: 2963 °C
Density	12.0 g/cm³	g/cm³
Electronegativity (Pauling)	2.2
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 4d¹⁰
Phase at 25 °C	solid

Safety

Palladium in a ring or a dental crown is not poisonous: Haz-Map, quoted in PubChem’s safety record, calls the platinum-group metals generally nontoxic in the metallic state. Of 616 company reports to the European Chemicals Agency, 365 (59.3%) find it short of the Globally Harmonized System’s hazard criteria. Contact dermatitis, an allergic skin rash, has been linked to the metal and especially its alloys, and finely divided palladium black usually ignites by itself in air. Of 251 reports giving a hazard, 31.5% class palladium as a flammable solid, signal word Danger; the PubChem Laboratory Chemical Safety Summary carries the detail.

Questions

Is there palladium in white gold?

Los Alamos describes white gold as gold whose yellow colour has been taken away by alloying it with palladium. Like gold, palladium can be hammered into leaf just 1/250,000 of an inch thick, and Los Alamos ranks silver's ductility behind only gold and perhaps palladium. The US Geological Survey counts jewellery among the uses of the platinum-group metals.

How is palladium used to treat cancer?

Through radioactive isotopes. IUPAC's notes in PubChem's record say palladium-103 seeds are placed inside the body against prostate and other cancers. Its half-life is 16.99 days. Each 100 decays yield roughly 80 X-rays and 186 Auger electrons, low-energy electrons thrown from inner shells. Palladium-104 is the usual target in a cyclotron, a machine that spins charged particles into a target, for making it. Palladium-109, with a 13.5-hour half-life, has been attached to porphyrins, molecules that gather in tumours.

How does palladium help date meteorites?

Radioactive palladium-107, half-life 6.5 million years, beta-decays to silver-107. IUPAC's notes say their ratio can date when that decay began. Material holding excess silver-107 with its palladium-107 formed at some point after the parent had decayed. Separately, small anomalies in palladium's isotope mix in one group of iron meteorites may record an early solar nebula, the cloud the Sun and planets formed from, that varied in space or time, or chemical processing inside it.

Why did the price of palladium drop in 2024?

The US Geological Survey puts the 2024 annual average at 980 dollars per troy ounce, 27 per cent below the 2023 figure of 1,351.66 dollars and far under 2021's 2,419.18. It blames weaker demand, investor uncertainty and oversupply. Output fell as well, for reasons the Survey lists at length: deep-mining costs, labour disputes and power cuts in South Africa, and natural disasters, lower ore grades, the Russia-Ukraine conflict and a plant outage in Russia.