Atomic number 40

Zirconium

Zr
+ phase
− phase

Shells Orbitals 4d 5s

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40 electrons in 5 shells: 2, 8, 18, 10, 2.

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

Category: Transition metal
Period: 5
Group: 4
Block: d
Standard atomic weight: 91.222(3) Da
Electron configuration: [Kr] 4d² 5s²
Phase at 25 °C: solid
Melting point: 1855 °C
Boiling point: 4377 °C
Density: 6.52 g/cm³
Electronegativity (Pauling): 1.33
Discovered: 1789, Martin Heinrich Klaproth

Neighbours in the table

Y Nb

The name goes back to zargun, gold-like; Los Alamos calls the word Persian, while CIAAW, IUPAC’s atomic-weights commission, calls it Arabic. In 1789 Klaproth found a new element in a jargon, a zircon, from Ceylon; Werner named it zircon and Klaproth called it Zirkonertz, or zirconia. CIAAW’s account has him finding it in zirconia instead. Berzelius isolated impure metal in 1824, and D. Lely Jr. and L. Hamburger made it pure in 1914.

Zirconium belongs to the second transition series, the run from yttrium to silver in which ten d electrons fill the shell beneath the outermost. The atom’s configuration reads [Kr] 4d² 5s², putting 40 electrons in shells of 2, 8, 18, 10 and 2. A transition metal gives up its outer 5s pair before the 4d, OpenStax says. The +4 oxidation state counts all four as lost, bookkeeping, not a real charge; high-state oxides, OpenStax adds, are more covalent than ionic.

Jefferson Lab gives +4 as its oxidation state, and Los Alamos lists 4, 3, 2, 1 and −2. OpenStax finds that second-series metals generally hold high oxidation states more stably than first-series ones. The 4+ oxide is amphoteric, able to react as an acid or as a base, as OpenStax expects of metal oxides in that state. Los Alamos describes the metal as greyish-white and lustrous.

Where it is found

PubChem’s record puts zirconium at 165 milligrams in each kilogram of the Earth’s crust, while sea water carries only 3 × 10⁻⁵ milligrams per litre. Jefferson Lab names two minerals as the main sources: zircon, ZrSiO₄, and baddeleyite, ZrO₂. Los Alamos counts some 30 other mineral species that contain it, with zircon holding about 1 per cent hafnium.

Off the Earth, Los Alamos reports zirconium in the sun and in meteorites, and in abundance in a class of stars that Los Alamos calls S-type. Apollo lunar samples showed a surprisingly high zirconium oxide content set against rocks from Earth.

Mines produced an estimated 1.5 million tons of zirconium minerals, gross weight, in 2024, the US Geological Survey reports, 4 per cent more than a year earlier. Australia led with 500,000 tons, followed by South Africa with 300,000 and Mozambique with 160,000. In the United States one company recovered zircon from surface mines in Florida and Georgia as a heavy-mineral-sand coproduct, and a second reprocessed old tailings in California. Australia and South Africa were the leading exporters of concentrates, and China the leading importer.

Why it matters

Nuclear power takes most of the metal: more than 90 per cent of production on Los Alamos’s page, and nearly 90 per cent of each year’s output on Jefferson Lab’s. The reason is its low absorption of neutrons, which lets it clad fuel elements. Los Alamos says one reactor may hold up to half a million linear feet of alloy tubing, and names Zircaloy as the alloy developed for this work.

The cladding has one weakness in an accident. At Three Mile Island and at Fukushima, OpenStax records, zirconium alloy reacted with high-temperature steam, forming zirconium dioxide and hydrogen gas. Los Alamos names the Kroll process, reducing the chloride with magnesium, among other methods; OpenStax adds sodium as a reducing metal.

The US Geological Survey names the chemical process and nuclear energy industries as the leading consumers. Jefferson Lab describes a corrosion-resistant metal used in high-performance pumps and valves. Los Alamos lists it as a getter in vacuum tubes, a material that removes trace gases, and in surgical appliances and rayon spinnerets. Its carbonate goes into poison ivy lotions, Los Alamos says, because it combines with urushiol, the plant’s irritant. Zircon itself goes mainly to ceramics, foundry sand, opacifiers (additives that make a material opaque) and refractories, materials that hold up at furnace heat.

Properties

Properties of zirconium 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	40
Standard atomic weight	91.222(3) Da	Da	IUPAC/CIAAW 2024
Melting point	1855 °C	°C	Los Alamos: 1855 °C
Boiling point	4377 °C	°C	Los Alamos: 4377 °C
Density	6.52 g/cm³	g/cm³
Electronegativity (Pauling)	1.33
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d²
Phase at 25 °C	solid

Safety

A zirconium valve, pump or tube is essentially harmless: the bulk metal is much harder to ignite than its powder, and Haz-Map records no work-related disease from it. The hazard lies in fine powder, dust and shavings, and the finer the particles, the more vigorously they react. The EU classes pyrophoric powder as catching fire on its own in air and giving off flammable gas with water. The PubChem Laboratory Chemical Safety Summary covers those forms.

Questions

Why is zirconium used in nuclear reactors?

Neutrons pass through it. Jefferson Lab, on PubChem's record, says zirconium does not easily absorb them, and Los Alamos calls its neutron absorption cross section, a measure of how readily it captures neutrons, low, which suits it to cladding fuel. IUPAC's isotope table goes further: zirconium enriched in zirconium-90, which CIAAW puts at 51.47 per cent of natural atoms, has been proposed for cladding because it absorbs fewer neutrons than the natural mix.

Is zircon the same as zirconium?

No. Zircon is a mineral, ZrSiO₄, and the principal ore of the metal. Clear crystals of it can be cut to pass for diamond, Jefferson Lab notes, and Los Alamos notes its high refractive index, the degree to which it bends light, alongside its use as a gem. Los Alamos gives the stone's other names: jargon, hyacinth, jacinth and ligure. Zirconia, ZrO₂, is a different substance, the oxide, which stands up to heat well enough for crucibles.

Is zirconium radioactive?

Only very slightly. Los Alamos counts five isotopes in natural zirconium; Jefferson Lab counts four stable ones. The odd one out is zirconium-96, 2.78 per cent of the atoms, which decays by double beta decay, giving off two electrons at once. PubChem's isotope table gives it a half-life of 23.4 exayears, about 2.3 × 10¹⁹ years, while PubChem's safety record states only that it exceeds 3.6 × 10¹⁷ years.

Why is zirconium always found with hafnium?

The two elements share the same ores and are chemically very similar. Los Alamos says hafnium is invariably present in zirconium ores and hard to separate, and Jefferson Lab traces the difficulty to their chemical similarity. The US Geological Survey puts the ratio in zircon at about 50 to 1. Commercial-grade zirconium keeps 1 to 3 per cent hafnium, Los Alamos adds; reactor grade is essentially free of it.