Zirconium (symbol Zr, atomic number 40) is a lustrous, silvery-gray transition metal in group 4 of the periodic table (period 5, d-block). At standard conditions (near 20°C and 1 atm), it is a tough solid with hexagonal close-packed (hcp) crystal structure. Zirconium’s most common oxidation state is +4 (as in ZrO₂ or ZrSiO₄), though it can also form +3 and +2 compounds. Its high melting point (~1855 °C) and strong affinity for oxygen lead it to form a tight oxide layer that makes the metal highly corrosion-resistant. These properties – plus a very low thermal neutron-capture cross-section – explain why zirconium (especially in its nuclear-grade alloy form, Zircaloy) is widely used for corrosion-resistant applications and for cladding fuel rods in nuclear reactors. The element’s name derives from the mineral zircon (zirconium silicate, ZrSiO₄), whose name comes from the Persian “zargun” meaning “gold-colored.”

Atomic Structure and Electron Configuration

Zirconium
Atomic number	40
Symbol	Zr
Group	4
Electron configuration	[Kr] 4d2 5s2
Density	6.52 g/cm^3
Period	5
Melting point	1855 °C
Phase STP	Solid
Block	d
Oxidation states	+4, +3, +2
Wikidata	Q1038

A neutral zirconium atom has 40 protons and (in its ground state) 40 electrons. Its electron configuration is `[Kr] 4d² 5s²`, filling the krypton core and placing two electrons each in the 4d and 5s subshells. The 4d and 5s electrons constitute its valence shell, which is responsible for bonding behavior in compounds. As an element in the titanium group (Ti, Zr, Hf), zirconium exhibits the expected trend of increasing atomic size and decreasing ionization energy going down the group. Its atomic radius is on the order of 160–175 picometers (pm) (covalent radius ~175 pm), larger than that of titanium, reflecting its higher principal quantum number. The Pauling electronegativity of Zr is about 1.33 (low, as typical for metals) and its first ionization energy is about 640 kJ/mol. These values are lower (weaker binding) than the lighter group member Ti, as expected by periodic trends. In summary, zirconium’s outer electrons Kr] 4d² 5s²) and its position in the periodic table give it characteristics similar to the other group-4 transition metals: metallic bonding, ductility, and a tendency to lose electrons to form positive ions (especially Zr⁴⁺).

Isotopes and Nuclear Properties

Naturally occurring zirconium consists of five isotopes. Four of them (⁹⁰Zr, ⁹¹Zr, ⁹²Zr, ⁹⁴Zr) are effectively stable, and a fifth (³⁹⁶Zr) is very long-lived (a primordial radionuclide undergoing double-beta decay with a half-life ~2.3×10¹⁹ years). The stable isotopes occur in approximate natural abundances: ⁹⁰Zr (≈51.5%), ⁹²Zr (≈17.2%), ⁹⁴Zr (≈17.4%), ⁹¹Zr (≈11.2%), and ⁹⁶Zr (≈2.8%). The even-even (even-proton, even-neutron) isotopes ⁹⁰, ⁹², ⁹⁴, ⁹⁶ have nuclear spin zero; ⁹¹Zr (odd-neutron) has spin 5/2.

Of the less abundant radioisotopes, zirconium-93 (Zr-93) is notable: it has a half-life of about 1.6×10^6 years and decays by beta emission to niobium-93. Zr-93 is a long-lived fission product in nuclear reactors (yield ≈6% per U-235 fission) Another useful isotope is Zr-89 (half-life ≈78.4 hours, decays by positron emission), which is produced by proton irradiation of yttrium-89 and used in medical PET imaging of antibodies (‘immuno-PET’) Shorter-lived isotopes like Zr-88 (83.4 days) and Zr-95 (64 days) are encountered in reactor waste and experimental work, but have few practical applications due to their short lifetimes. In nuclear physics, zirconium isotopes illustrate decay systematics: lighter Zr isotopes (A<90) decay by positron emission to yttrium, while heavier ones (A>92) undergo beta-minus decay to niobium. Historically, zirconium’s low neutron-capture cross-section (around 0.2–0.7 barns) and lack of radioactive activity explain its choice as reactor cladding material

Allotropes and Typical Compounds

Metallic zirconium has two common allotropes. At room temperature it is the α phase, hexagonal close-packed (hcp). Upon heating above about 865 °C it transforms to the β phase, a body-centered cubic structure, which remains stable until melting. (These structural forms are analogous to those of titanium.) No other allotropes are stable at ambient pressure.

Zirconium most commonly exhibits a +4 oxidation state. Its characteristic compounds reflect this. The most important oxide is zirconia (ZrO₂), a hard refractory solid with very high melting point (~2700 °C). Zirconia is insoluble in water and many acids, which means metallic Zr forms a passive ZrO₂ film that resists corrosion. Zirconia itself has several crystal forms: monoclinic at room temperature, transforming to tetragonal and then cubic at high temperatures (phases often stabilized by adding yttria or magnesia).

The silicate mineral zircon (natural ZrSiO₄) is the principal ore of zirconium. Zircon is dense and hard, with high melting point, and is used in foundry sands, ceramics and glazes. It often contains small amounts of hafnium.

Important binary compounds of zirconium include:

Halides: The tetrahalides ZrCl₄, ZrBr₄, ZrI₄ are volatile colorless solids (or liquids) that hydrolyze readily in moist air (ZrCl₄, for example, fuming in humidity). These Zr(IV) halides are key precursors in metal extraction. Lower halides (ZrCl₃, ZrCl₂, ZrBr₂, etc.) exist but are less stable and typically disproportionate.
Hydrides: Zirconium reacts with hydrogen to form zirconium hydrides, typically ZrH₂ (though non-stoichiometric forms ZrH₁–₂ exist). These brittle hydrides are used in some reactor designs as moderators (due to high hydrogen content).
Carbides and Nitrides: ZrC and ZrN are extremely hard, high-melting refractory compounds (melting points >3000 °C) used as abrasive or protective coatings (e.g. ZrN is golden and used in hard coatings for tools).
Oxychlorides: Zirconium oxychloride (ZrOCl₂·8H₂O) and related salts are water-soluble Zr(IV) compounds formed by carefully controlling hydrolysis of halides. They are used in water treatment, tanning and proppants.
Organometallics: In organic chemistry, Zr forms complexes like zirconocene dichloride (Cp₂ZrCl₂) and related metallocenes used as catalysts for olefin polymerization (Ziegler–Natta catalysts), illustrating its ability to attain +4 in coordination complexes.

In aqueous solution, Zr⁴⁺ is strongly polarizing: it hydrolyzes to zirconium hydroxide (often written Zr(OH)₄) and precipitates, and it forms high-coordination complexes (e.g. [Zr(OH)₂(H₂O)₆]²⁺). Zirconium compounds are amphoteric; for example, ZrO₂ dissolves in strong concentrated alkali (forming ZrO₃²⁻ or [Zr(OH)₆]²⁻), and Zr also forms fluoride complexes (ZrF₆²⁻ in HF).

Physical Properties

Zirconium is a moderately dense, silvery-metallic solid. Its density is about 6.52 g/cm³ at 20 °C. It is harder and stronger than many common metals (toughness varies with phase: α-Zr is strong and somewhat brittle, while β-Zr at high temperature is more ductile). At ambient temperature, zirconium is paramagnetic (unpaired d-electrons) and electrically conductive (bulk resistivity ~4.2×10⁻⁷ Ω·m at 20 °C Its thermal conductivity is about 22.7 W·m⁻¹·K⁻¹ comparable to stainless steel.

Key thermophysical constants (at 1 atm) are: melting point about 1855 °C (2128 K); boiling point around 4400 °C (≈4670 K) The solid state alpha phase is HCP (with lattice parameters depending on temperature), transforming at ≈865 °C to beta-Zr (BCC). Zirconium’s heat capacity at 25 °C is roughly 0.27 J/(g·K). Under electron spectroscopy, neutral Zr atoms have prominent emission lines in the ultraviolet and visible (used in analytical spectrometry), but no lines of striking cultural importance; most applications exploit its bulk physical properties rather than discrete atomic spectra.

Zirconium displays excellent refractoriness: for example, zirconia (ZrO₂) tiles can withstand 2000 °C, making them common in high-temperature crucibles. As a metal, Zr’s high melting point and low vapor pressure give it stability in vacuum or flame at temperatures far above most steels.

Chemical Reactivity and Trends

Zirconium’s chemistry closely parallels that of titanium: both are fairly reactive metallic elements that readily achieve the +4 oxidation state. In air, a fresh Zr surface rapidly oxidizes to ZrO₂ (often requiring only moderate heat to initiate combustion). This passive oxide layer makes bulk zirconium remarkably corrosion-resistant under many conditions For instance, it is essentially impervious to hydrochloric and sulfuric acids (even boiling, moderately concentrated acids), and to hot alkalis, because the ZrO₂ film stays intact. Zirconium’s corrosion resistance is often compared to titanium’s, though Zr is generally less affected by organic acids (acetic, citric, etc) than Ti

However, zirconium dissolves in strongly complexing media. Hydrofluoric acid (HF) aggressively attacks Zr by breaking down ZrO₂ to form soluble zirconium fluoride complexes. Similarly, oxidizing mixtures (like aqua regia) or hot (>80%) sulfuric acid can dissolve Zr. The metal does not react with pure water or steam at room temperature, but at high temperatures it is oxidized by steam (a reaction relevant in nuclear safety: Zr + 2 H₂O → ZrO₂ + 2 H₂, which produced hydrogen gas in reactor accidents). Zirconium also reacts with halogens: it burns with chlorine, bromine or fluorine to give the corresponding tetrahalide (ZrCl₄, etc). With hydrogen gas, Zr forms zirconium hydride (ZrH₂) when heated or under pressure.

Liquid ammonia or oxidizers likewise cause rapid excursion to ZrO₂ or nitrides (Zr + ½N₂ → ZrN at high T). In summary, zirconium behaves as a strongly electropositive metal (standard potential Zr⁴⁺/Zr ≈ –1.5 V), but its corrosion is usually self-limited by the stable Zr⁴⁺-oxide layer.

In aqueous solutions, zirconium ions (Zr⁴⁺) are very highly charged and strongly polarizing. They hydrolyze in water to form colloidal Zr(OH)₄, and only in strongly acidic solution do Zr⁴⁺ salts remain soluble. Zirconium forms high-coordination complexes (often coordination number 7–8) with oxygen-donors; for example, zirconyl chloride (ZrOCl₂·8H₂O) is a common starting compound. It also forms fluoro-complexes (e.g. ZrF₆²⁻ in HF). In mixed oxide or carbide, Zr tends to bond strongly to electronegative atoms (O, C, N).

Occurrence and Production

Zirconium is fairly abundant in Earth’s crust (~130 ppm by weight – more common than tin or lead – but it is never found as a free metal. Its major natural carriers are the minerals zircon (ZrSiO₄) and, less commonly, baddeleyite (natural ZrO₂). Zircon typically occurs in igneous and metamorphic rocks; over geological time it collects in heavy-mineral sand deposits. The mineral zircon often contains 1–2% of hafnium (Hf), which is chemically similar to Zr. For many applications (especially nuclear reactors), Hf must be removed because it absorbs neutrons strongly. Separating Zr from Hf is a key industrial challenge.

Major zirconium-bearing ores and mining centers worldwide include deposits in Australia, South Africa, Brazil, India, Malaysia, the United States, and several other countries Australia is the largest producer of zircon minerals. Concentration of the ore is typically done by gravity and electrostatic methods (e.g. removing lighter materials and other minerals).

To extract the metal, zircon or baddeleyite is first converted to zirconium dioxide (ZrO₂) by fusion and chemical processing. The ZrO₂ is then typically chlorinated (often with chlorine gas and carbon) to make zirconium tetrachloride (ZrCl₄), which is purified by distillation. Critically, purified ZrCl₄ is reduced with liquid magnesium (the “Kroll process”) or sodium at high temperature to yield metallic zirconium sponge and magnesium chloride

<code>ZrCl₄ (gas) + 2 Mg (melt) → Zr (solid) + 2 MgCl₂ (melt).
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The porous zirconium “sponge” is then leached of residual salts and consolidated, often by vacuum-arc remelting. In the Kroll method, about 100–150 kg of zirconium are produced from a ton of ore concentrate. While the Kroll (chloride–Mg) route is dominant for commercial and nuclear-grade zirconium, other methods exist: fluoride-based electrolysis (electrolysing K₂ZrF₆ melts) or the van Arkel–de Boer iodide process (decomposition of ZrI₄ on a hot filament) can yield ultra-pure zirconium, albeit at smaller scale

Modern zirconium metal is produced by only a few large facilities in the world (e.g. in the USA, France, India, China, Japan, and others) under strict purity and quality controls, because nuclear and aerospace uses demand very low Hf content and controlled impurity levels. As of the early 21st century, leading producers of zirconium ore and concentrate were Australia, South Africa, China, Indonesia, Mozambique, India and Sri Lanka

Applications and Technology

Zirconium’s combination of reactivity (when oxygen-free) and inertness in many environments has led to several major applications:

Nuclear Industry: By far the largest use of zirconium is in nuclear reactors. Zirconium alloys (known as Zircaloys, typically Zr-Sn-Fe-Cr-Ni) serve as fuel rod cladding and internal components in light-water reactors. Zirconium’s transparency to thermal neutrons and its corrosion resistance in hot water make it ideal for containing uranium fuel. (In contrast, its sister element hafnium is kept to very low levels in reactor-grade zirconium because Hf would absorb neutrons.) In naval reactors and power plants around the world, zirconium alloys encase uranium dioxide pellets.
Ceramics and Refractories: Zirconium dioxide (zirconia) is a key ceramic material. It is used to make very hard, extremely heat-resistant tiles, crucibles and coatings (e.g. thermal barrier coatings on turbine blades). Zirconia, often stabilized with yttrium (Y₂O₃) or magnesia, is also used in oxygen sensors, fuel cell electrolytes (solid-oxide fuel cells), and radiative infrared reflectors (in furnaces). The mineral zircon (ZrSiO₄) is used as a refractory sand in foundries and as a glaze opacifier in ceramics and glassmaking.
Gemstones and Jewelry: Natural zircon gemstones (ranging in color from colorless to yellow-brown or green) have been prized in jewelry for centuries. More famously, synthetic cubic zirconia (a stabilized cubic phase of ZrO₂) is a major diamond simulant in the jewelry industry, valued for its brilliance and hardness.
Chemical Industry: Zirconium compounds serve as catalysts and stabilizers in organic chemistry. For example, zirconocene dichloride (Cp₂ZrCl₂) and related metallocenes are important catalysts for polymerizing olefins to make plastics (a type of Ziegler–Natta catalyst). Zirconium hydroxy compounds (often marketed as zirconium oxychloride or sulfate) are used as high-temperature cauterizing agents, tanning agents in textiles and leather, and corrosion-resistant coatings.
Metallurgical Applications: Zirconium improves certain alloys: small amounts are added to magnesium and nickel alloys to refine grain size and improve strength. Zirconium-based alloys (like N102, Zr-2, Zr-4) are used in chemical processing equipment (pipe, heat exchangers) for acid-resistant service. Metallic zirconium also serves as a getter (gas-scavenger) in vacuum tubes and incandescent lamps, and as an alloying element in nuclear fuels (e.g. Pu-Zr alloys) and in superalloys.
Biomedical: Zirconia ceramics are biocompatible and have high strength, so Y-TZP (yttria-stabilized zirconia) is widely used for dental crowns and bridges, hip/knee prostheses, and other medical implants. Zirconium metal and its alloys are not known to provoke biological reactions, which makes them safe for implants (ZrO₂ has a “ceramic steel” reputation).
Electronics and Photonics: Zirconia and zircon compounds are used in advanced electronics. ZrO₂ is studied as a high-permittivity (“high-κ”) gate dielectric in microelectronics. Barium-zirconate titanate (Ba(Zr,Ti)O₃) ceramics are components in capacitors and piezoelectrics. In optics, ZrO₂ is transparent in parts of the infrared and used in lenses; ZrOCl₂ is a precursor for some coatings and nuclear imaging contrast agents.
Other Uses: The lustrous sparks of burning zirconium powder are pushed in fireworks (as incendiary, similar to magnesium). Zirconium hydride moderates neutrons or stores hydrogen in research applications. ZrO₂ is also used in cutting tools and abrasives.

Biology, Environment, and Safety

Zirconium has no known essential role in biology; it is not a nutrient. In fact, zirconium compounds are so inert that they have been considered biologically non-toxic. In practice, zirconium is of low toxicity: it is not absorbed readily by the body, and its salts (except strong acids or fluorides) are mostly insoluble. Zirconium dioxide ceramics are used in implants with no adverse effects in patients.

The main health hazard of zirconium is inhalation of fine particles. Inhaled Zr dust or fumes can cause pulmonary granulomas (a condition sometimes called “zirconium lung”), similar to other refractory dust exposures. Chronic exposure to high concentrations of zirconium compounds (usually industrial) may cause skin or lung irritation. Accordingly, occupational exposure limits are set to avoid dust inhalation; for example, the U.S. OSHA permissible exposure limit is 5 mg/m³ (as Zr) over an 8-hour day Zirconium is not classified as a carcinogen by major health agencies (it is typically listed as “not classifiable” or “A4” – not considered a human carcinogen).

Environmentally, zirconium behaves like other lithophile elements. It is mostly locked in soils and rocks in the form of zircon or oxides, with almost no mobility in water or biological systems. Zr is essentially absent from natural waters (sea water contains only ~0.03 ppm and it tends to adsorb onto sediments or form insoluble hydroxides. Zirconium mining and processing generate waste (tailings rich in zircon or baddeleyite), but elemental Zr itself is not a pollutant of major concern. Proper industrial hygiene (dust control, protective gear) generally suffices for safety when working with zirconium metal or compounds.

History and Etymology

The element zirconium was first recognized in 1789 by the German chemist Martin Heinrich Klaproth Klaproth analyzed the mineral zircon and found an oxide he identified as a new element. He named it “zirconia” for the mineral, based on the mineral’s name (from Persian “zargun” meaning “gold-colored,” referring to zircon’s yellowish crystals The metal itself was first isolated in impure form in 1824 by Jöns Jacob Berzelius. Early zirconium metal (even 99% pure) was hard and brittle.

High-purity zirconium was produced for the first time in 1925 by the Dutch chemists Anton van Arkel and Jan de Boer, using the so-called iodide process (they thermally decomposed ZrI₄ on a hot filament to obtain pure Zr metal) In the 1940s, William Justin Kroll developed a more economical method (the magnesium reduction of ZrCl₄) which remains the basis of modern industrial production.

For most of its history, zirconium was a laboratory curiosity or used in refractory ceramics. It was only after World War II that it gained prominence: its very low neutron absorption made it strategically important for nuclear reactors. Aluminium-magnesium zirconium alloys (Zircaloy) were introduced as fuel-cladding materials in the 1950s. Around the same time, ZrO₂ began to be used in dental and industrial ceramics. More recently, synthetic cubic zirconia (discovered in the 1970s) became widely known as a diamond substitute.

Throughout history, zircon (the mineral) also had cultural roles: it was used as a gemstone in various colors (blue, green, red, gold) long before its chemical composition was known. Its use for radiometric dating (U–Pb dating in zircon crystals) has been fundamental to geology, revealing some of the oldest ages of Earth’s crust (on the order of 4.4 billion years). But as an element, zirconium’s career is tied to nuclear technology and advanced materials rather than antiquity.

Data Table

Property	Value
Symbol	Zr
Atomic number (Z)	40
Standard atomic weight	91.224
Electron configuration	[Kr] 4d² 5s²
Group / Period / Block	4 / 5 / d-block (transition metal)
Common oxidation state	+4 (also +2, +3)
Phase at STP	Solid
Density (20°C)	6.52 g/cm³
Melting point	1855 °C (2128 K)
Boiling point	~4400 °C (~4670 K)
Covalent radius	~175 pm
Electronegativity	1.33 (Pauling scale)
First ionization energy	~640 kJ/mol
Crystal structure (α-Zr)	Hexagonal close-packed (hcp)
Major stable isotopes	⁹⁰Zr (51.5% natural), ⁹²Zr (17.2%), ⁹⁴Zr (17.4%), ⁹¹Zr (11.2%), ⁹⁶Zr (2.8%, radioactive double-β)
Notable radioisotopes	⁹³Zr (t½≈1.6×10⁶ y), ⁸⁹Zr (t½≈78.4 h; used in immuno-PET)
Crystal colours	Silver-gray (metal); white (ZrO₂)
Abundance (Earth’s crust)	~0.013% by weight (130 ppm)