Zr · Element 40 · Transition metal
Effectively transparent to neutrons, which is why it lines the fuel rods in nearly every nuclear reactor.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where zirconium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All zirconium atoms have the same number of protons, 40, and differ only in their number of neutrons: those variants are its isotopes. The most common, Zr-90, has 50 neutrons and makes up 51.45% of natural zirconium.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Zr-90 most common | 50 | 51.45% | Stable | — |
| Zr-91 | 51 | 11.22% | Stable | — |
| Zr-92 | 52 | 17.15% | Stable | — |
| Zr-94 | 54 | 17.38% | Stable | — |
| Zr-96 | 56 | 2.80% | 2.0 × 10¹⁹ yrs | — |
The classroom shortcut, rounded atomic mass minus atomic number, gives 51 for zirconium, but the most common zirconium atom has 50 neutrons: the atomic mass, 91.224, is an average over all natural isotopes.
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 160 pm — that is 0.16 nm, so about 3.13 million of them side by side would span a millimetre.
Thermal range
Zirconium is liquid over a 2554 K window, from 2128 K to 4682 K.
The story
Zirconium is a tough, corrosion-resistant metal used to clad nuclear reactor fuel; its oxide, cubic zirconia, is a common diamond simulant. This lustrous, gray-white transition metal has the remarkable property of being nearly transparent to neutrons while being extremely resistant to corrosion, making it essential for nuclear power generation. Named after the Persian word "zargun" meaning gold-colored, zirconium has become one of the most strategically important elements in modern technology. Discovered in 1789 by German chemist Martin Heinrich Klaproth while analyzing the gemstone zircon, zirconium remained largely a laboratory curiosity for over 150 years. An impure form of the metal was first isolated in 1824 by Jöns Jacob Berzelius, and high-purity zirconium only became available in the 1940s when the nuclear industry desperately needed materials that wouldn't interfere with nuclear reactions. What sets zirconium apart is its unique nuclear properties. Unlike most metals, zirconium is almost transparent to neutrons - they pass right through it without being absorbed. This makes zirconium alloys perfect for nuclear reactor fuel cladding, containing radioactive fuel while allowing neutrons to move freely and sustain the nuclear chain reaction. Over 90% of nuclear reactors worldwide use zirconium alloy fuel rods. Zirconium has high corrosion resistance, surpassing even stainless steel in harsh environments. It forms an ultra-thin, self-healing oxide layer that protects it from acids, alkalis, and seawater. This makes zirconium perfect for chemical processing equipment, medical implants, and marine applications where other metals would quickly corrode. Here's something dazzling: cubic zirconia (synthetic zirconium dioxide) is one of the most popular diamond simulants ever created. It's hard (8 to 8.5 on the Mohs scale, against diamond's 10), has brilliant optical properties, and can be produced in any color, making it the foundation of a billion-dollar jewelry industry.
Martin Heinrich Klaproth discovered zirconium in 1789 while analyzing zircon gemstones. Jöns Jakob Berzelius first isolated impure zirconium metal in 1824, though pure zirconium was first made by the van Arkel-de Boer crystal bar process in 1925.
Applications
Zirconium's exceptional corrosion resistance and low neutron absorption make it central for nuclear reactors, while its biocompatibility and strength enable advanced medical implants and aerospace applications.
Zircaloy fuel cladding encases uranium fuel pellets in nuclear reactors, providing corrosion resistance in high-temperature water while allowing neutron transmission for sustained nuclear fission. Zirconium's low neutron absorption cross-section makes it ideal for reactor core components where neutron economy is critical.
Zirconium equipment handles highly corrosive chemicals including hydrochloric acid, sulfuric acid, and organic acids at temperatures and concentrations that destroy stainless steel within hours. Pharmaceutical and fine chemical manufacturing rely on zirconium's chemical inertness for high-purity processing.
Zirconium alloys provide exceptional strength-to-weight ratios for aircraft components operating in corrosive environments. Space applications utilize zirconium's resistance to atomic oxygen and thermal cycling in satellite structural components.
Where it comes from
165 mg/kg of Earth's crust · more abundant than 85% of elements
Zirconium occurs primarily in zircon (ZrSiO₄) and baddeleyite (ZrO₂) minerals. Zircon represents one of Earth's most durable minerals, surviving billions of years and providing crucial information about early Earth history and planetary formation.
Handling
Zirconium metal and most compounds exhibit low toxicity and excellent biocompatibility. Zirconium powder presents fire and explosion hazards requiring appropriate storage and handling procedures.
Standard metalworking safety procedures apply for zirconium machining and fabrication. Dry zirconium powder can ignite spontaneously or from static, so it is kept wetted with at least 25% water or under inert gas, and a burning powder must never be hit with water.
Quick answers
Zirconium (symbol Zr) is element 40 on the periodic table, a transition metal in period 5, group 4. Effectively transparent to neutrons, which is why it lines the fuel rods in nearly every nuclear reactor. At room temperature it is a solid.
[Kr] 4d² 5s², giving 5 occupied shells with the distribution 2, 8, 18, 10, 2.Zirconium melts at 2128 K (1854.9 °C) and boils at 4682 K (4408.9 °C). That puts it among the most refractory elements — it stays solid at temperatures that vaporise most metals.
The standard atomic weight of zirconium is 91.224 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.
The most common zirconium atom, Zr-90, has 50 neutrons: its mass number, 90, minus its atomic number, 40. It makes up 51.45% of natural zirconium. The classroom shortcut, rounded atomic mass (91.224) minus atomic number, gives 51 instead, because the atomic mass is an average over all natural isotopes of zirconium.
Zirconium has a density of 6.52 g/cm³. Water is 1.0 g/cm³, so a block of zirconium is about 6.5× heavier.
Zirconium has a Pauling electronegativity of 1.33. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.
Zirconium was discovered in 1789 by Martin Heinrich Klaproth. Origin of the name: Persian zargun, "gold-coloured".
Zirconium makes up about 165 mg/kg of the Earth's crust — common enough to be mined at scale.