Zirconium (Zr)

Zirconium is nearly transparent to neutrons, which is why every water-cooled reactor in the world clads its fuel in a zirconium alloy — steel would soak up too many neutrons to sustain the chain reaction. That choice has a sting: above about 1,200 °C zirconium strips oxygen out of steam and releases hydrogen, which is what produced the explosions at Fukushima Daiichi. Zircon crystals from Jack Hills in Western Australia are the oldest known terrestrial material at about 4.4 billion years, because zircon is tough enough to survive being eroded, buried and re-eroded for most of Earth's history.

Properties

Atomic number

40

Symbol

Zr

Standard atomic weight

91.222 u

Category

Transition metal

Group

4

Period

5

Block

d-block

Electron configuration

[Kr] 5s² 4d²

Electrons per shell

2, 8, 18, 10, 2

Melting point

2,128 K — 1854.8 °C — 3370.7 °F

Boiling point

4,682 K — 4408.9 °C — 7967.9 °F

Density

6.52 g/cm³

Electronegativity

1.33 (Pauling)

Oxidation states

+4

Discovered

1789 by Martin Heinrich Klaproth

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d²

Noble gas shorthand

[Kr] 5s² 4d²

Shells

K: 2 · L: 8 · M: 18 · N: 10 · O: 2

What it is used for

• Fuel rod cladding in nuclear reactors, as zircaloy
• Cubic zirconia as a diamond simulant
• Zirconia dental crowns and hip joint ceramics
• Foundry sand and refractory linings
• Chemical plant pipework handling hot acids

CIAAW revised the standard atomic weight to 91.222 in 2023; tables printed before then show 91.224.

Neighbours in the table

Group 4

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Zirconium sits in group 4 with titanium, hafnium, rutherfordium.