Zirconium – facts and properties
- Symbol
- Zr
- Atomic number
- 40
- Atomic mass
- 91.224 u
- Period
- 5
- Group
- 4
- Category
- transition metal
- Electron configuration
- [Kr] 4d² 5s²
- Electronegativity
- 1.33
- Atomic radius
- 206 pm
- Valence electrons
- 4
- State at standard conditions
- solid
- Melting point
- 1855 °C
- Boiling point
- 4409 °C
- Density
- 6.52 g/cm³
- Discovered
- 1789
Zirconium is a silver-grey, corrosion-resistant metal with atomic number 40. Its name goes back to the mineral zircon, a gemstone. Zirconium absorbs few neutrons and withstands hot water, so the cladding of fuel rods in nuclear power plants is made from a zirconium alloy. The gem called cubic zirconia, by contrast, is linked to the metal only through its oxide.
Position in the periodic table
Occurrence and extraction
Zirconium is more abundant in the Earth's crust than many familiar metals, but never occurs pure. The main mineral is zircon, a zirconium silicate. It forms small, very hard grains in many rocks and collects in beach sands, where it is mined together with titanium minerals, chiefly in Australia and South Africa. A second ore is baddeleyite, a natural zirconium oxide.
The element was discovered in 1789 by the German chemist Martin Heinrich Klaproth. He examined a zircon from Ceylon, today's Sri Lanka, and found in it a previously unknown "earth", zirconium oxide. The metal itself could be obtained only later, as an impure powder, and very pure zirconium came in the twentieth century with the iodide process of van Arkel and de Boer.
Today the metal is produced by the Kroll process. Zircon or zirconium oxide is converted with chlorine and carbon into zirconium tetrachloride, which is then reduced with liquid magnesium to the metal. A difficulty is that zirconium ores always contain hafnium, which is chemically almost identical. For nuclear reactors it must be removed, because hafnium captures neutrons very well and zirconium hardly at all.
Properties and reactions
Zirconium is in period 5 and group 4, below titanium. Its electron configuration is [Kr] 4d² 5s² and its electronegativity is 1.33. The metal has a density of 6.52 g/cm³, melts at 1855 °C and boils at 4409 °C. It is tough, easy to shape and very heat-resistant. In compounds it nearly always has oxidation number plus four.
Its corrosion resistance rests on a wafer-thin oxide layer. It forms instantly in air, sticks firmly and protects the metal against most acids, alkalis and seawater; only a few acids such as hydrofluoric acid attack it. A piece of zirconium is therefore hard to attack as long as the layer is intact. Fine powder or turnings are different: they ignite easily and burn fiercely, so zirconium powder is off limits in school.
At very high temperatures the protection fails. Zirconium then reacts with steam to give zirconium oxide and hydrogen, a reaction that must be avoided in a reactor. In the accident at the Fukushima Daiichi nuclear plant in Japan in 2011, hydrogen formed mainly in this way after the cooling failed. It collected in the buildings and led to explosions. Since then the reaction has been examined very closely in safety analyses.
Uses in everyday life and technology
The largest share goes into nuclear technology. Fuel rods consist of uranium oxide pellets in thin tubes of Zircaloy, an alloy of zirconium with small amounts of tin, iron and chromium. The metal lets the neutrons of the chain reaction pass, withstands heat and hot water, and separates the fuel from the cooling water. A reactor core holds many tonnes of it.
The oxide matters greatly too. Zirconium dioxide, ZrO₂, is a hard, heat-resistant ceramic. It is used for dental crowns and bridges, artificial hip joints, sharp ceramic knives and the lining of crucibles and furnaces.
Gemstones are often confused. Zircon is a natural mineral and gem found in many colours, such as colourless, brown, yellow or, after heating, blue. Cubic zirconia is something else: a crystal grown in the laboratory from zirconium oxide, stabilised by adding yttrium oxide. It resembles diamond in sparkle and is very cheap, but it is softer.
Zirconium is also installed in chemical plants where acids would attack stainless steel, for example in heat exchangers and pumps. Zircon helps geology as well: when it grows it takes up traces of uranium but almost no lead, and it is extremely durable. The lead formed since then by radioactive decay reveals its age. The oldest known mineral grains on Earth are zircons, more than four billion years old.
Molecules containing this element
- Zirconium dioxide ZrO2
- Zirconium silicate ZrSiO4
- Potassium hexafluorozirconate K2ZrF6
Quick question
What is the atomic number of zirconium (Zr)?
FAQ
What is the difference between zircon and cubic zirconia?
Zircon is a natural mineral, a silicate of zirconium, cut as a gemstone. Cubic zirconia is an artificially grown crystal of zirconium oxide that looks like diamond. Both contain zirconium but are chemically different substances.
Why is zirconium used for fuel rods?
Zirconium hardly captures neutrons, so it does not slow the chain reaction. It also withstands hot water and radiation and forms a protective oxide layer. An alloy such as Zircaloy meets these demands very well in normal operation.
What does zirconium have to do with Fukushima?
In the 2011 accident the cooling failed and the fuel rods became extremely hot. Zirconium then reacts with steam and forms hydrogen. This gas collected in the buildings and exploded. The reaction is therefore an important point in reactor safety.