Hafnium – facts and properties
- Symbol
- Hf
- Atomic number
- 72
- Atomic mass
- 178.49 u
- Period
- 6
- Group
- 4
- Category
- transition metal
- Electron configuration
- [Xe] 4f¹⁴ 5d² 6s²
- Electronegativity
- 1.3
- Atomic radius
- 216 pm
- Valence electrons
- 4
- State at standard conditions
- solid
- Melting point
- 2233 °C
- Boiling point
- 4603 °C
- Density
- 13.3 g/cm³
- Discovered
- 1923
Hafnium is a lustrous, silvery-grey transition metal with atomic number 72. It resembles zirconium so closely that the two always turn up together in nature and are very hard to separate. Yet in one respect they are opposites: zirconium lets neutrons pass almost freely, while hafnium captures them. That is why hafnium goes into control rods for nuclear reactors, and as hafnium oxide it sits inside the transistors of modern microchips.
Position in the periodic table
Occurrence and extraction
Hafnium is not especially scarce, but it forms no deposits of its own. The Earth's crust holds a few grams per tonne of rock, and nearly all of it hides in zirconium ores, above all zircon, a zirconium silicate, and baddeleyite.
The element was found only in 1923, although chemists had been looking for it for a long time. The periodic system had a gap at atomic number 72, and many expected a rare-earth metal to fill it. Niels Bohr reasoned from his model of the atom that this element should belong to the fourth group and resemble zirconium. At Bohr's institute in Copenhagen, Dirk Coster and George de Hevesy examined zircon by X-ray spectroscopy and found the lines that had been predicted. They named the element after Hafnia, the Latin name of Copenhagen.
The hardest step in production is the separation from zirconium. The two kinds of atom are almost the same size, and their compounds behave almost identically. Their compounds are therefore separated by elaborate methods such as liquid-liquid extraction or distillation. Hafnium appears as a by-product and is then reduced to the metal, usually with magnesium.
Properties and reactions
Hafnium is in period 6 and group 4, below zirconium, which belongs to period 5. In its period it comes right after the lanthanides, which the periodic table usually draws as a separate row underneath. Its electron configuration is [Xe] 4f¹⁴ 5d² 6s² and its electronegativity is 1.3. The metal has a density of 13.3 g/cm³, melts at 2233 °C and boils at 4603 °C.
That hafnium is almost exactly the size of zirconium, despite all its extra electrons, is the work of the lanthanide contraction. From lanthanum to lutetium, fourteen electrons are added to the 4f subshell. They shield the growing nuclear charge poorly, so the outer electrons are pulled in ever more strongly. The atoms shrink, and by the end the radius of hafnium comes very close to that of its neighbour in period 5.
In compounds hafnium almost always has oxidation number plus four. Its most important oxide, HfO₂, is a white solid that withstands great heat. In air the metal develops a thin, firm oxide skin that protects it from corrosion. Acids and alkalis hardly touch it, and above all hydrofluoric acid dissolves it. The compact metal is considered of low toxicity. Fine powder, however, can ignite by itself in air and is a fire hazard.
Uses in everyday life and technology
In pressurised-water reactors, control rods steer the chain reaction by soaking up surplus neutrons. Hafnium is very good at this because several of its isotopes capture neutrons and turn into new isotopes that absorb as well, so the effect lasts a long time. It is found above all in the reactors of nuclear submarines. The cladding tubes around the fuel are made of zirconium alloys instead, precisely because zirconium barely absorbs neutrons.
The second major use is electronics. In a transistor, a wafer-thin insulating layer separates the gate from the channel. For decades it was made of silicon dioxide, but as the structures shrank it became only a few atoms thick, and current leaked straight through. Hafnium oxide has a much higher dielectric constant and is called a high-k dielectric. The layer can be thicker and still act like a very thin one, so less current is lost. Chip makers have used hafnium oxide in processors since 2007.
Other applications include superalloys for turbines and rocket nozzles, and electrodes in plasma torches for cutting metal. Hafnium carbide and mixed carbides with tantalum rank among the substances with the highest known melting points.
Quick question
What is the atomic number of hafnium (Hf)?
FAQ
What is hafnium used for?
Mainly for control rods in nuclear reactors, where it absorbs neutrons, and as hafnium oxide in the transistors of microchips.
Why are hafnium and zirconium so hard to separate?
The atoms of both metals are almost the same size, because the lanthanide contraction shrinks the atoms before hafnium. As ions they also carry the same charge. Their compounds dissolve almost equally well, so only elaborate multi-stage processes can tell them apart.
Why is hafnium called hafnium?
The name goes back to Hafnia, the Latin word for Copenhagen. The element was identified there at the institute of Niels Bohr.