Hafnium
[Xe] 4f¹⁴ 5d² 6s² · 2 · 8 · 18 · 32 · 10 · 2
Zirconium's twin, differing from it in practically nothing but its attitude to neutrons: zirconium lets them through, hafnium devours them. Reactor engineering is built on that contrast — one makes the cladding, the other the control rods.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 72
- Atomic mass
- 178.49 а.е.м.
- Electron configuration
- [Xe] 4f¹⁴ 5d² 6s²
- Electrons per shell
- 2 · 8 · 18 · 32 · 10 · 2
- Block
- d-block
- Group
- Titanium group
- Period
- 6
- Electronegativity (Pauling)
- 1.3
- Electronegativity (Allen)
- 1.16
- Atomic radius
- 208 pm
- Covalent radius
- 175 pm
- Van der Waals radius
- 212 pm
- Ionisation energy 1
- 658.5 kJ/mol
- Ionisation energy 2
- 1 440 kJ/mol
- Ionisation energy 3
- 2 250 kJ/mol
- Ionisation energy 4
- 3 216 kJ/mol
- Electron affinity
- 17.2 kJ/mol
- Common oxidation states
- +4
- Oxidation states
- -2, +1, +2, +3, +4
Physical properties
- State at 25 °C
- solid
- Density
- 13.31 g/cm³
- Melting point
- 2 506 K · 2 232.9 °C
- Boiling point
- 4 876 K · 4 602.9 °C
- Speed of sound
- 3 010 m/s
Thermal properties
- Heat of fusion
- 27.2 kJ/mol
- Heat of vaporisation
- 648 kJ/mol
- Specific heat capacity
- 0.144 J/(g·K)
- Thermal conductivity
- 23 W/(m·K)
Mechanical properties
- Young's modulus
- 78 GPa
- Shear modulus
- 30 GPa
- Bulk modulus
- 110 GPa
- Poisson ratio
- 0.37
- Mohs hardness
- 5.5
- Brinell hardness
- 1 700 MPa
Electrical and magnetic properties
- Electrical resistivity
- 331 nΩ·m
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- 0.128 K
Crystal structure
- Crystal structure
- hexagonal close-packed (hcp)
- Lattice constants
- a = 319.64 pm · c = 505.11 pm
Abundance
- In the crust
- 3 mg/kg
- In the ocean
- 7·10⁻⁶ mg/L
- In the universe
- 0.7 mg/kg
- In the human body
- 0 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 174Hf | 0.16 % | 2.218·10⁶ Gyr | α |
| 176Hf | 5.26 % | stable | — |
| 177Hf | 18.6 % | stable | — |
| 178Hf | 27.28 % | stable | — |
| 179Hf | 13.62 % | stable | — |
| 180Hf | 35.08 % | stable | — |
Hafnium-178m2 is a nuclear isomer with record stored energy; attempts to release it on demand failed, despite loud claims in the 1990s.
Discovery
- Year of discovery
- 1923
- Discovered by
- Dirk Coster and George de Hevesy
- Where
- Denmark
- Origin of the name
- from Hafnia, the Latin name for Copenhagen
History
Hafnium was the last stable element found in nature, in 1923. Coster and Hevesy in Copenhagen hunted it by X-ray spectra, following Bohr's prediction about atomic structure, and found it in ordinary zirconium ore.
Where it occurs
3 mg/kg of the crust, always with zirconium: zircon holds about 2 % hafnium relative to its zirconium.
How it is produced
Separated from zirconium by multi-stage solvent extraction — the most expensive part of zirconium processing. The metal comes from the Kroll process.
Role in living things
No biological role.
Safety
The metal is harmless. The powder is pyrophoric.
Uses
- Nuclear reactor control rods, especially in naval reactors
- Hafnium oxide as the gate dielectric in modern transistors
- Heat-resistant superalloys for turbine blades
- Plasma cutting electrodes
- Hafnium carbide, among the most refractory substances known at about 4200 K
Curiosities
- Hafnium is named after Copenhagen and was the last stable element discovered in nature.
- Hafnium dioxide replaced silicon oxide in transistor gates and kept Moore's law going after 2007.
- Zirconium and hafnium are chemically almost identical but opposite in neutron behaviour.
- Hafnium carbide alloyed with tantalum carbide has the highest known melting point.