72 Hf 178.49

Hafnium

Transition metal Hafnium solid

[Xe] 4f¹⁴ 5d² 6s² · 2 · 8 · 18 · 32 · 10 · 2

← Lu Lutetium Ta Tantalum →

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: no data

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.