Hafnium (Hf)

Hafnium and zirconium sit directly above and below each other and are so alike that separating them is one of the harder industrial separations there is — yet the separation is essential, because zirconium is transparent to neutrons and hafnium absorbs them greedily. Reactor cladding must be hafnium-free zirconium; reactor control rods are the hafnium that was taken out. Since about 2007 every leading-edge processor has used a hafnium oxide gate dielectric, because silicon dioxide had been thinned to a few atoms and was leaking; hafnium was what let Moore's law continue past 45 nm.

Properties

Atomic number

72

Symbol

Hf

Standard atomic weight

178.49 u

Category

Transition metal

Group

4

Period

6

Block

d-block

Electron configuration

[Xe] 6s² 4f¹⁴ 5d²

Electrons per shell

2, 8, 18, 32, 10, 2

Melting point

2,506 K — 2232.8 °C — 4051.1 °F

Boiling point

4,876 K — 4602.9 °C — 8317.1 °F

Density

13.3 g/cm³

Electronegativity

1.3 (Pauling)

Oxidation states

+4

Discovered

1923 by Dirk Coster and George de Hevesy

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d²

Noble gas shorthand

[Xe] 6s² 4f¹⁴ 5d²

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 10 · P: 2

What it is used for

• Control rods in naval reactors, where neutron absorption matters most
• Hafnium oxide as the high-k gate dielectric in modern processors
• Nickel superalloys for turbine blades
• Plasma cutting torch electrodes

CIAAW gives 178.486; 178.49 is the abridged value. Named after Hafnia, the Latin name for Copenhagen, where it was found in Bohr's institute — and its discovery confirmed Bohr's prediction that element 72 would be a transition metal rather than a rare earth.

Neighbours in the table

Group 4

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