Hafnium

Hafnium has the configuration [Xe]4f14 5d2 6s2 and sits directly below zirconium in group 4. Normally an element is clearly larger than the one above it, but between zirconium and hafnium lie the fourteen lanthanides, whose poorly shielding 4f electrons pull the outer shells inward. This lanthanide contraction makes the Hf4+ and Zr4+ ions almost exactly the same size, so the two elements are among the hardest pair in the periodic table to separate. Every zirconium mineral contains a few percent of hafnium.

The twins differ sharply in one respect: hafnium absorbs thermal neutrons about 600 times more strongly than zirconium. Zirconium used to clad nuclear fuel must therefore have its hafnium removed, and that hafnium is put to work in control rods. Hafnium also forms a very stable oxide, HfO2, whose high dielectric constant let Intel shrink transistors in 2007 without electrical leakage. Its carbide, HfC, melts at about 3900 °C, among the highest of any compound.

Discovery

In 1923 Dirk Coster and George de Hevesy, working at Niels Bohr's institute in Copenhagen, found element 72 by X-ray spectroscopy in Norwegian zircon. Bohr's new atomic theory had predicted that element 72 would resemble zirconium rather than the rare earths, which is why they searched zirconium ores. They named it after Hafnia, the Latin name for Copenhagen. The discovery strongly supported Bohr's model of electron shells.