Two dozen years before halfnium was discovered and identified, its existence was already predicted by Dmitri Mendeleev. The same Russian scientist also predicted that this element will have properties that are similar to another metallic element, the zirconium. In the early 1900s, Niels Bohr, a scientist who agreed with Mendeleev’s predictions, used his own atomic theory to predict the electron structure of a still-undiscovered element that is similar to zirconium. Then, in 1923, when the Dutch physicist Dirk Coster and the Hungarian-Swedish chemist George Charles von Hevesy were working in Bohr’s laboratory in Norway, they discovered halfnium. They found halfnium while conducting an x-ray spectroscopy on an ore of zirconium. The name is taken from “Hafnia,” the Latin word for Copenhagen, the capital of Norway.
Today, it is already well-known by the scientific community that any zirconium mineral will contain about 2 to 5% of halfnium. Similar to zirconium, halfnium is metallic with a shiny silver gray color. The estimated atomic radius of an atom of halfnium (0.71 Angstrom) is also similar to that zirconium (0.72 Angstrom). The lack of atomic radii difference is primarily the reason why the separation halfnium from zirconium is an energy-intensive process. Fortunately, the density of halfnium (13.3 g/cc) is about twice the density of zirconium (6.52 g/cc). Solvent extraction processes and repetitive crystallization of potassium fluorides are some of the known methods of separating halfnium from zirconium. The purest halfnium metal, (with a little bit zirconium as an impurity) can be produced by conducting the Kroll process, in which the tetrachloride compound of halfnium undergoes a chemical reduction reaction using magnesium or sodium.
Located in period 6 and group 4 of the periodic table, halfnium is ductile and can conduct electricity. It is solid at room temperatures but when it is in its powder form, it can catch fire and react explosively with air. Some of halfnium’s properties include:
Atomic number: 72
Atomic weight: 178.49
Melting point: 2,506 K
Boiling point: 4,876 K
Oxidation state: +4
Standard potential : -1.68 V
Electronegativity: 1.3 (Pauling)
Hardness: 5.5 (Moh’s scale)
Although powdered halfnium is dangerous and a health hazard to the eyes, skin, and liver, the solid alloyed halfnium is very stable and useful. It does not corrode because it forms a thin but impenetrable outer layer of oxide. It also has the unique ability of absorbing neutrons. This is why halfnium is used as the primary material for the control rods of nuclear reactors inside nuclear-armed submarines. Halfnium is also utilized in producing alloys and ceramics at high temperatures and in removing oxygen and nitrogen gases from vacuum tubes.