Science / Periodic Table of Elements / Hafnium (Hf): Atomic number: 72, Atomic mass: 178.49 g.mol -1, Electronegativity: 1.3, Density: 13.07 g.cm-3 at 20°C, Melting point: 2200 °C, Boiling point: 5200 °C, Vanderwaals radius: 0.161 nm, Ionic radius: 0.075 nm, Isotopes: 10, Electronic shell: [ Xe ] 4f14 5d2 6s2, Energy of first ionisation: 530 kJ.mol -1, Energy of second ionisation: 1425.5 kJ.mol -1, Energy of third ionisation: 2244.3 kJ.mol -1, Energy of fourth ionisation: 3207.5 kJ.mol -1, Standard potential: - 1.68 V ( HfO2+/ Hf ), Discovered by: Dirk Coster in 1923. Hafnium is a lustrous, silvery, ductile metal. It restists corrosion due to formation of a tough, impenetrable oxide film on its surface. The metal is unaffected by alkalis and acids, except hydrofluoric acid. Hafnium is difficult to separate form its group 4 partner, zirconium, bacause the two elements have atoms that are the same size. Applications: Hafnium and its alloys are used for control rods in nuclear reactors and nuclear submarines because hafnium is excellent at absorbing neutrons and it has a very high melting point and is corrosion resistant. It is used in high-temperature alloys and ceramics, since some of its compounds are very refractory: they will not melt except under the most extreme temperatures. Hafnium in the enviroment: Hafnium ores are rare, but two are known: hafnon and alvite. Industrial production of hafnium metal is not much more than 50 tonnes a year. Known reserves are not recorded, but can be estimated from those of zirconium.