|Boiling Point: 4876°K, 4603°C, 8317°F
Melting Point: 2500°K, 2227°C, 4041°F
Electrons Energy Level: 2, 8, 18, 32, 10, 2
Isotopes: 28 + 5 Stable
Heat of Vaporization: 575 kJ/mol
Heat of Fusion: 24.06 kJ/mol
Density: 13.31 g/cm3 @ 300°K
Specific Heat: 0.14 J/gK
Atomic Radius: 2.16Å
Ionic Radius: 0.71Å
Electronegativity: 1.3 (Pauling); 1.23 (Allrod Rochow)
Vapor Pressure: 0.00112 Pa @ 2227°C
1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d2 6s2
The 1869 periodic table by Mendeleev had implicitly predicted the existence of a heavier analog of titanium and zirconium, but in 1871 Mendeleev placed lanthanum in that spot.
The existance of a gap in the periodic table for an as-yet undiscovered element 72 was predicted by Henry Moseley in 1914. Hafnium (Latin Hafnia for "Copenhagen", the home town of Niels Bohr) was discovered by Dirk Coster and Georg Charles von Hevesy in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev. Soon after, the new element was predicted to be associated with zirconium by using the Bohr theory and was finally found in zircon through X-ray spectroscopy analysis in Norway.
It was separated from zirconium through repeated recrystallization of double ammonium or potassium fluorides by Jantzen and von Hevesey. Metallic hafnium was first prepared by Anton Eduard van Arkel and Jan Hendrik de Boer by passing hafnium tetraiodide vapor over a heated tungsten filament. This process for differential purification of Zr and Hf is still in use today.
The Faculty of Science of the University of Copenhagen uses in its seal a stylized image of hafnium.
Hafnium is a lustrous, shiny silvery-gray, tetravalent, ductile transition metal that is corrosion resistant and chemically similar to zirconium. The properties of hafnium are markedly affected by zirconium impurities and these two elements are amongst the most difficult to separate. A notable physical difference between them is their density (zirconium is about half as dense as hafnium), but chemically the elements are extremely similar. Nevertheless, separation of them becomes important in the nuclear power industry, as Zr is common fuel-rod cladding alloy material, with the desirable properties of low neutron cross section and high chemical stability at high temperatures. However, because of Hf's neutron absorbing properties, hafnium impurities in zirconium cause it to be far less useful for nuclear reactor materials applications.
Hafnium carbide is the most refractory binary compound known and hafnium nitride is the most refractory of all known metal nitrides with a melting point of 3310oC. This has led to proposals that hafnium or Hf carbides might be useful as construction materials subjected to very high temperatures.
The metal is resistant to concentrated alkalis, but halogens react with it to form hafnium tetrahalides. At higher temperatures hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon.
The nuclear isomer Hf-178-m2 is also a source of cascades of gamma rays whose energies total to 2.45MeV per decay. It is notable because it has the highest excitation energy of any comparably long-lived isomer of any element. One gram of pure Hf-178-m2 would contain approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT. Possible applications requiring such highly concentrated energy storage are of interest. For example, it has been studied as a possible power source for gamma ray lasers.
Hafnium is estimated to make up about 0.00058% of the Earth's upper crust by weight. It is found combined in natural zirconium compounds but it does not exist as a free element in nature. Minerals that contain zirconium, such as alvite [(Hf, Th, Zr) SiO4·H2O, thortveitite and zircon (ZrSiO4), usually contain between 1 and 5% hafnium. Hafnium and zirconium have nearly identical chemistry, which makes the two difficult to separate. About half of all hafnium metal manufactured is produced by a by-product of zirconium refinement. This is done through reducing hafnium (IV) chloride with magnesium or sodium in the Kroll process.
Hafnium is used to make control rods for nuclear reactors because of its ability to absorb neutrons (its thermal neutron absorption cross section is nearly 600 times that of zirconium), excellent mechanical properties and exceptional corrosion-resistance properties.
Melting near 3890°C, hafnium carbide, HfC, has the highest melting point of any known two-element compound. Hafnium nitride, HfN, also has a high melting point, around 3305°C. Other hafnium compounds include: hafnium chloride, HfCl4, hafnium fluoride, HfF4, and hafnium oxide, HfO2.
|Hafnium Chloride, HfCl4||Hafnium Fluoride, HfF4|
|Hafnium Oxide, HfO2||Hafnium Nitride, HfN|
|Hafnium Carbide, HfC|
|Hf174||173.94||2.0E 15 years|
|Hf182||181.9506||9E 6 years|
|Care needs to be taken when machining hafnium because, like its sister metal zirconium, when hafnium is divided into fine particles, it is pyrophoric and can ignite spontaneously in air.|
Compounds that contain this metal are rarely encountered by most people and the pure metal is not normally toxic but all its compounds should be handled as if they are toxic (although there appears to be limited danger to exposed individuals).
|Ionization Energy (eV): 6.825 eV
Estimated Crustal Abundance: 3.0 milligrams per kilogram
Estimated Oceanic Abundance: 7×10-6 milligrams per liter