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General | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Name, Symbol, Number | hafnium, Hf, 72 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Chemical series | transition metals | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Group, Period, Block | 4, 6, d | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Appearance | gray steel |
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Atomic mass | 178.49(2) g/mol | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Electron configuration | [Xe] 4f14 5d2 6s2 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Electrons per shell | 2, 8, 18, 32, 10, 2 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Physical properties | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Phase | solid | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Density (near r.t.) | 13.31 g·cm−3 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Liquid density at m.p. | 12 g·cm−3 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Melting point | 2506 K (2233 °C, 4051 °F) |
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Boiling point | 4876 K (4603 °C, 8317 °F) |
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Heat of fusion | 27.2 kJ·mol−1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Heat of vaporization | 571 kJ·mol−1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Heat capacity | (25 °C) 25.73 J·mol−1·K−1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Atomic properties | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Crystal structure | hexagonal | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Oxidation states | 4 (amphoteric oxide) |
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Electronegativity | 1.3 (Pauling scale) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Ionization energies (more) |
1st: 658.5 kJ·mol−1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2nd: 1440 kJ·mol−1 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
3rd: 2250 kJ·mol−1 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Atomic radius | 155 pm | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Atomic radius (calc.) | 208 pm | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Covalent radius | 150 pm | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Miscellaneous | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Magnetic ordering | no data | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Electrical resistivity | (20 °C) 331 nΩ·m | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Thermal conductivity | (300 K) 23.0 W·m−1·K−1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Thermal expansion | (25 °C) 5.9 µm·m−1·K−1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Speed of sound (thin rod) | (20 °C) 3010 m/s | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Young's modulus | 78 GPa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Shear modulus | 30 GPa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Bulk modulus | 110 GPa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Poisson ratio | 0.37 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Mohs hardness | 5.5 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Vickers hardness | 1760 MPa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Brinell hardness | 1700 MPa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
CAS registry number | 7440-58-6 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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References |
Hafnium (IPA: /ˈhæfniəm/) is a chemical element in the periodic table that has the symbol Hf and atomic number 72. A lustrous, silvery gray tetravalent transition metal, hafnium resembles zirconium chemically and is found in zirconium minerals. Hafnium is used in tungsten alloys in filaments and electrodes and also acts as a neutron absorber in control rods in nuclear power plants.
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Hafnium is a shiny silvery, ductile 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, halfnium 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 3310 °C.[1] This has led to proposals that halfnium or Hf carbides might be useful as construction materials in reactors subjected to very high temperatures, such as fusion reactors (see below).
The metal is resistant to concentrated alkalis, but halogens react with it to form hafnium tetrahalides.[1] 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 2.45 MeV per decay.[2] 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.[3]
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.
Other uses:
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 von Hevesy in 1923 in Copenhagen, Denmark. 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 spectroscope 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 tetraiodide vapor over a heated tungsten filament.
The Faculty of Science of the University of Copenhagen uses in its seal a stylized image of hafnium.
Hafnium 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.
Care needs to be taken when machining hafnium because when it 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).