Hafnium

Hafnium, 72Hf
Hf-crystal bar.jpg
Hafnium
Pronunciationm/ (HAF-nee-əm)
Appearancesteel gray
Standard atomic weight Ar, std(Hf)178.49(2)[1]
Hafnium in the periodic table
HydrogenHelium
LithiumBerylliumBoronCarbonNitrogenOxygenFluorineNeon
SodiumMagnesiumAluminiumSiliconPhosphorusSulfurChlorineArgon
PotassiumCalciumScandiumTitaniumVanadiumChromiumManganeseIronCobaltNickelCopperZincGalliumGermaniumArsenicSeleniumBromineKrypton
RubidiumStrontiumYttriumZirconiumNiobiumMolybdenumTechnetiumRutheniumRhodiumPalladiumSilverCadmiumIndiumTinAntimonyTelluriumIodineXenon
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon
FranciumRadiumActiniumThoriumProtactiniumUraniumNeptuniumPlutoniumAmericiumCuriumBerkeliumCaliforniumEinsteiniumFermiumMendeleviumNobeliumLawrenciumRutherfordiumDubniumSeaborgiumBohriumHassiumMeitneriumDarmstadtiumRoentgeniumCoperniciumNihoniumFleroviumMoscoviumLivermoriumTennessineOganesson
Zr

Hf

Rf
lutetiumhafniumtantalum
Atomic number (Z)72
Groupgroup 4
Periodperiod 6
Blockd-block
Element category  Transition metal
Electron configuration[Xe] 4f14 5d2 6s2
Electrons per shell2, 8, 18, 32, 10, 2
Physical properties
Phase at STPsolid
Melting point2506 K ​(2233 °C, ​4051 °F)
Boiling point4876 K ​(4603 °C, ​8317 °F)
Density (near r.t.)13.31 g/cm3
when liquid (at m.p.)12 g/cm3
Heat of fusion27.2 kJ/mol
Heat of vaporization648 kJ/mol
Molar heat capacity25.73 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)268929543277367941944876
Atomic properties
Oxidation states−2, +1, +2, +3, +4 (an amphoteric oxide)
ElectronegativityPauling scale: 1.3
Ionization energies
  • 1st: 658.5 kJ/mol
  • 2nd: 1440 kJ/mol
  • 3rd: 2250 kJ/mol
Atomic radiusempirical: 159 pm
Covalent radius175±10 pm
Color lines in a spectral range
Spectral lines of hafnium
Other properties
Natural occurrenceprimordial
Crystal structurehexagonal close-packed (hcp)
Hexagonal close packed crystal structure for hafnium
Speed of sound thin rod3010 m/s (at 20 °C)
Thermal expansion5.9 µm/(m·K) (at 25 °C)
Thermal conductivity23.0 W/(m·K)
Electrical resistivity331 nΩ·m (at 20 °C)
Magnetic orderingparamagnetic[2]
Magnetic susceptibility+75.0·10−6 cm3/mol (at 298 K)[3]
Young's modulus78 GPa
Shear modulus30 GPa
Bulk modulus110 GPa
Poisson ratio0.37
Mohs hardness5.5
Vickers hardness1520–2060 MPa
Brinell hardness1450–2100 MPa
CAS Number7440-58-6
History
Namingafter Hafnia. Latin for: Copenhagen, where it was discovered
PredictionDmitri Mendeleev (1869)
Discovery and first isolationDirk Coster and George de Hevesy (1922)
Main isotopes of hafnium
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
172Hfsyn1.87 yε172Lu
174Hf0.16%2×1015 yα170Yb
176Hf5.26%stable
177Hf18.60%stable
178Hf27.28%stable
178m2Hfsyn31 yIT178Hf
179Hf13.62%stable
180Hf35.08%stable
182Hfsyn8.9×106 yβ182Ta
| references

Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869, though it was not identified until 1923, by Coster and Hevesy, making it the last stable element to be discovered. Hafnium is named after Hafnia, the Latin name for Copenhagen, where it was discovered.[4][5]

Hafnium is used in filaments and electrodes. Some semiconductor fabrication processes use its oxide for integrated circuits at 45 nm and smaller feature lengths. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten.

Hafnium's large neutron capture cross section makes it a good material for neutron absorption in control rods in nuclear power plants, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant zirconium alloys used in nuclear reactors.

Characteristics

Physical characteristics

Pieces of hafnium

Hafnium is a shiny, silvery, ductile metal that is corrosion-resistant and chemically similar to zirconium[6] (due to its having the same number of valence electrons, being in the same group, but also to relativistic effects; the expected expansion of atomic radii from period 5 to 6 is almost exactly cancelled out by the lanthanide contraction). The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity.[6]

A notable physical difference between these metals is their density, with zirconium having about one-half the density of hafnium. The most notable nuclear properties of hafnium are its high thermal neutron capture cross section and that the nuclei of several different hafnium isotopes readily absorb two or more neutrons apiece.[6] In contrast with this, zirconium is practically transparent to thermal neutrons, and it is commonly used for the metal components of nuclear reactors – especially the cladding of their nuclear fuel rods.

Chemical characteristics

Hafnium dioxide

Hafnium reacts in air to form a protective film that inhibits further corrosion. The metal is not readily attacked by acids but can be oxidized with halogens or it can be burnt in air. Like its sister metal zirconium, finely divided hafnium can ignite spontaneously in air. The metal is resistant to concentrated alkalis.

The chemistry of hafnium and zirconium is so similar that the two cannot be separated on the basis of differing chemical reactions. The melting points and boiling points of the compounds and the solubility in solvents are the major differences in the chemistry of these twin elements.[7]

Isotopes

At least 34 isotopes of hafnium have been observed, ranging in mass number from 153 to 186.[8][9] The five stable isotopes are in the range of 176 to 180. The radioactive isotopes' half-lives range from only 400 ms for 153Hf,[9] to 2.0 petayears (1015 years) for the most stable one, 174Hf.[8]

The nuclear isomer 178m2Hf was at the center of a controversy for several years regarding its potential use as a weapon.

Occurrence

Zircon crystal (2×2 cm) from Tocantins, Brazil

Hafnium is estimated to make up about 5.8 ppm of the Earth's upper crust by mass. It does not exist as a free element on Earth, but is found combined in solid solution with zirconium in natural zirconium compounds such as zircon, ZrSiO4, which usually has about 1–4% of the Zr replaced by Hf. Rarely, the Hf/Zr ratio increases during crystallization to give the isostructural mineral hafnon (Hf,Zr)SiO4, with atomic Hf > Zr.[10] An obsolete name for a variety of zircon containing unusually high Hf content is alvite.[11]

A major source of zircon (and hence hafnium) ores is heavy mineral sands ore deposits, pegmatites, particularly in Brazil and Malawi, and carbonatite intrusions, particularly the Crown Polymetallic Deposit at Mount Weld, Western Australia. A potential source of hafnium is trachyte tuffs containing rare zircon-hafnium silicates eudialyte or armstrongite, at Dubbo in New South Wales, Australia.[12]

Hafnium reserves have been infamously estimated to last under 10 years by one source if the world population increases and demand grows.[13] In reality, since hafnium occurs with zirconium, hafnium can always be a byproduct of zirconium extraction to the extent that the low demand requires.