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Element Fact
Iridium is the most corrosion-resistant metal known. Together with platinum, it is used to make the international prototype of the kilogram, which is the standard definition for the unit of mass.

Hafnium, Hf

hafnium sample

Atomic number: 72
Atomic mass: 178.49(2)
Natural abundance: 2.8 ppm
174Hf (173.940040) - 0.162%
176Hf (175.941402) - 5.206%
177Hf (176.943220) - 18.606%
178Hf (177.943698) - 27.297%
179Hf (178.945815) - 13.629%
180Hf (179.946549) - 35.1%

History and Production

From Hafnia, Latin name for Copenhagen. In 1911, G. Urbain claimed to have isolated the new element from a sample of rare earth residue and named it celtium. In 1922 he, together with A. Dauvillier claimed to have spectroscopic evidence to support the discovery. However, the new atomic thoery, developed by Neils Bohr refute such claim. The element number 72 should in fact more likely to be found along with zirconium. In 1923, this was confirmed by D. Coster and G. von Hevesy which used X-ray spectroscopy technique to show that the element was indeed present in zircon (ZrSiO4). Subsequently, it was named hafnium, as the element was correctly discovered and identified in Bohr's Laboratory in Copenhagan. It is usually obtained by repeat crystallization process from zircon that ususally contains about 1-5% of zirconium content. The metal can be obtained by reducing the chloride (HfCl4) with magnesium or with sodium (Kroll process). The extreme mechanical and chemical properties and ability to adsorp neutron has found its use for reactor control rods in nuclear submarines. The nitride and carbide are some of the most refractory materials ever known. It is also used as a getter for scavenging oxygen and nitrogen.

Physical Data

It is a lustrous silvery melting with high melting point. The mechanical properties are greatly affected by impurities (such as oxygen, nitrogen and zirconium), making it difficult to fabricate. Zirconium is invariably present even in pure hafnium. Indeed, both elements are among the most difficult to separate. Hafnium consists of 2.8 ppm of the earth's crustal rocks. It is usually found in zirconium-containing minerals such as zircon, baddekeyite (ZrO2) and alvite (MSiO4.xH2O, M=Zr,Hf and Th). It has a good absorption cross section of thermal neuton and finely divided metal is easily ignited spontaneously in air.

Interatomic distance: 312.8 pm
Melting point: 2233C
Boiling point: 4603C
Thermal conductivity/Wm-1K-1: 23.0 (27C)
Density/kgm-3: 13310 (20C), 12000 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 619.2 kJ/mol
Gibbs free energy of formation: 576.5 kJ/mol
Entropy: 186.9 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Xe] 4f14 5d2 6s2
Term symbol: 3F2
Electron affinity: 0 kJ/mol   Electronegativity (Pauline): 1.3
Ionization energy (first, second, third): 658.52, 1437.63, 2248.11 kJ/mol

Chemical properties

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