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Element Fact
The liquid helium-4 possesses the most unusual physical properties that defy common sense. Below 2.2 K it becomes a superfluid that can flow, against gravity, up the wall of a container.

Uranium, U

uranium sample

Atomic number: 92
Atomic mass: 238.02891(3)
Natural abundance: 2.3 ppm
234U (234.040946) - 0.0055%
235U (235.043923) - 0.72%
238U (238.050783) - 99.2745%

History and Production

The word uranium is derived from the planet Uranus. The element was discovered by M.H. Klaproth while he was examining pitchblende (U3O8) who at first thought that pitchblende to be a mixture of iron, zinc and tungsten oxides ore. He named the new element uranium aften the recently discovered planet, Uranus. The element was first isolated by B. Peligot in 1841, who reduced the anhydrous chloride with potassium. Important sources can be found in pitchblende and also carnotite (K2(UO2)2(VO4)2.3H2O). Detailed metal extractions depend on the nature of the ore. However, the first stage usually involves roasting of the crushed and concentrated ore follows by leaching with sulfuric acid. Final metal extraction can be achieved by reduction of uranium fluoride (UF4) with magnesium at 700C. Very pure metal can be isolated by heating uranium fluoride or chloride on a hot filament. The most significant use of the element is as a nuclear fuel. Natural uranium, which is slightly enriched (0.72%) with 235U isotope can be used in nuclear power reactors for the electricity generation. The isotope can be further concentrated by means of gaseous diffusion, a complicated and expensive process, for nuclear fuel and also as an explosive. Alternatively, if a moderator is used 238U can be converted into fissionable 239Pu. Such process is used in the 'fast breeder' reactors which can extract up to 60 times more energy from the natural uranium. The future use of nuclear energy depends very much on social, political and environmental considerations. Other use of the metal is for X-ray targets for producing high energy X-rays. Uranium salts are also used, since 79 AD, for producing yellow-colored glass and ceramics.

Physical Data

It is a heavy, silvery-white metal. It is pyrophoric when finely divided. It is malleable, ductile and slightly paramagnetic. Naturally occurring uranium contains, by weight, 99.2745% 238U (half-life=4.47 x 109 y), 0.720% 235U (half-life=7.04 x 108 y) and 0.0055% 234U (half-life=2.45 x 105 y). The metal consists of 2.3 ppm of the earth's crust and can be found in a wide varity of minerals: pitchblende, carnotite, autunite, davidite etc.

Interatomic distance: 277 pm
Melting point: 1135C
Boiling point: 4131C
Thermal conductivity/Wm-1K-1: 27.6 (27C)
Density/kgm-3: 18950 (20C), 17907 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 533 kJ/mol
Gibbs free energy of formation: 484.4 kJ/mol
Entropy: 199.8 J/mol K
Heat capacity: 23.7 J/mol K

Electronic data

Electronic configuration: [Rn] 5f3 6d1 7s2
Term symbol: 5L6
Electron affinity: -   Electronegativity (Pauline): 1.7
Ionization energy (first, second, third): 597.635, -, - kJ/mol

Chemical properties

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