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Element Fact
Selenium is an essential trace element in the diets that acts as antioxidants.

Helium, He

helium sample

Atomic number: 2
Atomic mass: 4.002602(2)
Natural abundance: 0.003 ppm
3He (3.01602931) - 0.000137%
4He (4.00260325) - 99.999863%

History and Production

From Greek word helios, the sun. The element first detected by Janssen during the solar eclipse of 1868 when a new yellow line was observed in the spectrum of the sun's chromosphere. J.N. Lockyer and E. Frankland suggested the existence of a new element. In 1895, Ramsey discovered helium in the uranium mineral clevite, and it was independently discovered in clevite by the Swedish chemists Cleve and Langlet about the same time. Helium is extracted from natural gas which contains at least trace quantities of helium. It has its use in arc welding as an inert gas shield; a cooling medium for nuclear reactor; artifial atmosphere for divers (mixing with oxygen) and also in cryogenic research.

Physical Data

Colorless and odorless gas, helium is the most abundant element in the universe, after hydrogen. Helium has the lowest melting point of any element which is close to absolute zero (-273C). It is the only liquid that cannot be solidified by lowering the temperature, but can be solidified by increasing the pressure. Both isotopes in solid form can be changed in volume by more than 30% by application of pressure. The liquid helium-4 possesses the most unusual physocal properties that defy common sense. Below 2.18 K at 1 atmospheric pressure, the liquid has a very high thermal conductivity and expands on cooling. It is also a superfluid that can flow, against the gravity, up the wall of an open container and down the outside.

Interatomic distance: 280.0 pm (van der Waal diameter)
Melting point: -272.2C (26 atm)
Boiling point: -268.93C
Thermal conductivity/Wm-1K-1: 0.152 (27C), 0.142 (0C)
Density/kgm-3: 124.8 (b.p.), 0.1785 (0C)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 0 kJ/mol
Gibbs free energy of formation: -
Entropy: 126.2 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: 1s2 = [He]
Term symbol: 1S0
Electron affinity: (not stable)   Electronegativity (Pauline): -
Ionization energy (first, second, third): 2372.3, 5250.4, - kJ/mol

Chemical property

Helium's outer-most electron shell has already achieved the most stable configuration and hence the atom is particularly resistant to chemical reaction. In fact, no known chemical compound of helium has been prepared so far. However, under extreme temperatures and pressures, helium can initiate a thermnuclear (fusion) reaction known as the helium burning. It is believed to occur in the core of gaint stars at around 108 K and after which the supply of hydrogen is exhausted. At this temperature, the following reactions occur:

helium burning

The net effect is the use of 3 helium to produce one carbon nucleus in excited states. After that, heavier nuclei (such as 16O, 20Ne etc.) can be synthesized within the core. For instance, fusion of another helium nuclei with 12C to give 16O and 7.15 MeV of energy is released.

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