An inert gaseous element. The atom is characterised by possessing two protons in its nucleus. Usually there two neutrons as well. This abundant form of helium is about four times as heavy as hydrogen. Helium accounts for just under one quarter of the atomic matter in the universe and was principally formed during the first few minutes after the big bang. The isotope, helium-3 very short lived with only 1 neutron, is also important in nucleosynthesis (of elements). Helium is also produced by the nuclear fusion which takes place at the centre of stars on the main sequence.


In the sun (hence its name) by Norman Lockyer studying the spectral lines in 1868.

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Atomic number 2 Clip
Mass number 4 Clip
Atomic / Molecular Weight 4.002602 gmol-1Clip
Density 0.166 kgm-3Clip
Lattice constant 356 fmClip
Melting Point 3.5 KClip
Boiling Point 4.22 KClip
Specific heat capacity 0.00519 Jkg-1K-1Clip
Latent heat of evaporation 25000 Jkg-1Clip
Ratio of Specific Heats 1.667 Clip
Thermal conductivity 0.1415 Wm-1K-1Clip
Refractive index 1.000036 at 589.3nm 101.3kPa 0CClip
Velocity of sound 972.5ms-1 @ 0C ms-1Clip
Viscosity 0.0000186 Nsm-2Clip
Kinematic Viscosity 1.12048192771084E-04 m2s-1Clip
Critical Temperature 5.3 KClip
Critical Pressure 230000 PaClip
Critical Volume 0.000058 m3mol-1Clip
de Broglie wavelength 0.000000000081 mClip
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See also: Alpha Particle, Helium Burning, Noble Gases, Periodic Table.

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Subjects: Chemistry