Helium

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.

Symbol

He

Discovered

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

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Atomic number	2		Clip
Mass number	4		Clip
Atomic / Molecular Weight	4.002602	gmol-1	Clip
Density	0.166	kgm-3	Clip
Lattice constant	356	fm	Clip
Melting Point	3.5	K	Clip
Boiling Point	4.22	K	Clip
Specific heat capacity	0.00519	Jkg-1K-1	Clip
Latent heat of evaporation	25000	Jkg-1	Clip
Ratio of Specific Heats	1.667		Clip
Thermal conductivity	0.1415	Wm-1K-1	Clip
Refractive index	1.000036	at 589.3nm 101.3kPa 0°C	Clip
Velocity of sound	972.5ms-1 @ 0°C	ms-1	Clip
Viscosity	0.0000186	Nsm-2	Clip
Kinematic Viscosity	1.12048192771084E-04	m2s-1	Clip
Critical Temperature	5.3	K	Clip
Critical Pressure	230000	Pa	Clip
Critical Volume	0.000058	m3mol-1	Clip
de Broglie wavelength	0.000000000081	m	Clip

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See also: Alpha Particle, Helium Burning, Noble Gases, Periodic Table.