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Spectral lines of helium
General properties
Name, symbol, number helium, He, 2
Pronunciation /ˈhliəm/ HEE-lee-əm
Element category noble gases
Group, period, block 181, s
Standard atomic weight 4.002602(2)g·mol−1
Electron configuration 1s2
Electrons per shell 2 (Image)
Physical properties
Phase gas
Density (0 °C, 101.325 kPa)
0.1786 g/L
Liquid density at m.p. 0.145 g·cm−3
Liquid density at b.p. 0.125 g·cm−3
Critical point 5.19 K, 0.227 MPa
Heat of fusion 0.0138 kJ·mol−1
Heat of vaporization 0.0829 kJ·mol−1
Vapor pressure (defined by ITS-90)
at T (K)     1.23 1.67 2.48 4.21
Atomic properties
Oxidation states 0
Electronegativity no data (Pauling scale)
Ionization energies 1st: 2372.3 kJ·mol−1
2nd: 5250.5 kJ·mol−1
Covalent radius 28 pm
Van der Waals radius 140 pm
Crystal structure hexagonal close-packed
Magnetic ordering diamagnetic[1]
Speed of sound 972 m/s
CAS registry number 7440-59-7
Most stable isotopes
Main article: Isotopes of helium
iso NA half-life DM DE (MeV) DP
3He 0.000137%* 3He is stable with 1 neutron
4He 99.999863%* 4He is stable with 2 neutrons
*Atmospheric value, abundance may differ elsewhere.

Helium is a chemical element. It has the chemical symbol He, atomic number 2 and atomic weight of about 4. There are 9 isotopes of helium, only two of which are stable. These are 3He and 4He. 4He is by far the most common isotope.


Helium is used to fill balloons and airships because it is lighter than air, and does not burn or react, meaning it is normally safe for using it in that way. It is also used in some kinds of light bulbs. People also breathe it in to make their voices sound higher than they normally do as a joke, but this is extremely dangerous if they breathe in too much, since it can kill them because they are not breathing normal air. Breathing too much helium can also cause long-term effects to vocal cords.

It can be created through the process of nuclear fusion in the sun (we are currently trying to harness nuclear fusion as a form of power but this is still at an early stage because the sun can quantum tunnel enough because of it's huge size). During this process, four Hydrogen atoms are fused together to form one Helium atom.

