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| colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field|
Spectral lines of helium
|Name, symbol, number||helium, He, 2|
|Element category||noble gases|
|Group, period, block||18, 1, s|
|Standard atomic weight||4.002602(2) g·mol−1|
|Electrons per shell||2 (Image)|
|Density|| (0 °C, 101.325 kPa)|
|Liquid density at m.p.||0.145 g·cm−3|
|Liquid density at b.p.||0.125 g·cm−3|
|Melting point||(at 2.5 MPa) 0.95 K, −272.20 °C, −457.96 °F|
|Boiling point||4.22 K, −268.93 °C, −452.07 °F|
|Critical point||5.19 K, 0.227 MPa|
|Heat of fusion||0.0138 kJ·mol−1|
|Heat of vaporization||0.0829 kJ·mol−1|
|Specific heat capacity||(25 °C) 5R/2 = 20.786 J·mol−1·K−1|
|Vapor pressure (defined by ITS-90)|
|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|
|Thermal conductivity||(300 K) 0.1513 W·m−1·K−1|
|Speed of sound||972 m/s|
|CAS registry number||7440-59-7|
|Most stable isotopes|
|Main article: Isotopes of helium|
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 called a noble gas, because it does not regularly mix with other chemicals and form new compounds. It has the lowest boiling point of all the elements. It is the second most common element in the universe, after hydrogen, and has no color or smell. However, Helium has a blue-ish, neon color when being burnt with fire. Helium does not usually react with anything else. Astronomers discovered helium in 1868. They found that it was in the Sun before it was found on Earth. Because of where it was found, its name comes from the Greek word for Sun, helios.
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.
- Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.