Helium is a chemical element. It has the chemical symbol He, atomic number 2, and atomic weight of about 4.002602. 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.
|Appearance||colorless gas, exhibiting a red-orange glow when placed in an electric field|
|Standard atomic weight (Ar, standard)||4.002602(2)|
|Helium in the periodic table|
|Atomic number (Z)||2|
|Group||group 18 (noble gases)|
|Element category||noble gas|
Electrons per shell
|Phase at STP||gas|
|Melting point||0.95 K (−272.20 °C, −457.96 °F) (at 2.5 MPa)|
|Boiling point||4.222 K (−268.928 °C, −452.070 °F)|
|Density (at STP)||0.1786 g/L|
|when liquid (at m.p.)||0.145 g/cm3|
|when liquid (at b.p.)||0.125 g/cm3|
|Triple point||2.177 K, 5.043 kPa|
|Critical point||5.1953 K, 0.22746 MPa|
|Heat of fusion||0.0138 kJ/mol|
|Heat of vaporization||0.0829 kJ/mol|
|Molar heat capacity||20.78 J/(mol·K)|
|Vapor pressure (defined by ITS-90)|
|Electronegativity||Pauling scale: no data|
|Covalent radius||28 pm|
|Van der Waals radius||140 pm|
|Spectral lines of helium|
|Crystal structure||hexagonal close-packed (hcp)|
|Speed of sound||972 m/s|
|Thermal conductivity||0.1513 W/(m·K)|
|Magnetic susceptibility||−1.88·10−6 cm3/mol (298 K)|
|Naming||after Helios, Greek Titan of the Sun|
|Discovery||Pierre Janssen, Norman Lockyer (1868)|
|First isolation||William Ramsay, Per Teodor Cleve, Abraham Langlet (1895)|
|Main isotopes of helium|
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 red-orange glow when placed in an electric field. Helium does not usually react with anything else. Astronomers detected the presence of helium in 1868, when its spectrum was identified in light from the Sun. This was before its discovery on Earth.
Helium is used to fill balloons and airships because its density is lighter than air. It does not burn, so is safe for that kind of use. It is also used in some kinds of light bulbs. People can breathe in helium: it makes their voices sound higher than it normally does. This is a joke, but is dangerous as if they breathe in too much, hypoxia can injure or kill them as they are not breathing normal air. Breathing too much helium can also cause long-term effects to vocal cords.
Helium is created through the process of nuclear fusion in the Sun, and in similar stars. During this process, four hydrogen atoms are fused together to form one helium atom. On Earth it is made by the natural radioactive decay of heavy radioactive elements like thorium and uranium, although there are other examples. The alpha particles emitted by such decays consist of helium-4 nuclei.
Helium has become rare on Earth. If it gets free into the air it leaves the planet. Unlike hydrogen, which reacts with oxygen to form water, helium is not reactive. It stays as a gas. For many years after the 1925 Helium Act, the USA collected helium in a National Helium Reserve. American helium comes from wells in the great plains area. At present, more helium is supplied by Qatar than by the USA.
Several research organisations have released statements on the scarcity and conservation of helium. These organisations released policy recommendations as early as 1995 and as late as 2016 urging the United States government to store and conserve helium because of the natural limits to the helium supply and the unique nature of the element. For researchers, helium is irreplaceable because it is essential for producing very low temperatures. Helium at low temperatures is used in cryogenics, and in certain cryogenics applications. Liquid helium is used to cool certain metals to the extremely low temperatures required for superconductivity, such as in superconducting magnets for magnetic resonance imaging.
- Meija, J.; Coplen, T. B.; Berglund, M.; Brand, W.A.; De Bièvre, P.; Gröning, M.; Holden, N.E.; Irrgeher, J. et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry 88 (3): 265-91. doi:10.1515/pac-2015-0305. https://www.degruyter.com/downloadpdf/j/pac.2016.88.issue-3/pac-2015-0305/pac-2015-0305.xml.
- Shuen-Chen Hwang, Robert D. Lein, Daniel A. Morgan (2005). "Noble Gases". Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383. doi:10.1002/0471238961.0701190508230114.a01.
- Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
- Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
- Based on this location, its name was derived from the Greek word for Sun, helios.
- American Physical Society (1995). “National Policy”. https://www.aps.org/policy/statements/95_3.cfm
- Epple, Dennis (1982). "The Helium Storage Controversy: Modeling Natural Resource Supply: The complex issue of helium storage provides a case study of the difficult decisions involved in using natural resources". American Scientist.