Helium

Helium, ₂He

Helium
Pronunciation	/ˈhiːliəm/ ​(HEE-lee-əm)
Appearance	colorless gas, exhibitin a reid-orange glow when placed in a heich-voltage electric field
Standard atomic weight Ar, std(He)	7000400260200000000♠4.002602(2)[1]
Helium in the periodic cairt
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Airn Cobalt Nickel Capper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Siller (element) Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gowd Mercur (element) Thallium Leid (element) Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Ununtrium Flerovium Ununpentium Livermorium Ununseptium Ununoctium - ↑ He ↓ Ne hydrogen ← helium → lithium
Atomic nummer (Z)	2
Group	group 18 (noble gases)
Period	period 1
Block	s-block
Element category	Noble gas
Electron confeeguration	1s2
Electrons per shell	2
Pheesical properties
Phase at STP	gas
Meltin pynt	(at 2.5 MPa) 0.95 K ​(−272.20 °C, ​−457.96 °F)
Bylin pynt	4.22 K ​(−268.93 °C, ​−452.07 °F)
Density (at STP)	0.1786 g/L
when liquid (at m.p.)	0.145 g/cm3
whan liquid (at b.p.)	0.125 g/cm3
Creetical pynt	5.19 K, 0.227 MPa
Heat o fusion	0.0138 kJ/mol
Heat o vapourisation	0.0829 kJ/mol
Molar heat capacity	5R/2 = 20.786 J/(mol·K)
Vapour pressur (defined bi ITS-90) P (Pa) 1 10 100 1 k 10 k 100 k at T (K)     1.23 1.67 2.48 4.21
Atomic properties
Oxidation states	0
Electronegativity	Pauling scale: no data
Covalent radius	28 pm
Van der Waals radius	140 pm
Spectral lines o helium
Ither properties
Naitural occurrence	primordial
Creestal structur	​hexagonal close-packed (hcp)
Speed o soond	972 m/s
Thermal conductivity	0.1513 W/(m·K)
Magnetic orderin	diamagnetic[2]
CAS Nummer	7440-59-7
History
Diskivery	Pierre Janssen, Norman Lockyer (1868)
First isolation	William Ramsay, Per Teodor Cleve, Abraham Langlet (1895)
Main isotopes o helium
Iso­tope Abun­dance Hauf-life (t1/2) Decay mode Pro­duct 3He 0.000137%* stable 4He 99.999863%* stable
Atmospheric value, abundance mey differ elsewhaur.
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Helium is a chemical element wi seembol He an atomic nummer 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heids the noble gas group in the periodic table. Its boilin an meltin pynts are the lawest amang the elements an it exists anly as a gas except in extreme condeetions.

Helium is the seicont lichtest element an is the seicont maist abundant element in the observable universe, bein present at aboot 24% o the tot elemental mass, which is mair nor 12 times the mass o aw the hivier elements combined. Its abundance is seemilar tae this figure in the Sun an in Jupiter. This is due tae the very heich nuclear bindin energy (per nucleon) o helium-4 wi respect tae the next three elements efter helium. This helium-4 bindin energy an aa accoonts for why it is a product o baith nuclear fusion an radioactive decay. Maist helium in the universe is helium-4, an is believed tae hae been fairmed durin the Big Bang. Lairge amoonts o new helium are bein creatit bi nuclear fusion o hydrogen in starns.

Helium is named for the Greek God o the Sun, Helios. It wis first detectit as an unkent yellae spectral line signatur in sunlicht durin a solar eclipse in 1868 bi French astronomer Jules Janssen. Janssen is jyntly creditit wi detectin the element alang wi Norman Lockyer. Jannsen observed durin the solar eclipse o 1868 while Lockyer observed frae Breetain. Lockyer wis the first tae propose that the line wis due tae a new element, which he named. The formal discovery o the element wis made in 1895 bi twa Swadish chemists, Per Teodor Cleve an Nils Abraham Langlet, who foond helium emanatin frae the uranium ore cleveite. In 1903, muckle reserves o helium wur foond in naitural gas fields in pairts o the Unitit States, which is bi far the mucklest supplier o the gas the day.

Helium is uised in cryogenics (its mucklest single uise, absorbin aboot a quarter o production), parteecularly in the cuilin o superconductin magnets, wi the main commercial application bein in MRI scanners. Helium's ither industrial uises—as a pressurizin an purge gas, as a protective atmosphere for arc waldin an in processes sic as growin crystals tae mak silicon wafers—accoont for hauf o the gas produced. A well-kent but minor uise is as a liftin gas in balloons an airships.^[3] As wi ony gas wi differin density frae air, inhalin a smaa volume o helium temporarily chynges the timbre an quality o the human vyce. In scienteefic research, the behaviour o the twa fluid phases o helium-4 (helium I an helium II), is important tae researchers studyin quantum mechanics (in parteecular the property o superfluidity) an tae those leukin at the phenomena, sic as superconductivity, that temperaturs near absolute zero produce in matter.

On Yird it is relatively rare—0.00052% bi volume in the atmosphere. Maist terrestrial helium present the day is creatit bi the naitural radioactive decay o hivy radioactive elements (thorium an uranium, awtho thare are ither examples), as the alpha particles emitted bi sic decays conseest o helium-4 nuclei. This radiogenic helium is trapped wi naitural gas in concentrations up tae 7% bi volume, frae which it is extractit commercially bi a law-temperatur separation process cried fractional distillation.

1. ↑ Meija, Juris; 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.
2. ↑ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
3. ↑ Helium: Up, Up and Away? Melinda Rose, Photonics Spectra, Oct. 2008. Accessed Feb 27, 2010. For a more authoritative but older 1996 pie chart showing U.S. helium use by sector, showing much the same result, see the chart reproduced in "Applications" section of this article.