helium helium helium (from greek: ἥλιος, romanized: helios, lit. 'sun') is a chemical element with the symbol he and atomic number 2. it is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. its boiling point is the lowest among all the elements, and it does not have a melting point at standard pressure. it is the second lightest and second most abundant element in the observable universe, after hydrogen. it is present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. its abundance is similar to this in both the sun and in jupiter, due to the very high nuclear binding energy (per nucleon) of helium-4, with respect to the next three elements after helium. this helium-4 binding energy also accounts for why it is a product of both nuclear fusion and radioactive decay. the most common isotope of helium in the universe is helium-4, the vast majority of which was formed during the big bang. large amounts of new helium are created by nuclear fusion of hydrogen in stars. helium, 2heheliumpronunciation/ˈhiːliəm/ ​(hee-lee-əm)appearancecolorless gas, exhibiting a gray, cloudy glow (or reddish-orange if an especially high voltage is used) when placed in an electric fieldstandard atomic weight ar°(he)4.002602±0.0000024.0026±0.0001 (abridged) helium in the periodic table 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 iron cobalt nickel copper zinc gallium germanium arsenic selenium bromine krypton rubidium strontium yttrium zirconium niobium molybdenum technetium ruthenium rhodium palladium silver 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 gold mercury (element) thallium lead 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 nihonium flerovium moscovium livermorium tennessine oganesson –↑he↓ne hydrogen ← helium → lithium atomic number (z)2groupgroup 18 (noble gases)periodperiod 1block s-blockelectron configuration1s2electrons per shell2physical propertiesphase at stpgasmelting point0.95 k ​(−272.20 °c, ​−457.96 °f) (at 2.5 mpa)boiling point4.222 k ​(−268.928 °c, ​−452.070 °f) density (at stp)0.1786 g/lwhen liquid (at m.p.)0.145 g/cm3 when liquid (at b.p.)0.125 g/cm3 triple point2.177 k, ​5.043 kpa critical point5.1953 k, 0.22746 mpa heat of fusion0.0138 kj/mol heat of vaporization0.0829 kj/mol molar heat capacity20.78 j/(mol·k) vapor pressure (defined by 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 propertiesoxidation states0electronegativitypauling scale: no data ionization energies1st: 2372.3 kj/mol 2nd: 5250.5 kj/mol covalent radius28 pm van der waals radius140 pm spectral lines of heliumother propertiesnatural occurrenceprimordialcrystal structure ​hexagonal close-packed (hcp)speed of sound972 m/s thermal conductivity0.1513 w/(m⋅k) magnetic orderingdiamagnetic molar magnetic susceptibility−1.88×10−6 cm3/mol (298 k)cas number7440-59-7 historynamingafter helios, greek god of the sundiscoverypierre janssen, norman lockyer (1868)first isolationwilliam ramsay, per teodor cleve, abraham langlet (1895)isotopes of helium main isotopes decay abun­dance half-life (t1/2) mode pro­duct 3he 0.0002% stable 4he 99.9998% stable helium was first detected as an unknown, yellow spectral line signature in sunlight during a solar eclipse in 1868 by georges rayet, captain c. t. haig, norman r. pogson, and lieutenant john herschel, and was subsequently confirmed by french astronomer jules janssen. janssen is often jointly credited with detecting the element, along with norman lockyer. janssen recorded the helium spectral line during the solar eclipse of 1868, while lockyer observed it from britain. lockyer was the first to propose that the line was due to a new element, which he named after the sun. the formal discovery of the element was made in 1895 by chemists sir william ramsay, per teodor cleve, and nils abraham langlet, who found helium emanating from the uranium ore, cleveite, which is now not regarded as a separate mineral species, but as a variety of uraninite. in 1903, large reserves of helium were found in natural gas fields in parts of the united states, by far the largest supplier of the gas today. liquid helium is used in cryogenics (its largest single use, consuming about a quarter of production), and in the cooling of superconducting magnets, with its main commercial application in mri scanners. helium's other industrial uses—as a pressurizing and purge gas, as a protective atmosphere for arc welding, and in processes such as growing crystal to make silicon wafers—account for half of the gas produced. a small but well-known use is as a lifting gas in balloons and airships. as with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre and quality of the human voice. in scientific research, the behavior of the two fluid phases of helium-4 (helium i and helium ii) is important to researchers studying quantum mechanics (in particular the property of superfluidity) and to those looking at the phenomena, such as superconductivity, produced in matter near absolute zero. on earth, it is relatively rare—5.2 ppm by volume in the atmosphere. most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei. this radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation. terrestrial helium is a non-renewable resource because once released into the atmosphere, it promptly escapes into space. its supply is thought to be rapidly diminishing. however, some studies suggest that helium produced deep in the earth by radioactive decay can collect in natural gas reserves in larger than expected quantities, in some cases, having been released by volcanic activity. history scientific discoveries the first evidence of helium was observed on august 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the sun. the line was detected by french astronomer jules janssen during a total solar eclipse in guntur, india. this line was initially assumed to be sodium. on october 20 of the same year, english astronomer, norman lockyer, observed a yellow line in the solar spectrum, which, he named the d3 because it was near the known d1 and d2 fraunhofer line lines of sodium. he concluded that it was caused by an element in the sun unknown on earth. lockyer and english chemist edward frankland named the element with the greek word for the sun, ἥλιος (helios). spectral lines of helium in 1881, italian physicist luigi palmieri detected helium on earth for the first time through its d3 spectral line, when he analyzed a material that had been sublimated during a recent eruption of mount vesuvius. sir william ramsay, the discoverer of terrestrial helium the cleveite sample from which ramsay first purified helium on march 26, 1895, scottish chemist sir william ramsay isolated helium on earth by treating the mineral cleveite (a variety of uraninite with at least 10% rare-earth elements) with mineral acids. ramsay was looking for argon but, after separating nitrogen and oxygen from the gas, liberated by sulfuric acid, he noticed a bright yellow line that matched the d3 line observed in the spectrum of the sun. these samples were identified as helium by lockyer and british physicist william crookes. it was independently isolated from cleveite, in the same year, by chemists, per teodor cleve and abraham langlet, in uppsala, sweden, who collected enough of the gas to accurately determine its atomic weight. helium was also isolated by the american geochemist, william francis hillebrand, prior to ramsay's discovery, when he noticed unusual spectral lines while testing a sample of the mineral uraninite. hillebrand, however, attributed the lines to nitrogen. his letter of congratulations to ramsay offers an interesting case of discovery, and near-discovery, in science. in 1907, ernest rutherford and thomas royds demonstrated that alpha particles are helium nuclei, by allowing the particles to penetrate the thin, glass wall of an evacuated tube, then creating a discharge in the tube, to study the spectrum of the new gas inside. in 1908, helium was first liquefied by dutch physicist heike kamerlingh onnes by cooling the gas to less than 5 k (−268.15 °c; −450.67 °f). he tried to solidify it, by further reducing the temperature, but failed, because helium does not solidify at atmospheric pressure. onnes' student willem hendrik keesom was eventually able to solidify 1 cm3 of helium in 1926 by applying additional external pressure. in 1913, niels bohr published his "trilogy" on atomic structure that included a reconsideration of the pickering–fowler series as central evidence in support of his model of the atom. this series is named for edward charles pickering, who in 1896 published observations of previously unknown lines in the spectrum of the star ζ puppis (these are now known to occur with wolf–rayet and other hot stars). pickering attributed the observation (lines at 4551, 5411, and 10123 å) to a new form of hydrogen with half-integer transition levels. in 1912, alfred fowler managed to produce similar lines from a hydrogen-helium mixture, and supported pickering's conclusion as to their origin. bohr's model does not allow for half-integer transitions (nor does quantum mechanics) and bohr concluded that pickering and fowler were wrong, and instead assigned these spectral lines to ionised helium, he+. fowler was initially skeptical but was ultimately convinced that bohr was correct, and by 1915 "spectroscopists had transferred definitively to helium." bohr's theoretical work on the pickering series had demonstrated the need for "a re-examination of problems that seemed already to have been solved within classical theories" and provided important confirmation for his atomic theory. in 1938, russian physicist pyotr leonidovich kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. this phenomenon is related to bose–einstein condensation. in 1972, the same phenomenon was observed in helium-3, but at temperatures much closer to absolute zero, by american physicists douglas d. osheroff, david m. lee, and robert c. richardson. the phenomenon in helium-3 is thought to be related to pairing of helium-3 fermions to make bosons, in analogy to cooper pairs of electrons producing superconductivity. extraction and use historical marker, denoting a massive helium find near dexter, kansas after an oil drilling operation in 1903 in dexter, kansas produced a gas geyser that would not burn, kansas state geologist erasmus haworth collected samples of the escaping gas and took them back to the university of kansas at lawrence where, with the help of chemists hamilton cady and david mcfarland, he discovered that the gas consisted of, by volume, 72% nitrogen, 15% methane (a combustible percentage only with sufficient oxygen), 1% hydrogen, and 12% an unidentifiable gas. with further analysis, cady and mcfarland discovered that 1.84% of the gas sample was helium. this showed that despite its overall rarity on earth, helium was concentrated in large quantities under the american great plains, available for extraction as a byproduct of natural gas. this enabled the united states to become the world's leading supplier of helium. following a suggestion by sir richard threlfall, the united states navy sponsored three small experimental helium plants during world war i. the goal was to supply barrage balloons with the non-flammable, lighter-than-air gas. a total of 5,700 m3 (200,000 cu ft) of 92% helium was produced in the program even though less than a cubic meter of the gas had previously been obtained. some of this gas was used in the world's first helium-filled airship, the u.s. navy's c-class blimp c-7, which flew its maiden voyage from hampton roads, virginia, to bolling field in washington, d.c., on december 1, 1921, nearly two years before the navy's first rigid helium-filled airship, the naval aircraft factory-built uss shenandoah, flew in september 1923. although the extraction process using low-temperature gas liquefaction was not developed in time to be significant during world war i, production continued. helium was primarily used as a lifting gas in lighter-than-air craft. during world war ii, the demand increased for helium for lifting gas and for shielded arc welding. the helium mass spectrometer was also vital in the atomic bomb