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lium atoms in a ground state which are superfluid and flow with exactly zero viscosity and a proportion of helium atoms in an excited state which behave more like an ordinary fluid 103 in the fountain effect a chamber is constructed which is connected to a reservoir of helium ii by a sintered disc through which superfluid helium leaks easily but through which non superfluid helium cannot pass if the interior of the container is heated the superfluid helium changes to non superfluid helium in order to maintain the equilibrium fraction of superfluid helium superfluid helium leaks through and increases the pressure causing liquid to fountain out of the container 104 the thermal conductivity of helium ii is greater than that of any other known substance a million times that of helium i and several hundred times that of copper 31 this is because heat conduction occurs by an exceptional quantum mechanism most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat helium ii has no such valence band but nevertheless conducts heat well the flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air when heat is introduced it moves at 20 meters per second at 1 8 k through helium ii as waves in a phenomenon known as second sound 31 helium ii also exhibits a creeping effect when a surface extends past the level of helium ii the helium ii moves along the surface against the force of gravity helium ii will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates it moves in a 30 nm thick film regardless of surface material this film is called a rollin film and is named after the man who first characterized this trait bernard v rollin 31 105 106 as a result of this creeping behavior and helium ii s ability to leak rapidly through tiny openings it is very difficult to confine unless the container is carefully constructed the helium ii will creep along the surfaces and through valves until it reaches somewhere warmer where it will evaporate waves propagating across a rollin film are governed by the same equation as gravity waves in shallow water but rather than gravity the restoring force is the van der waals force 107 these waves are known as third sound 108 solid phases helium remains liquid down to absolute zero at atmospheric pressure but it freezes at high pressure solid helium requires a temperature of 1 1 5 k about 272 c or 457 f at about 25 bar 2 5 mpa of pressure 109 it is often hard to distinguish solid from liquid helium since the refractive index of the two phases are nearly the same the solid has a sharp melting point and has a crystalline structure but it is highly compressible applying pressure in a laboratory can decrease its volume by more than 30 110 with a bulk modulus of about 27 mpa 111 it is 100 times more compressible than water solid helium has a density of 0 214 0 006 g cm 3 at 1 15 k and 6 7 mpa 66 atm the projected density at 0 k and 2 5 mpa 25 atm is 0 187 0 009 g cm 3 112 at higher temperatures helium will solidify with sufficient pressure at room temperature this requires about 11 600 mpa 114 000 atm 113 helium 4 and helium 3 both form several crystalline solid phases all requiring at least 25 bar they both form an α phase which has a hexagonal close packed hcp crystal structure a β phase which is face centered cubic fcc and a γ phase which is body centered cubic bcc 114 isotopes main article isotopes of helium there are eight known isotopes of helium of which two helium 3 and helium 4 are stable in the earth s atmosphere one atom is 3 he for every million that are 4 he 29 unlike most elements helium s isotopic abundance varies greatly by origin due to the different formation processes the most common isotope helium 4 is produced on earth by alpha decay of heavier radioactive elements the alpha particles that emerge are fully ionized helium 4 nuclei helium 4 is an unusually stable nucleus because its nucleons are arranged into complete shells it was also formed in enormous quantities during big bang nucleosynthesis 115 helium 3 is present on earth only in trace amounts most of it has been present since earth s formation though some falls to earth trapped in cosmic dust 116 trace amounts are also produced by the beta decay of tritium 117 rocks from the earth s crust have isotope ratios varying by as much as a factor of ten and these ratios can be used to investigate the origin of rocks and the composition of the earth s mantle 116 3 he is much more abundant in stars as a product of nuclear fusion thus in the interstellar medium the proportion of 3 he to 4 he is about 100 times higher than on earth 118 extraplanetary material such as lunar and asteroid regolith have trace amounts of helium 3 from being bombarded by solar winds the moon s surface contains helium 3 at concentrations on the order of 10 ppb much higher than the approximately 5 ppt found in the earth s atmosphere 119 120 a number of people starting with gerald kulcinski in 1986 121 have proposed to explore the moon mine lunar regolith and use the helium 3 for fusion liquid helium 4 can be cooled to about 1 k 272 15 c 457 87 f using evaporative cooling in a 1 k pot similar cooling of helium 3 which has a lower boiling point can achieve about 0 2 kelvin in a helium 3 refrigerator equal mixtures of liquid 3 he and 4 he below 0 8 k separate into two immiscible phases due to their dissimilarity they follow different quantum statistics helium 4 atoms are bosons while helium 3 atoms are fermions 31 dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins 122 it is possible to produce exotic helium isotopes which rapidly decay into other substances the shortest lived heavy helium isotope is the unbound helium 10 with a half life of 2 6 4 10 22 s 7 helium 6 decays by emitting a beta particle and has a half life of 0 8 seconds helium 7 and helium 8 are created in certain nuclear reactions 31 helium 6 and helium 8 are known to exhibit a nuclear halo 31 properties table of thermal and physical properties of helium gas at atmospheric pressure 123 124 temperature k density kg m 3 specific heat kj kg c dynamic viscosity kg m s kinematic viscosity m 2 s thermal conductivity w m c thermal diffusivity m 2 s prandtl number 100 5 193 9 63e 06 1 98e 05 0 073 2 89e 05 0 686 120 0 406 5 193 1 07e 05 2 64e 05 0 0819 3 88e 05 0 679 144 0 3379 5 193 1 26e 05 3 71e 05 0 0928 5 28e 05 0 7 200 0 2435 5 193 1 57e 05 6 44e 05 0 1177 9 29e 05 0 69 255 0 1906 5 193 1 82e 05 9 55e 05 0 1357 1 37e 04 0 7 366 0 1328 5 193 2 31e 05 1 74e 04 0 1691 2 45e 04 0 71 477 0 10204 5 193 2 75e 05 2 69e 04 0 197 3 72e 04 0 72 589 0 08282 5 193 3 11e 05 3 76e 04 0 225 5 22e 04 0 72 700 0 07032 5 193 3 48e 05 4 94e 04 0 251 6 66e 04 0 72 800 0 06023 5 193 3 82e 05 6 34e 04 0 275 8 77e 04 0 72 900 0 05451 5 193 4 14e 05 7 59e 04 0 33 1 14e 03 0 687 1000 5 193 4 46e 05 9 14e 04 0 354 1 40e 03 0 654 compounds main article helium compounds structure of the helium hydride ion 125 heh structure of the suspected fluoroheliate anion ohef helium has a valence of zero and is chemically unreactive under all normal conditions 110 it is an electrical insulator unless ionized as with the other noble gases helium has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential 31 helium can form unstable compounds known as excimers with tungsten iodine fluorine sulfur and phosphorus when it is subjected to a glow discharge to electron bombardment or reduced to plasma by other means the molecular compounds hene hghe 10 and whe 2 and the molecular ions he 2 he 2 2 heh and hed have been created this way 126 heh is also stable in its ground state but is extremely reactive it is the strongest brønsted acid known and therefore can exist only in isolation as it will protonate any molecule or counteranion it contacts this technique has also produced the neutral molecule he 2 which has a large number of band systems and hghe which is apparently held together only by polarization forces 31 van der waals compounds of helium can also be formed with cryogenic helium gas and atoms of some other substance such as lihe and he 2 127 theoretically other true compounds may be possible such as helium fluorohydride hhef which would be analogous to harf discovered in 2000 128 calculations show that two new compounds containing a helium oxygen bond could be stable 129 two new molecular species predicted using theory csfheo and n ch 3 4 fheo are derivatives of a metastable fheo anion first theorized in 2005 by a group from taiwan 130 helium atoms have been inserted into the hollow carbon cage molecules the fullerenes by heating under high pressure the endohedral fullerene molecules formed are stable at high temperatures when chemical derivatives of these fullerenes are formed the helium stays inside 131 if helium 3 is used it can be readily observed by helium nuclear magnetic resonance spectroscopy 132 many fullerenes containing helium 3 have been reported although the helium atoms are not attached by covalent or ionic bonds these substances have distinct properties and a definite composition like all stoichiometric chemical compounds under high pressures helium can form compounds with various other elements helium nitrogen clathrate he n 2 11 crystals have been grown at room temperature at pressures ca 10 gpa in a diamond anvil cell 133 the insulating electride na 2 he has been shown to be thermodynamically stable at pressures above 113 gpa it has a fluorite structure 134 occurrence and production natural abundance although it is rare on earth helium is the second most abundant element in the known universe constituting 23 of its baryonic mass only hydrogen is more abundant 29 the vast majority of helium was formed by big bang nucleosynthesis one to three minutes after the big bang as a result measurements of its abundance contribute to cosmological models the first molecular bonds were formed when primordial helium atoms combined with protons to form helium hydride ions 135 heh in stars helium is formed by the nuclear fusion of hydrogen in proton proton chain reactions and the cno cycle part of stellar nucleosynthesis 115 in the earth s atmosphere the concentration of helium by volume is only 5 2 parts per million 136 137 the concentration is low and fairly constant despite the continuous production of new helium because most helium in the earth s atmosphere escapes into space by several processes 138 139 140 in the earth s heterosphere a part of the upper atmosphere helium and hydrogen are the most abundant elements most helium on earth is a result of radioactive decay helium is found in large amounts in minerals of uranium and thorium such as uraninite and its varieties cleveite and pitchblende 21 141 as well as carnotite and monazite a group name monazite usually refers to monazite ce 142 143 because they emit alpha particles helium nuclei he 2 to which electrons immediately combine as soon as the particle is stopped by the rock in this way an estimated 3000 metric tons of helium are generated per year throughout the lithosphere 144 145 146 in the earth s crust the concentration of helium is 8 parts per billion in seawater the concentration is only 4 parts per trillion there are also small amounts in mineral springs volcanic gas and meteoric iron because helium is trapped in the subsurface under conditions that also trap natural gas the greatest natural concentrations of helium on the planet are found in natural gas from which most commercial helium is extracted the concentration varies in a broad range from a few ppm to more than 7 in a small gas field in san juan county new mexico 147 148 as of 2021 update the world s helium reserves were estimated at 31 000 000 000 cubic metres 1 1 10 12 cu ft with a third of that being in qatar 149 in 2015 and 2016 additional probable reserves were announced to be under the rocky mountains in north america 150 and in the east african rift 27 the bureau of land management blm has proposed an october 2024 plan for managing natural resources in western colorado the plan involves closing 543 000 acres to oil and gas leasing while keeping 692 300 acres open among the open areas 165 700 acres have been identified as suitable for helium recovery the united states possesses an estimated 8 7 10 9 m 3 306 000 000 000 cu ft of recoverable helium sufficient to meet current consumption rates of 61 000 000 m 3 2 150 000 000 cu ft per year for approximately 150 years 151 modern extraction and distribution see also helium production in the united states extracting helium from air is not economical 152 for large scale use helium is extracted by fractional distillation from natural gas which can contain as much as 7 helium 153 since helium has a lower boiling point than any other element low temperatures and high pressure are used to liquefy nearly all the other gases mostly nitrogen and methane the resulting crude helium gas is purified by successive exposures to lowering temperatures in which almost all of the remaining nitrogen and other gases are precipitated out of the gaseous mixture activated charcoal is used as a final purification step usually resulting in 99 995 pure grade a helium 31 the principal impurity in grade a helium is neon in a final production step most of the helium that is produced is liquefied via a cryogenic process this is necessary for applications requiring liquid helium and also allows helium suppliers to reduce the cost of long distance transportation as the largest liquid helium containers have more than five times the capacity of the largest gaseous helium tube trailers 83 154 in 2008 approximately 169 million standard cubic meters scm 6 0 10 9 cubic feet of helium were extracted from natural gas or withdrawn from helium reserves with approximately 78 from the united states 10 from algeria and most of the remainder from russia poland and qatar 155 by 2013 increases in helium production in qatar under the company qatargas managed by air liquide had increased qatar s fraction of world helium production to 25 making it the second largest exporter after the united states 156 in 2024 the united states surpassed qatar as the world s largest producer of the gas having extracted 68 million scm of helium that year compared to qatar s 64 million scm 157 an estimated 54 billion cubic feet 1 5 10 9 m 3 deposit of helium was found in tanzania in 2016 158 and a large scale helium plant was opened in ningxia china in 2020 159 in the united states most helium is extracted from the natural gas of the hugoton and nearby gas fields in kansas oklahoma and the panhandle field in texas 83 160 much of this gas was once sent by pipeline to the national helium reserve but since 2005 this reserve has been depleted and so...