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se diagram of helium 4 atmospheric pressure is about 0 1 mpa 0 99 atm liquefied helium this helium is not only liquid but has been cooled to the point of superfluidity the drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side to empty out of the container the energy to drive this process is supplied by the potential energy of the falling helium helium liquifies when cooled below 4 2 k 268 95 c 452 11 f at atmospheric pressure unlike any other element however helium remains liquid down to a temperature of absolute zero this is a direct effect of quantum mechanics specifically the zero point energy of the system is too high to allow freezing pressures above about 2 5 mpa 25 atm are required to freeze it there are two liquid phases helium i is a conventional liquid and helium ii which occurs at a lower temperature is a superfluid helium i below its boiling point of 4 22 k 268 93 c 452 07 f and above the lambda point of 2 1768 k 270 9732 c 455 7518 f the isotope helium 4 exists in a normal colorless liquid state called helium i 31 like other cryogenic liquids helium i boils when it is heated and contracts when its temperature is lowered below the lambda point however helium does not boil and it expands as the temperature is lowered further helium i has a gas like index of refraction of 1 026 which makes its surface so hard to see that floats of styrofoam are often used to show where the surface is 31 this colorless liquid has a very low viscosity and a density of 0 145 0 125 g ml between about 0 and 4 k 102 which is only one fourth the value expected from classical physics 31 quantum mechanics is needed to explain this property and thus both states of liquid helium helium i and helium ii are called quantum fluids meaning they display atomic properties on a macroscopic scale this may be an effect of its boiling point being so close to absolute zero preventing random molecular motion thermal energy from masking the atomic properties 31 helium ii main article superfluid helium 4 liquid helium below its lambda point called helium ii exhibits very unusual characteristics due to its high thermal conductivity when it boils it does not bubble but rather evaporates directly from its surface helium 3 also has a superfluid phase but only at much lower temperatures as a result less is known about the properties of the isotope 31 unlike ordinary liquids helium ii will creep along surfaces in order to reach an equal level after a short while the levels in the two containers will equalize the rollin film also covers the interior of the larger container if it were not sealed the helium ii would creep out and escape 31 helium ii is a superfluid a quantum mechanical state of matter with strange properties for example when it flows through capillaries as thin as 10 to 100 nm it has no measurable viscosity 29 however when measurements were done between two moving discs a viscosity comparable to that of gaseous helium was observed existing theory explains this using the two fluid model for helium ii in this model liquid helium below the lambda point is viewed as containing a proportion of helium 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 mo...