which makes its surface so hard to see that floats of styrofoam are often used to show where the surface is. this colorless liquid has a very low viscosity and a density of 0.145–0.125 g/ml (between about 0 and 4 k), which is only one-fourth the value expected from classical physics. 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. helium ii 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. 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. helium ii is a superfluid, a quantum mechanical state (see: macroscopic quantum phenomena) of matter with strange properties. for example, when it flows through capillaries as thin as 10−7 to 10−8 m it has no measurable viscosity. however, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. current 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. 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. 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. 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. 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. 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. these waves are known as third sound. isotopes there are nine known isotopes of helium of which two, helium-3 and helium-4, are stable. in the earth's atmosphere, one atom is 3he for every million that are 4he. 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. 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. trace amounts are also produced by the beta decay of tritium. 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. 3he is much more abundant in stars as a product of nuclear fusion. thus in the interstellar medium, the proportion of 3he to 4he is about 100 times higher than on earth. 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. a number of people, starting with gerald kulcinski in 1986, 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 3he and 4he 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). dilution refrigerators use this immiscibility to achieve temperatures of a few millikelvins. 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. helium-6 decays by emitting a beta particle and has a half-life of 0.8 second. helium-7 and helium-8 are created in certain nuclear reactions. helium-6 and helium-8 are known to exhibit a nuclear halo. properties table of thermal and physical properties of helium gas at atmospheric pressure: 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 structure of the helium hydride ion, hhe+ structure of the suspected fluoroheliate anion, ohef− helium has a valence of zero and is chemically unreactive under all normal conditions. 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. 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, hghe10, and whe2, and the molecular ions he+2, he2+2, heh+, and hed+ have been created this way. 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 he2, which has a large number of band systems, and hghe, which is apparently held together only by polarization forces. van der waals compounds of helium can also be formed with cryogenic helium gas and atoms of some other substance, such as lihe and he2. theoretically, other true compounds may be possible, such as helium fluorohydride (hhef) which would be analogous to harf, discovered in 2000. calculations show that two new compounds containing a helium-oxygen bond could be stable. two new molecular species, predicted using theory, csfheo and n(ch3)4fheo, are derivatives of a metastable fheo− anion first theorized in 2005 by a group from taiwan. if confirmed by experiment, the only remaining element with no known stable compounds would be neon. 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. if helium-3 is used, it can be readily observed by helium nuclear magnetic resonance spectroscopy. 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(n2)11) crystals have been grown at room temperature at pressures ca. 10 gpa in a diamond anvil cell. the insulating electride na2he has been shown to be thermodynamically stable at pressures above 113 gpa. it has a fluorite structure. 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. the vast majority of helium was formed by big bang nucleosynthesis one to three minutes after the big bang. as such, measurements of its abundance contribute to cosmological models. in stars, it is formed by the nuclear fusion of hydrogen in proton–proton chain reactions and the cno cycle, part of stellar nucleosynthesis. in the earth's atmosphere, the concentration of helium by volume is only 5.2 parts per million. 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. in the earth's heterosphere, a part of the upper atmosphere, helium and other lighter gases 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, including uraninite and its varieties cleveite and pitchblende, carnotite and monazite (a group name; "monazite" usually refers to monazite-(ce)), because they emit alpha particles (helium nuclei, he2+) 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. 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. as of 2021 the world's helium reserves were estimated at 31 billion cubic meters, with a third of that being in qatar. in 2015 and 2016 additional probable reserves were announced to be under the rocky mountains in north america and in the east african rift. modern extraction