manhattan project. the government of the united states set up the national helium reserve in 1925 at amarillo, texas, with the goal of supplying military airships in time of war and commercial airships in peacetime. because of the helium act of 1925, which banned the export of scarce helium on which the us then had a production monopoly, together with the prohibitive cost of the gas, german zeppelins was forced to use hydrogen as lifting gas, which would gain infamy in the hindenburg disaster. the helium market after world war ii was depressed but the reserve was expanded in the 1950s to ensure a supply of liquid helium as a coolant to create oxygen/hydrogen rocket fuel (among other uses) during the space race and cold war. helium use in the united states in 1965 was more than eight times the peak wartime consumption. after the "helium acts amendments of 1960" (public law 86–777), the u.s. bureau of mines arranged for five private plants to recover helium from natural gas. for this helium conservation program, the bureau built a 425-mile (684 km) pipeline from bushton, kansas, to connect those plants with the government's partially depleted cliffside gas field near amarillo, texas. this helium-nitrogen mixture was injected and stored in the cliffside gas field until needed, at which time it was further purified. by 1995, a billion cubic meters of the gas had been collected and the reserve was us$1.4 billion in debt, prompting the congress of the united states in 1996 to discontinue the reserve. the resulting helium privatization act of 1996 (public law 104–273) directed the united states department of the interior to empty the reserve, with sales starting by 2005. helium produced between 1930 and 1945 was about 98.3% pure (2% nitrogen), which was adequate for airships. in 1945, a small amount of 99.9% helium was produced for welding use. by 1949, commercial quantities of grade a 99.95% helium were available. for many years, the united states produced more than 90% of commercially usable helium in the world, while extraction plants in canada, poland, russia, and other nations produced the remainder. in the mid-1990s, a new plant in arzew, algeria, producing 17 million cubic meters (600 million cubic feet) began operation, with enough production to cover all of europe's demand. meanwhile, by 2000, the consumption of helium within the u.s. had risen to more than 15 million kg per year. in 2004–2006, additional plants in ras laffan, qatar, and skikda, algeria were built. algeria quickly became the second leading producer of helium. through this time, both helium consumption and the costs of producing helium increased. from 2002 to 2007 helium prices doubled. as of 2012, the united states national helium reserve accounted for 30 percent of the world's helium. the reserve was expected to run out of helium in 2018. despite that, a proposed bill in the united states senate would allow the reserve to continue to sell the gas. other large reserves were in the hugoton in kansas, united states, and nearby gas fields of kansas and the panhandles of texas and oklahoma. new helium plants were scheduled to open in 2012 in qatar, russia, and the us state of wyoming, but they were not expected to ease the shortage. in 2013, qatar started up the world's largest helium unit, although the 2017 qatar diplomatic crisis severely affected helium production there. 2014 was widely acknowledged to be a year of over-supply in the helium business, following years of renowned shortages. nasdaq reported (2015) that for air products, an international corporation that sells gases for industrial use, helium volumes remain under economic pressure due to feedstock supply constraints. (also, see the sub-section conservation advocates in this article’s section occurrence and production, below.) characteristics atom the helium atom. depicted are the nucleus (pink) and the electron cloud distribution (black). the nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case. in quantum mechanics in the perspective of quantum mechanics, helium is the second simplest atom to model, following the hydrogen atom. helium is composed of two electrons in atomic orbitals surrounding a nucleus containing two protons and (usually) two neutrons. as in newtonian mechanics, no system that consists of more than two particles can be solved with an exact analytical mathematical approach (see 3-body problem) and helium is no exception. thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electrons. such computational chemistry methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps. such models show that each electron in helium partly screens the nucleus from the other, so that the effective nuclear charge zeff which each electron sees is about 1.69 units, not the 2 charges of a classic "bare" helium nucleus. related stability of the helium-4 nucleus and electron shell the nucleus of the helium-4 atom is identical with an alpha particle. high-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own electron cloud. this symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these fermions fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin. adding another of any of these particles would require angular momentum and would release substantially less energy (in fact, no nucleus with five nucleons is stable). this arrangement is thus energetically extremely stable for all these particles, and this stability accounts for many crucial facts regarding helium in nature. for example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements. in a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions that involve either heavy-particle emission or fusion. some stable helium-3 (two protons and one neutron) is produced in fusion reactions from hydrogen, but it is a very small fraction compared to the highly favorable helium-4. binding energy per nucleon of common isotopes. the binding energy per particle of helium-4 is significantly larger than all nearby nuclides. the unusual stability of the helium-4 nucleus is also important cosmologically: it explains the fact that in the first few minutes after the big bang, as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. owing to the relatively tight binding of helium-4 nuclei, its production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and thus few neutrons were available to form heavier atoms such as lithium, beryllium, or boron. helium-4 nuclear binding per nucleon is stronger than in any of these elements (see nucleogenesis and binding energy) and thus, once helium had been formed, no energetic drive was available to make elements 3, 4 and 5. it is barely energetically favorable for helium to fuse into the next element with a lower energy per nucleon, carbon. however, due to lack of intermediate elements, this process requires three helium nuclei striking each other nearly simultaneously (see triple alpha process). there was thus no time for significant carbon to be formed in the few minutes after the big bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. this left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4. all heavier elements (including those necessary for rocky planets like the earth, and for carbon-based or other life) have thus been created since the big bang in stars which were hot enough to fuse helium itself. all elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. helium-4, by contrast, makes up about 23% of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen. gas and plasma phases helium discharge tube shaped into 'he', the element's symbol. helium is the second least reactive noble gas after neon, and thus the second least reactive of all elements. it is chemically inert and monatomic in all standard conditions. because of helium's relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen. for these reasons and the small size of helium monatomic molecules, helium diffuses through solids at a rate three times that of air and around 65% that of hydrogen. helium is the least water-soluble monatomic gas, and one of the least water-soluble of any gas (cf4, sf6, and c4f8 have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x2/10−5, respectively, versus helium's 0.70797 x2/10−5), and helium's index of refraction is closer to unity than that of any other gas. helium has a negative joule–thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand. only below its joule–thomson inversion temperature (of about 32 to 50 k at 1 atmosphere) does it cool upon free expansion. once precooled below this temperature, helium can be liquefied through expansion cooling. most extraterrestrial helium is plasma in stars, with properties quite different from those of atomic helium. in a plasma, helium's electrons are not bound to its nucleus, resulting in very high electrical conductivity, even when the gas is only partially ionized. the charged particles are highly influenced by magnetic and electric fields. for example, in the solar wind together with ionized hydrogen, the particles interact with the earth's magnetosphere, giving rise to birkeland currents and the aurora. liquid phase 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. unlike any other element, helium will remain liquid down to absolute zero at normal pressures. this is a direct effect of quantum mechanics: specifically, the zero point energy of the system is too high to allow freezing. 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. 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%. with a bulk modulus of about 27 mpa it is ~100 times more compressible than water. solid helium has a density of 0.214±0.006 g/cm3 at 1.15 k and 66 atm; the projected density at 0 k and 25 bar (2.5 mpa) is 0.187±0.009 g/cm3. at higher temperatures, helium will solidify with sufficient pressure. at room temperature, this requires about 114,000 atm. 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. 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