Helium
Helium is a chemical element with atomic number 2 and symbol He, a colorless, odorless, tasteless, non-toxic, inert, monatomic noble gas that is the second most abundant element in the universe after hydrogen.[1][2][3] Its cosmic prevalence arises mainly from Big Bang nucleosynthesis, forming about 24% of the universe's baryonic mass, while on Earth it is scarce, making up roughly 5 parts per million by volume in the atmosphere and primarily sourced from natural gas deposits via alpha decay of radioactive elements.[4][1] Helium was the first element identified extraterrestrially, detected in 1868 via a distinctive yellow spectral line (587.49 nm) in the Sun's chromosphere during a solar eclipse by French astronomer Pierre Janssen and English spectroscopist Norman Lockyer, who coined the name from the Greek helios for sun.[5][6] Terrestrial isolation followed in 1895 when Scottish chemist William Ramsay and English chemist Morris Travers extracted it from the uranium-bearing mineral cleveite, confirming its presence on Earth through spectroscopic matching.[7] Notable for the lowest boiling point of any substance (4.2 K at standard pressure) and lacking a melting point under normal conditions due to quantum effects preventing solidification, liquid helium below 2.17 K displays superfluidity, flowing without viscosity and exhibiting phenomena like film creep over container walls.[8][2] Practically, helium enables cryogenic cooling of superconducting magnets in MRI scanners and particle accelerators, serves as a lifting gas in balloons and dirigibles owing to its low density (0.1786 kg/m³), and acts as an inert shield in arc welding to inhibit oxidation.[2][9]Physical and Chemical Properties
Atomic and Nuclear Structure
Helium has atomic number 2 and consists of a nucleus containing two protons and typically two neutrons (in the predominant isotope, ^4He), orbited by two electrons in the ground state configuration 1s^2.[10] This closed-shell electron arrangement, with both electrons paired in the lowest-energy 1s orbital, imparts helium's characteristic chemical inertness due to the lack of available orbitals for bonding without significant energy input.[11] The atomic radius of helium is approximately 31 pm, reflecting the tight binding of electrons to the Z=2 nucleus.[12] The nucleus of ^4He, known as the alpha particle, comprises two protons and two neutrons in a highly stable configuration, with a total binding energy of 28.3 MeV, or 7.07 MeV per nucleon—the highest among light nuclei.[13] This exceptional stability results from the saturation of the strong nuclear force in the symmetric spin-0, isospin-0 state, minimizing energy through pairing effects and overcoming proton-proton repulsion.[14] In contrast, the rarer isotope ^3He (two protons, one neutron) has a binding energy of 7.72 MeV per nucleon but lower overall stability due to its odd nucleon count and fermionic nature (spin-1/2).[15] Quantum mechanically, the helium atom poses a three-body problem intractable analytically because of electron-electron Coulomb repulsion, requiring approximations like the variational method, which yields a ground-state energy of -79.0 eV by optimizing an effective nuclear charge of Z_eff = 27/16 ≈ 1.69 for each electron.[12] The exact non-relativistic ground-state energy, computed numerically, is -2.9037 hartrees (-79.0 eV), underscoring the dominance of kinetic and potential energies in this simplest multi-electron system.[16]Phase Transitions and States of Matter
Helium-4, the predominant isotope, transitions from gas to liquid at its boiling point of 4.222 K under standard atmospheric pressure of 101.325 kPa, with a critical point at 5.1953 K and 0.227 MPa beyond which distinct gas and liquid phases do not coexist.[17] Unlike nearly all other elements, helium-4 does not solidify upon cooling to absolute zero at low pressures due to its extremely weak van der Waals interatomic forces and high zero-point energy, which prevent atoms from forming a stable lattice; solidification requires pressures above approximately 2.5 MPa at temperatures near 0 K.[18][19] At saturated vapor pressure, liquid helium-4 undergoes a second-order phase transition at the λ-point of 2.17 K, separating the normal-fluid He I phase above this temperature from the superfluid He II phase below, where quantum mechanical effects lead to macroscopic coherence akin to Bose-Einstein condensation since helium-4 atoms are bosons with integer spin zero.[17][20] The He II phase exhibits zero viscosity, allowing it to flow through narrow capillaries without resistance, and a two-fluid model describes its behavior as a mixture of superfluid and normal components, with the superfluid fraction increasing as temperature decreases toward 0 K.[20] The λ-transition line in the phase diagram extends to higher pressures and temperatures, terminating at the solid-liquid boundary. Under sufficient pressure, solid helium-4 forms, initially in a body-centered cubic (BCC) structure at lower pressures and temperatures, transitioning to a hexagonal close-packed (HCP) structure at higher pressures above about 6 MPa near the melting curve minimum at 1.1 K and 2.9 MPa.[20] The melting curve of helium-4 rises steeply with pressure, reaching solidification temperatures up to around 80 K at extreme pressures beyond 10 GPa, though such high-pressure phases are less relevant to typical low-temperature studies.[21] Helium-3, being a fermion, displays distinct behavior with no superfluid transition until millikelvin temperatures under pressure, where p-wave pairing leads to anisotropic superfluid phases A and B, but its phase diagram lacks the λ-point characteristic of helium-4.[17]Isotopic Properties
Helium possesses two stable isotopes: helium-3 (³He) and helium-4 (⁴He), with all others being short-lived radioisotopes.[22] Eight isotopes of helium are known in total, though only ³He and ⁴He occur naturally in significant quantities.[22] In natural terrestrial helium, ⁴He dominates with an abundance of 99.999863%, while ³He accounts for the remaining 0.000137%.[23] These abundances reflect distinct formation mechanisms: ⁴He arises predominantly from alpha particle emission during the radioactive decay of heavy elements like uranium and thorium in Earth's crust, accumulating over geological time.[22] In contrast, ³He is largely primordial, originating from Big Bang nucleosynthesis, with minor contributions from cosmic ray interactions, tritium decay, and spallation of lithium.[24] Nuclear properties further distinguish the isotopes. ⁴He has an atomic mass of 4.002603 u and zero nuclear spin (0⁺), rendering its atoms composite bosons that follow Bose-Einstein statistics, which facilitates phenomena like Bose-Einstein condensation at low temperatures.[25][26] ³He, with an atomic mass of approximately 3.01603 u and nuclear spin ½, consists of an odd number of fermions (two protons and one neutron), obeying Fermi-Dirac statistics and exhibiting distinct quantum behaviors, such as requiring lower temperatures for superfluidity compared to ⁴He.[27] Both isotopes are stable against beta decay due to their low proton-to-neutron ratios and high binding energies relative to lighter nuclides, as evidenced by the peak in nuclear binding energy per nucleon near mass number 4.[28]| Isotope | Atomic Mass (u) | Natural Abundance (%) | Nuclear Spin | Particle Statistics | Primary Terrestrial Source |
|---|---|---|---|---|---|
| ³He | 3.016029 | 0.000137 | ½ | Fermionic | Primordial + cosmogenic |
| ⁴He | 4.002603 | 99.999863 | 0 | Bosonic | Radiogenic (alpha decay) |
Occurrence in Nature
Terrestrial Abundance and Sources
Helium constitutes a negligible fraction of Earth's atmosphere, with a concentration of 5.24 parts per million by volume.[29] This scarcity arises from helium's low atomic mass, which allows atoms to achieve escape velocity from the planet's gravitational field over geological timescales, limiting atmospheric retention.[30] In the Earth's crust, helium abundance is even lower, approximately 8 parts per billion.[31] Terrestrial helium primarily derives from radiogenic processes rather than primordial sources inherited from Earth's formation. Alpha particles emitted during the radioactive decay of uranium and thorium isotopes in crustal rocks produce helium-4 nuclei, which accumulate in natural gas reservoirs due to helium's chemical inertness and ability to migrate through porous formations before becoming trapped by impermeable cap rocks.[32] While trace amounts of primordial helium-3 persist in the mantle, surface-level helium is overwhelmingly radiogenic, with atmospheric inputs balanced by continuous escape to space.[33] Commercial extraction occurs exclusively from natural gas fields containing helium concentrations exceeding 0.3% by volume, as lower levels render recovery uneconomical.[34] The United States has historically dominated production, drawing from fields such as the Hugoton-Panhandle in Texas, Oklahoma, and Kansas, and the LaBarge field in Wyoming, though domestic reserves face depletion, with the Federal Helium Reserve exhausted by 2021.[35][36] Qatar, leveraging its North Field, emerged as the top producer in 2023 with 66 million cubic meters annually, followed by Algeria's Hassi R'Mel field and Russian operations.[37][38] Global reserves are concentrated in these nations, with the U.S. holding 20.6 billion cubic meters, Qatar 10.1 billion, Algeria and Russia trailing.[39] Production reached approximately 6.5 billion cubic feet worldwide in 2025, amid ongoing supply constraints from field maturation.[40]Cosmic and Astrophysical Distribution
Helium constitutes the second most abundant element in the observable universe by mass, following hydrogen, with a primordial mass fraction of approximately 0.24 resulting from Big Bang nucleosynthesis (BBN) in the first few minutes after the universe's origin.[41] [4] During BBN, helium-4 nuclei formed through fusion of protons and neutrons under conditions of extreme temperature and density, yielding nearly all primordial helium as the stable isotope ^4He, with trace amounts of deuterium, helium-3, and lithium.[42] This primordial abundance, denoted Y_p, has been measured spectroscopically from metal-poor extragalactic H II regions by analyzing the ratio of He I to H I emission lines, extrapolated to zero metallicity to isolate pre-stellar contributions; recent determinations place Y_p at 0.2446 ± 0.0019 (statistical) ± 0.0009 (systematic).[42] Stellar nucleosynthesis supplements the primordial helium, producing additional ^4He via the proton-proton chain and CNO cycle in low- to intermediate-mass stars, and through helium burning stages in massive stars, which convert hydrogen cores into helium ash.[43] In main-sequence stars, helium accumulates in convective cores of higher-mass objects (>1.2 solar masses), while post-main-sequence evolution in red giants and asymptotic giant branch stars dredges helium to surfaces or ejects it via winds and planetary nebulae. Supernovae from massive stars (>8 solar masses) disperse helium-enriched material into the interstellar medium (ISM), contributing to galactic chemical evolution where helium mass fractions rise from primordial levels to 0.25–0.30 in present-day disks, correlating with increasing metallicity.[44] Astrophysically, helium distribution varies by environment: in the ISM of spiral galaxies like the Milky Way, neutral and ionized helium traces follow hydrogen but with enhancements from nearby stellar feedback; diffuse intergalactic helium, detected via Lyman-alpha absorption, reflects reionization epochs around redshift z ≈ 2.5–3.5, where He II (doubly ionized helium) transitioned to He I.[45] In globular clusters and elliptical galaxies, helium-rich subpopulations (up to 40% of stars) exhibit enhanced Y ≈ 0.3–0.4, inferred from horizontal branch morphology and color spreads, indicating self-enrichment from asymptotic giant branch pollution or primordial variations.[46] Neutron star mergers and black hole formation further concentrate helium in compact remnants, though bulk cosmic helium remains diffusely distributed, comprising ~24–28% of baryonic mass overall when accounting for stellar recycling.[44]Production and Supply Economics
Extraction Techniques
Helium extraction primarily occurs as a byproduct during natural gas processing from subterranean reservoirs containing helium concentrations typically exceeding 0.3% by volume, the economic threshold for recovery.[47] Natural gas is first extracted via conventional drilling and pumping operations, then undergoes pretreatment to remove condensable hydrocarbons, water vapor, carbon dioxide, and hydrogen sulfide through compression, dehydration, and acid gas removal.[48] The helium-enriched stream, often termed the crude helium stream with 50-90% helium purity, is subsequently isolated using specialized separation technologies before final purification to grades exceeding 99.99%.[49] The dominant industrial method is cryogenic distillation, which exploits helium's exceptionally low boiling point of 4.2 K to separate it from other gas components. In this process, the pretreated natural gas is precooled, compressed, and expanded to achieve temperatures below -160°C, causing methane and heavier hydrocarbons to liquefy while helium remains gaseous; this non-liquefied fraction is then fractionated in distillation columns to concentrate helium.[50] Cryogenic systems often integrate turboexpanders for refrigeration and may operate in conjunction with liquefied natural gas (LNG) facilities, where helium is recovered from the nitrogen rejection unit tail gas, yielding high-purity product with recovery rates up to 95% from feeds as low as 0.5% helium.[51] This technique accounts for approximately 90% of global helium production due to its scalability and ability to handle variable feed compositions.[52] Alternative and complementary methods include pressure swing adsorption (PSA), which employs cyclic pressure variations over adsorbent beds—typically activated carbon or zeolites—to selectively adsorb impurities like methane, nitrogen, and carbon dioxide, desorbing purified helium during depressurization.[53] PSA is particularly effective for upgrading low-concentration helium streams (below 10%) or as a polishing step post-cryogenics, achieving purities over 99.9% with lower energy demands than full cryogenic cycles, though it requires multiple beds for continuous operation and may yield lower recovery rates from complex feeds.[54] Membrane separation, using polymer or inorganic membranes permeable to helium, serves niche applications for initial enrichment but is less common industrially due to lower selectivity and flux compared to cryogenics.[50] Hybrid processes combining these techniques, such as cryogenic-membrane cascades, are emerging to optimize efficiency and reduce costs in marginal fields.[55] Final purification often involves catalytic oxidation of trace impurities followed by additional PSA or cryogenic steps to meet specifications for end-use applications.[49]Global Reserves and Market Dynamics
Global helium reserves are estimated based on helium-bearing natural gas fields with concentrations typically exceeding 0.3%, as lower levels are uneconomical to extract given current technologies and prices. Recoverable reserves are unevenly distributed, with the United States possessing the largest share at approximately 20.6 billion cubic meters, more than double that of Qatar's 10.1 billion cubic meters; other significant holders include Algeria, Russia, and Canada.[39] The U.S. Geological Survey estimates world helium resources excluding the United States at about 31.3 billion cubic meters, though these figures represent identified resources rather than strictly proven reserves, and extraction feasibility depends on associated natural gas infrastructure and helium purity.[56] Annual global production reached 170 million cubic meters in 2023, an 8% increase from the previous year, driven by new facilities in Canada (four plants), Russia (one), and Qatar expansions.[57] The United States remains the top producer and exporter, accounting for roughly 40% of output from fields in Texas, Oklahoma, and Kansas, though its share has declined from historical dominance due to depleting federal reserves, which were fully privatized by 2024 with the sale of remaining stockpiles to private entities like Messer Group.[58] Qatar, Algeria, and Russia collectively supply over half of global helium, with Qatar's RasGas facility alone capable of 70 million cubic meters annually; emerging production in Iran began pilot operations in 2025 from the South Pars gas field.[38]| Country | Estimated Reserves (billion m³) | Approximate Production Share (2023) |
|---|---|---|
| United States | 20.6 | ~40% |
| Qatar | 10.1 | ~25-30% |
| Algeria | ~8.0 | ~15% |
| Russia | ~6.0-7.0 | ~10-15% |
| Others (e.g., Canada, Iran) | Variable, emerging | ~5-10% |