Helium - He

Helium (He) is a light, chemically inert gas used in cryogenics, analysis and leak detection. Dissolved helium also provides information about groundwater. Its low chemical reactivity should be distinguished from the physical hazards of oxygen displacement, pressure and extreme cold.

Chemical properties · Uses and occurrence · Health effects · Environmental effects · Water and treatment

Chemical properties of helium

Selected properties of helium

Symbol / atomic number	He / 2
Group / period / block	18 / 1 / s
Standard atomic weight	4.002602(2)
Electron configuration	1s2
State at 20 °C and 1 bar	Gas
Gas density (ideal, 20 °C, 1 bar)	0.164 g/L
Boiling point (at atmospheric pressure)	−268.928 °C (4.222 K)
Solidification	Requires elevated pressure; no normal atmospheric-pressure melting point
Non-bonded atomic radius	0.140 nm
First ionisation energy	2372.3 kJ mol−1
Naturally occurring stable isotopes	3He and 4He
Discovery	Detected in the solar spectrum in 1868; identified on Earth in 1895
CAS registry number	7440-59-7

Atomic weight follows CIAAW; parentheses indicate uncertainty in the final digits. Physical properties follow the Royal Society of Chemistry unless noted. Atomic radii depend on their definition. Gas density is an ideal-gas estimate from ρ = pM/(RT), using CIAAW atomic weight and the stated temperature and pressure.

Uses, occurrence and history

Helium is a colourless, odourless, very unreactive noble gas and the second-lightest element. Its very low boiling point makes it important in cryogenics. Unlike most substances, helium does not solidify at atmospheric pressure, even when cooled towards absolute zero; solidification requires additional pressure.

Uses include cooling superconducting magnets in MRI and scientific equipment, controlled atmospheres, semiconductor and fibre-optic manufacture, leak detection, gas chromatography and lifting applications. Helium is also used in specialised breathing mixtures containing oxygen, under professional control; pure helium is not breathable air.

In 1868, Pierre Janssen and Norman Lockyer independently observed the solar spectral feature associated with helium. William Ramsay identified terrestrial helium in 1895, with independent work by Per Teodor Cleve and Nils Abraham Langlet. Its discovery should therefore not be dated solely to 1895.

Helium-4 is produced by radioactive decay of heavier elements in the Earth. Commercial helium is recovered from suitable gas deposits; the existence of helium in the atmosphere does not mean that atmospheric extraction is its usual commercial source.

Health effects and handling

Helium has no established biological role. Although chemically non-toxic under ordinary conditions, it can displace oxygen and cause rapid suffocation. Inhaling helium for a voice-changing effect is unsafe, and gas from a pressurised supply adds pressure-related hazards. It should not be inhaled recreationally.

Leaks in confined or poorly ventilated areas require an oxygen-deficiency assessment. Compressed cylinders must be handled with appropriate rated equipment. Liquid helium and very cold equipment can cause severe cold burns, while rapid vaporisation or trapped liquid can create dangerous pressure. These are physical hazards even though helium is chemically inert.

Helium in the environment

Helium is abundant in the universe but much less abundant in Earth's atmosphere. Helium-4 generated within rocks can migrate into groundwater and gas accumulations or escape to the atmosphere. The relative amounts of helium-3 and helium-4 vary with their sources.

Recovery and recirculation can reduce losses from helium-using installations. CERN, for example, operates helium in closed cryogenic circuits. Environmental and operational assessment should consider resource use and release hazards rather than assuming that an inert gas requires no controls.

Helium in water and groundwater studies

Helium can occur as a dissolved gas in groundwater. Measurements of helium isotopes help distinguish atmospheric and geological contributions. Tritium/helium-3 dating uses helium-3 produced by radioactive decay of tritium to investigate the age of relatively young groundwater.

Interpretation requires corrections for helium from other sources and consideration of gas loss or water mixing. An apparent groundwater age is not, by itself, evidence that the water is suitable for drinking or a particular industrial process.

Helium is not a water disinfectant. Its main relevance to water investigations is as a tracer and, in suitable engineering applications, a leak-detection gas. See Helium in water (He + H₂O) for further background.

Helium page last reviewed: 24 October 2026 by Lenntech.

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