Helium
Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among all the elements.
Identity
- Atomic mass
- 4.003 u
- Category
- Noble gases
- Group
- 18
- Period
- 1
- Block
- s
- Oxidation States
- 0
- Phase (25 °C)
- Gas
- Appearance
- colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field
Electronic Structure
- Electron configuration
- 1s2
- Short configuration
- 1s2
- Electrons per shell
- 2
- Electronegativity (Pauling)
- —
- Electron affinity
- -48 kJ/mol
- 1st ionization energy
- 2.37e+3 kJ/mol
- Ionization energies
- 2372 · 5251 kJ/mol
Physical properties
- Melting point
- -272.2 °C (1 K)
- Boiling point
- -268.9 °C (4 K)
- Density
- 0.0001785 g/cm³
- Molar heat capacity
- —
Chemical behavior of helium
Helium has completed its only electron shell (1s²), an exceptionally stable configuration, with a very high ionization energy and no tendency to give up, capture or share electrons. That is why its oxidation state is always 0, it is not assigned electronegativity and under ordinary conditions it forms no bonds: it is an inert monatomic gas. The exception confirms the rule: only in plasmas, in the interstellar medium or at pressures of hundreds of gigapascals do fleeting species such as HHe⁺ or Na₂He appear.
Reactivity
H⁺ + He → HHe⁺
In plasmas and in the interstellar medium helium can combine with a proton to form the helium hydride cation, detected in 2019 in the nebula NGC 7027. It only exists isolated or in extreme conditions; it is not an ordinary chemical compound.
2 Na + He → Na₂He
Subjected to pressures above about 113 GPa, sodium and helium crystallise together in a stable compound. Helium acts as an electronic spacer without forming classical bonds: a purely high-pressure phenomenon.
He + He⁺ → He₂⁺
In electrical discharges, ionised helium can associate with another helium atom to form the He₂⁺ cation, which decays instantly under normal conditions. It illustrates that helium only interacts when a lot of energy is supplied.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Helium hydride | HHe⁺ | First molecule of the universe; only stable in plasmas and interstellar medium |
| Disodium helide | Na₂He | High-pressure compound; demonstrates chemistry under gigapascals |
| Helium endofullerene | He@C₆₀ | Helium trapped physically inside a fullerene, without chemical bond |
Electron shell distribution
View in the atom builderIsotopes
Helium-4 (⁴He), with two neutrons, constitutes 99.9999% of natural helium and originates largely from the alpha decay of uranium and thorium. Helium-3 (³He), with one neutron, accounts for only about 0.00014%, but is very valuable for cryogenics, neutron detectors and as a possible fusion fuel.
Occurrence and production
It is the second most abundant element in the universe (about 24% of mass), formed in the Big Bang and in stars. On Earth it is scarce because it escapes into space; it accumulates in some natural gas fields (up to 7%) through radioactive decay. It is obtained by cryogenic fractional distillation of natural gas, mainly in the United States, Qatar and Algeria.
History and origin of the name
In 1868, during a solar eclipse, Janssen and Lockyer observed an unknown spectral line and attributed it to a new element «solar»: helios. William Ramsay isolated it on Earth in 1895 from the mineral cleveite, and Heike Kamerlingh Onnes liquefied it in 1908, opening the field of low-temperature physics.
Applications and relevance of helium
Uses of the element
Its main use is cryogenics: it cools the superconducting magnets of magnetic resonance imaging and of accelerators such as the LHC. Cryogenics in welding, in breathing mixtures for deep diving, in balloons and airships, and for leak detection due to its ability to pass through microfissures.
Uses of its compounds
Its compounds have no industrial applications: HHe⁺ is of interest to astrochemistry as the first molecule of the universe, helium-3 is used in dilution refrigeration and neutron detectors, and endofullerenes are studied as model quantum systems.
Health, safety and environment
Biological role
It is chemically inert and does not participate in any biological process: it is not a nutrient or toxic by itself. The only known physiological effect is physical, not chemical.
Safety
The danger is asphyxiation: being odourless, it displaces oxygen in enclosed spaces without warning. Liquid helium (−269 °C) causes cryogenic burns and pressurized cylinders must be handled with care.
Environment
It is a non-renewable resource on a human timescale: once released, it rises and is lost in space. Its extraction linked to natural gas and losses in medical and industrial uses make its recycling a priority.
Frequently asked questions about Helium
Why is helium so inert?
Its only electron shell is complete with two electrons (1s²), an exceptionally stable configuration. Removing an electron requires a very high ionisation energy, and it has no place to accommodate extra electrons. With no tendency to give up, capture or share electrons, it forms no bonds under ordinary conditions and remains always as free He atoms. Only the violence of a plasma or pressures of hundreds of gigapascals manage to force it to interact.
Why are helium balloons not made with hydrogen?
Because hydrogen is extremely flammable and any spark turns the balloon into a fireball, as demonstrated by the tragedy of the airship Hindenburg in 1937. Helium is also lighter than air and makes balloons and airships float, but being inert it does not burn or explode. That safety far outweighs its much higher cost and scarcity.
What is special about helium-3?
It is the light isotope of helium, with a single neutron and only about 0.00014% abundance. It is extremely rare on Earth, but abundant in lunar regolith. It is of interest for two reasons: it enables temperatures close to absolute zero in dilution refrigeration and is an ideal fuel for nuclear fusion, since its reaction with deuterium produces barely any radioactive neutrons.
Does helium really have any compound?
Yes, but none ordinary. In space there is helium hydride (HHe⁺), the first molecule of the universe, detected in the NGC 7027 nebula. At pressures above 100 GPa sodium and helium crystallise as Na₂He. And helium atoms can be trapped inside fullerenes (He@C₆₀) without forming bonds. In a normal laboratory, helium does not combine with anything.
Why does helium change the voice?
Voice depends on the speed of sound in the gas of the respiratory tract, and in helium sound travels almost three times faster than in air. That shifts resonance frequencies upward and the voice sounds higher, like cartoons. The effect is reversible, but inhaling it directly from a pressurized bottle can cause asphyxiation or lung injury.
Is it true that helium is running out?
On a human scale, yes: helium released into the atmosphere ends up escaping into space and is not regenerated, since on Earth it is only slowly produced by radioactive decay. Natural-gas-linked reserves are limited and some uses (cryogenics, welding, festive balloons) waste it. That is why recycling is growing in hospitals and laboratories and there are calls to reserve it for non-substitutable uses.
Why does magnetic resonance need liquid helium?
MRI scanners use superconducting magnets that only work near absolute zero (−269 °C), and liquid helium is the only refrigerant capable of keeping them. Without it, the magnet loses superconductivity in a quench that vaporises hundreds of litres at once. Helium scarcity has driven recycling equipment and designs that consume much less.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.