Helium
1s² · 2
The second most abundant element in the universe and the most inert of all. Helium is the only substance that will not freeze at atmospheric pressure even at absolute zero, and the only one that turns superfluid.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 2
- Atomic mass
- 4.003 а.е.м.
- Electron configuration
- 1s²
- Electrons per shell
- 2
- Block
- s-block
- Group
- Noble gases
- Period
- 1
- Electronegativity (Pauling)
- no data
- Electronegativity (Allen)
- 4.16
- Atomic radius
- 31 pm
- Covalent radius
- 28 pm
- Van der Waals radius
- 140 pm
- Ionisation energy 1
- 2 372.3 kJ/mol
- Ionisation energy 2
- 5 250.5 kJ/mol
- Electron affinity
- 0 kJ/mol
- Common oxidation states
- 0
- Oxidation states
- 0
Physical properties
- State at 25 °C
- gas
- Density
- 0.00018 g/cm³
- Melting point
- 0.95 K · -272.2 °C
- Boiling point
- 4.22 K · -268.9 °C
- Speed of sound
- 972 m/s
Thermal properties
- Heat of fusion
- 0.0138 kJ/mol
- Heat of vaporisation
- 0.0829 kJ/mol
- Specific heat capacity
- 5.193 J/(g·K)
- Thermal conductivity
- 0.1513 W/(m·K)
Mechanical properties
- Young's modulus
- no data
- Shear modulus
- no data
- Bulk modulus
- no data
- Poisson ratio
- no data
- Mohs hardness
- no data
- Brinell hardness
- no data
Electrical and magnetic properties
- Electrical resistivity
- no data
- Magnetic ordering
- diamagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- hexagonal close-packed (hcp)
- Lattice constants
- a = 424 pm · c = 346 pm
Abundance
- In the crust
- 0.008 mg/kg
- In the ocean
- 7.2·10⁻⁶ mg/L
- In the universe
- 2.3·10⁸ mg/kg
- In the human body
- 0 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 3He | 0.0002 % | stable | — |
| 4He | 100 % | stable | — |
Helium-3 is only 0.0002 % of terrestrial helium, but it is what makes dilution refrigerators work, cooling to thousandths of a kelvin.
Discovery
- Year of discovery
- 1868
- Discovered by
- Pierre Janssen and Norman Lockyer
- Where
- India and England
- Origin of the name
- from Greek helios (Sun): first found in the spectrum of the solar corona
History
Helium was found on the Sun before it was found on Earth: in 1868 Janssen and Lockyer spotted a yellow line in the solar corona that belonged to no known element. It took another 27 years, until 1895, to isolate it here, from uranium ore.
Where it occurs
Terrestrial helium is almost all radiogenic — alpha particles from decaying uranium and thorium, accumulated in gas fields over hundreds of millions of years. The atmosphere holds only 5 parts per million: a light gas leaves for good.
How it is produced
By cryogenic distillation of natural gas; extraction pays only where the gas is at least about 0.3 % helium. The largest sources are the United States, Qatar and Algeria.
Role in living things
It has no biological role and does not linger in the body. Breathing helium raises the pitch of a voice because sound travels through it almost three times faster than through air.
Safety
Non-toxic in itself, but it displaces oxygen: inhaling pressurised helium straight from a cylinder has caused fatal lung barotrauma and asphyxiation.
Uses
- Cooling the superconducting magnets in MRI scanners and particle accelerators
- Shielding atmospheres for welding and for growing silicon crystals
- Filling balloons and airships — the non-flammable replacement for hydrogen
- Deep-diving breathing mixtures: helium causes no nitrogen narcosis
- Leak-testing vacuum systems — the smallest molecule finds the smallest gap
Curiosities
- Helium is named after the Sun — the only element discovered on another body before it was found on Earth.
- Below 2.17 K liquid helium turns superfluid: it flows with no viscosity and creeps up and over the walls of its container.
- The world's helium is finite and unrecoverable — once it reaches the atmosphere it is gone.
- The helium-4 nucleus is so stable that it is what heavy nuclei eject in alpha decay.
- Helium has the lowest boiling point of any substance: 4.2 K, or −269 °C.