Helium
Noble gas · Period 1 · Group 18 · s-block
Born in the Sun, too light for Earth to keep
- Atomic mass
- 4.003 u
- State at 25 °C
- Gas
- Electronegativity
- —
- Melting point
- -272.2 °C
- Boiling point
- -268.9 °C
- Configuration
- 1s2
Helium, explained
Overview
Helium's two electrons completely fill its only shell, 1s2, and they are held tightly by a nucleus very close by. The result is the highest first ionization energy of any element (2372 kJ/mol) and almost no tendency to form bonds; no stable neutral helium compound is known under ordinary conditions. The attractions between helium atoms are so feeble that it has the lowest boiling point of any substance, 4.2 K (-269 °C), and it never freezes at normal pressure, no matter how cold.
Helium is the second most abundant element in the universe, made in the Big Bang and by fusion in stars. Earth's atmosphere holds only about 5 parts per million, because helium atoms are light enough to escape into space. The helium we use comes from natural gas fields, where it has accumulated over millions of years from the alpha decay of uranium and thorium in rock. Cooled below 2.17 K, helium-4 becomes a superfluid that flows without friction, and liquid helium keeps the superconducting magnets of MRI scanners and particle accelerators cold.
Discovery
During a solar eclipse in 1868, the French astronomer Pierre Janssen saw a bright yellow line at 587.5 nm in the spectrum of the Sun's prominences. Norman Lockyer concluded it came from an unknown element and, with Edward Frankland, named it after the Greek helios, the Sun. In 1895 William Ramsay found the same line in gas released by treating the uranium mineral cleveite with acid, and Per Teodor Cleve and Nils Langlet independently isolated it in Uppsala that year.
- Where it’s found
- Helium is extracted from natural gas deposits in places such as the United States, Qatar and Algeria, where some wells contain up to a few percent helium produced by radioactive decay underground.
- What it’s used for
- MRI magnet cooling
- Balloons and airships
- Deep-sea diving gas
- Welding shielding gas
- Leak detection
- Semiconductor manufacturing
- In living things
- Helium has no biological role. It is non-toxic and passes through the body unchanged.
- Safety
- Helium is non-flammable and non-toxic, but breathing it in place of air causes rapid asphyxiation, and inhaling directly from a pressurized tank can rupture lung tissue. Liquid helium causes severe cold burns and expands about 750-fold as it boils.
How it compares
Where Helium sits among all 118 elements on the trends that shape the table.
- ElectronegativityNo data
Not measured for this element
- Ionization energy2,370kJ/mol
#1 of 104 elements with data
- Atomic radius120pm
#77 of 95 elements with data
- Electron affinity-48kJ/mol
#95 of 109 elements with data
- Density0g/cm³
#113 of 114 elements with data
- Melting point0.95K
#110 of 110 elements with data
- Density
- 0.179 g/L
- Atomic radius
- 120 pm
- Covalent radius
- 46 pm
- Molar heat
- —
- Appearance
- colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field
- Discovered by
- Pierre Janssen
Inside the atom
2 electrons arranged in 1 shell: 2.
Orbital diagram
1s2 · shorthand 1s2
- 1s2
Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Helium bonds.
Ionization energies
Signature reactions
The chemistry Helium is known for.
Nuclear · Fusion
Proton-proton chain (net)
In the Sun's core, four hydrogen nuclei are fused in several steps into one helium-4 nucleus, emitting two positrons and two neutrinos. About 0.7% of the mass is converted into energy, which eventually leaves the Sun as light. This is where most of the universe's helium beyond the Big Bang supply comes from.
Conditions: stellar core, about 15 million K
Nuclear · Alpha decay
Alpha decay of uranium-238
A uranium-238 nucleus ejects an alpha particle, which is simply a helium-4 nucleus; it soon picks up two electrons and becomes a helium atom. Over geological time this decay, along with that of thorium, fills pores in rock with helium. Natural gas traps concentrate it enough to be extracted commercially.
Conditions: spontaneous, half-life 4.5 billion years
Nuclear · Fusion
Triple-alpha process
When a star exhausts the hydrogen in its core, it contracts and heats until three helium nuclei can fuse into carbon-12, via a fleeting beryllium-8 intermediate. This step bridges the gap left by unstable nuclei of mass 5 and 8. Nearly all the carbon in living things was made this way.
Conditions: red giant core, about 100 million K
Where you’ll meet it
Gas balloon
Helium, at one-seventh the density of air, lifts balloons without the fire risk of hydrogen.
Photo: Wikimedia Commons
Goodyear Blimp
Goodyear's airships have flown on helium for decades, a direct lesson from hydrogen airship disasters.
Photo: Wikimedia Commons
MRI scanner
Most MRI machines bathe their superconducting coils in liquid helium at about 4 K.
Photo: Wikimedia Commons
Large Hadron Collider
About 120 tonnes of helium, much of it superfluid, cool the LHC's magnets to 1.9 K, colder than outer space.
Photo: Wikimedia Commons
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Research
No research on Helium yet
When Bhavya publishes a project that studies Helium, it will be linked here.