Helium
Atomic Data
| Atomic Number | 2 |
| Symbol | He |
| Atomic Weight | 4.0026 u |
| Density (STP) | 0.1786 g/L |
| Melting Point | N/A °C (None K) |
| Boiling Point | −268.93 °C (4.22 K) |
| Electronegativity | — |
| Electron Config. | 1s2 |
| Oxidation States | 0 |
| Phase at STP | Gas |
| Category | Noble Gas |
| Period / Group | 1 / 18 |
| CAS Number | 7440-59-7 |
Electron Configuration
1s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| Total | 2 | 2 | ||
Abundance & Occurrence
Helium is present in Earth's crust at approximately 0.008 ppm by mass and at approximately 230000 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Asphyxiation: Helium is a colourless, odourless gas that displaces oxygen in confined or poorly ventilated spaces; high concentrations can cause dizziness, loss of consciousness, and death without warning.
- Pressurised cylinders: Cylinders must be stored upright, chained securely, and kept away from heat sources; damaged valves can cause rapid, uncontrolled gas release with enough force to turn a cylinder into a projectile.
- Inhalation misuse: Inhaling helium directly from a pressurised cylinder — rather than a balloon — can rupture lung tissue and force a fatal gas embolism into the bloodstream; this misuse has caused deaths.
- Liquid helium: Liquid helium is stored at −269 °C; contact with skin or eyes causes severe cryogenic burns, and rapid vaporisation in a closed space can cause a sudden oxygen-deficient atmosphere.
Helium in the Real World
Real-World Uses
- MRI cooling — Liquid helium at −269 °C keeps the superconducting electromagnets in MRI scanners at near-absolute-zero, enabling the powerful magnetic fields required for medical imaging.
- Lifting gas — Helium's low density and non-flammability make it the safe choice for inflating weather balloons, scientific high-altitude research balloons, and airships.
- Pressurisation — Helium pressurises rocket propellant tanks and purges fuel lines; its chemical inertness prevents any reaction with liquid hydrogen or liquid oxygen propellants.
- Welding and semiconductor fabrication — Helium provides an inert shielding atmosphere in arc welding and in the growth of silicon and gallium arsenide crystals for microchips and solar cells.
- Diving gas mixtures — Heliox (helium-oxygen) and trimix (helium-nitrogen-oxygen) replace nitrogen in deep-sea diving to prevent nitrogen narcosis and decompression sickness at great depths.
- Cryogenic research — Physicists use liquid helium to reach temperatures below 4 K for studies of superconductivity, superfluidity, and quantum computing hardware.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is helium used for?
Helium is used primarily as a cryogenic coolant for the superconducting magnets in MRI scanners, as a lifting gas for weather and scientific balloons, as a pressurising gas in rockets, and as a shielding atmosphere in welding and semiconductor manufacturing. In medicine, helium-oxygen (heliox) gas mixtures help patients with severe airway obstruction breathe more easily.
Why is helium lighter than air?
Helium has an atomic weight of 4.0026 u, far less than the average molecular weight of air (about 29 u). Because helium atoms are so much lighter than nitrogen and oxygen molecules, a given volume of helium weighs roughly seven times less than the same volume of air, causing helium-filled balloons and blimps to rise.
Why does helium change the sound of your voice?
Sound travels about three times faster in helium than in air because helium is much less dense. When your vocal tract is filled with helium, the resonant frequencies of your voice shift upward, producing the characteristic high-pitched squeak. Inhaling helium is dangerous, however, as it displaces oxygen and can cause asphyxiation.
Is the world running out of helium?
Helium is a finite, non-renewable resource on Earth. Once released into the atmosphere it is too light to be retained by Earth's gravity and escapes into space. Known underground reserves are concentrated in a handful of countries, and global consumption for MRI machines, semiconductor manufacturing, and scientific research has raised concern about long-term supply. New discoveries and recycling programmes help extend reserves, but shortages and price spikes have already occurred.