Periodic table › Helium
1. Identity and classification
Colorless, odorless monatomic gas; glows peach-orange in high-voltage discharge tubes
- Name and symbol
- Helium (Helium) · He
- Atomic number
- 2
- Relative atomic weight / mass number
- 4.0026 ± 0.0001
- Group · period · block
- 18 · 1 · s
- Category
- Noble Gas
- State near room temperature
- Gas
- Stable isotopes
- At least one stable isotope
- CAS number
- 7440-59-7
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 2 · 2
- Neutrons in reference isotope
- 2 (4He)
- Full electron configuration
1s²- Abbreviated configuration
1s²- Electrons per shell
- 2
- Listed oxidation states
- 0
- Electronegativity
- Not established
- First ionization energy
- 2372.285 kJ/mol · 24.587 eV
- Electron affinity
- Not established
- Covalent radius
- 28 pm
- van der Waals radius
- 140 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 0.0001785 g/cm³
- Melting point
- Not established
- Boiling point
- 4.22 K · -268.93 °C · -452.074 °F
- Crystal structure
- HCP
- Specific heat
- 5.193 J/(g·K)
- Thermal conductivity
- 0.1513 W/(m·K)
- Enthalpy of fusion
- 0.0138 kJ/mol
- Enthalpy of vaporization
- 0.0829 kJ/mol
- Magnetic ordering
- Diamagnetic
- Mohs hardness
- Not established
- Speed of sound
- 972 m/s
Helium has no solid–liquid melting point at 1 atm; solidification requires higher pressure.
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
A closed 1s shell explains its very low chemical reactivity.
Selected compounds: None stable at standard conditions · HeH⁺ (Helium hydride ion in interstellar space) · Na₂He (high-pressure electride phase >113 GPa)
Reaction reference
No ordinary reaction with air.
No ordinary reaction with water.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 3He | Stable | 0.0002% | Stable |
| 4He | Stable | 99.9998% | Stable |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- 1868 CE
- Scientists associated with discovery
- Pierre Janssen & Norman Lockyer
- Location
- India (Guntur solar eclipse) / UK
- Origin
- Natural
- Listed crustal abundance
- 0.008 ppm
Minerals and naming reference
Extracted from natural gas reservoirs via radioactive alpha decay of U and Th in crustal rocks
From Greek 'Helios' (Sun), as its spectral line (587.49 nm) was first observed in the solar chromosphere during the 1868 eclipse in Guntur, India
7. Applications
Party and meteorological balloons, scuba diving breathing mixtures (Heliox/Trimix)
Detailed applications by field
- everyday
- Party and meteorological balloons, scuba diving breathing mixtures (Heliox/Trimix)
- industry
- Inert shielding gas for arc welding, supersonic wind tunnels, leak detection in vacuum systems
- electronics
- Carrier gas in gas chromatography, cooling fiber-optic drawing furnaces and semiconductor plasma etchers
- medicine
- Cryogenic cooling of superconducting magnets in hospital MRI scanners; Heliox respiratory therapy
- energy
- Coolant in high-temperature gas-cooled nuclear reactors (HTGR)
- aerospace
- Pressurizing liquid hydrogen and oxygen propellant tanks in orbital launch vehicles; airships
- agriculture
- Controlled-atmosphere research and analytical gas chromatography of soil gases
- science
- Dilution refrigerators reaching millikelvin temperatures, LHC superconducting magnet cooling
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Biologically inert; neither metabolized nor toxic, though breathing pure helium displaces oxygen and causes rapid asphyxiation.