Nuclides
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Chemical element · Atomic number 2
Helium (He)
Noble gases · Period 1 · Gas
Element identity
- Atomic number (Z)
- 2
- Symbol
- He
- Atomic weight / isotope mass number
- 4.002 602(2)
- Category
- Noble gases
- Physical state (reference)
- Gas
- Period
- 1
- Group (reference)
- 18
- Block
- s
Physical properties
- Appearance
- Colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field
- Density
- 0.1786×10⁻³ g·cm⁻³
- Melting point
- 0.95 K / −272.20 °C / −457.96 °F
- Boiling point
- 4.222 K / −268.928 °C / −452.070 °F
- Sublimation point
- Not available
Atomic and material properties
- Electron configuration
- 1s²
- Electrons per shell
- K2
- Oxidation states
- 0
- Electronegativity (Pauling)
- Not available
- Atomic radius
- 31 pm
- Covalent radius
- 28 pm
- Molar ionization energies
- 1st: 2372.3 kJ·mol⁻¹ 2nd: 5250.5 kJ·mol⁻¹
- Thermal conductivity
- 0.1513 W·m⁻¹·K⁻¹
- Molar heat capacity
- 20.78 J·mol⁻¹·K⁻¹
- Crystal structure
- Hexagonal close-packed
- CAS number
- 7440-59-7
Property source: Wikipedia — Helium, adapted from the Nuclides App snapshot. Quoted conditions and qualifiers are retained.
Sources and interpretation
Snapshot exported 24 October 2026 from the element assets used by Nuclides. Properties are a reference compilation, not a new evaluation. Missing values are shown as “Not available”. Density and phase depend on temperature, pressure and allotrope; retained conditions are part of the quoted property.
Atomic weight is dimensionless. Parentheses give uncertainty in the last quoted digits. A single integer in square brackets is an isotope mass number, not a standard atomic weight. The snapshot can differ from current evaluations; consult CIAAW standard atomic weights and the IUPAC periodic table for current values and conventions.
Element properties in the App snapshot are adapted from Wikipedia. Each element page links to its source article; text adaptations are available under CC BY-SA 4.0. No third-party images are included.
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