002Noble gas · Group 18 · Period 1
Helium
Helium (He) is element 2, a noble gas in the s-block — group 18, period 1. It is a gas at room temperature with 1 electron shells (2) and an atomic mass of 4.0026022 u.
- Atomic mass
- 4.003 u
- Phase · STP
- gas
- Melting
- 1 K · -272 °C
- Boiling
- 4 K · -269 °C
- Electronegativity
- —
- Density
- 0.1786 g/cm³
Photograph — Jurii, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons, source: https://images-of-elements.com/helium.php
Emission spectrum
The visible light Helium emits when energised — its unique spectral fingerprint.
Identity
- Atomic number
- 2
- Standard atomic weight
- 4.003 u
- Category
- Noble gas
- Group
- 18 · Noble gases
- Period
- 1
- Block
- s-block
- Oxidation states
- 0
Physical properties
- Melting point
- 1 K · -272 °C
- Boiling point
- 4 K · -269 °C
- Density
- 0.1786 g/cm³
- Molar heat capacity
- —
- Atomic radius
- 140 pm
- Electron affinity
- -48 kJ/mol
Atomic structure
2protons
2neutrons
2electrons
2valence e⁻
- Noble-gas notation
- 1s2
- Full configuration
- 1s2
- Electrons per shell
- 2
Periodic trends
Where Helium ranks across all 118 elements.
Ionization energies
Energy to remove each successive electron (kJ/mol).
2.4k1
5.3k2
Successive ionization energies (kJ/mol, log scale).
Discovery
- Discovered
- 1868
- Discovered by
- Pierre Janssen
- Named by
- —
- Appearance
- colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field