Periodic table / Noble gases
2Helium
Helium is a noble gase in group 18, period 1. At 25 °C it is a gas.
- Symbol
- He
- Atomic number
- 2 protons in the nucleus
- Standard atomic weight
- 4.0026 ± 0.0001
- Electron configuration
- 1s2
- Electrons in each shell
- 2
- Group, period, block
- 18, 1, s
- Family
- Noble gases
- State at 25 °C
- gas
- Electronegativity
- no value
- First ionisation energy
- 2372.3 kJ/mol (24.587389011 eV)
Where these numbers come from
Atomic weight: IUPAC CIAAW, abridged standard atomic weights 2024. Electron configuration and ionisation energy: NIST Atomic Spectra Database. Family, state and electronegativity: PubChem. See Sources. Melting points, boiling points and densities are left out for now: published values differ between sources, and we will add them once each has been checked.