Helium
Colourless, odourless gaseous nonmetallic element. Belongs to group 18 of the periodic table. Lowest boiling point of all elements and can only be solidified under pressure. Chemically inert, no known compounds. Discovered in the solar spectrum in 1868 by Lockyer.
Gallery
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
Physical Propertiesmass · density · phase · crystal
Atomic Mass
4.0026Da
Density
0.0002g/L
Phase (STP)
Gas
Melting Point
—
Boiling Point
4.2K
Molar Heat
5.1930J/(mol·K)
Crystal Structure
HEX
Lattice Constant
3.570Å
Atomic Radiiatomic · covalent · van der Waals
Atomic (Empirical)
120.00pm
Covalent
46.00pm
Van der Waals
140.00pm
Electronic Propertieselectronegativity · ionization · affinity
Electron Configuration
1s2
Electron Shells
2
Electronegativity
—
Electron Affinity
-19.7kJ/mol
1st Ionization Energy
24.6kJ/mol
Oxidation States
0
Ionization Energies (2 known)
Emission Spectrumvisible range · characteristic spectral lines
Abundanceby mass
Log scale · ppm = parts per million by mass
Biological Role
Inert noble gas. Used in deep-sea diving gas mixtures.
Discovery
Discovered By
Sir William Ramsey, Nils Langet, P.T.Cleve
Named By
—
Year
1895
Nuclear Data
Known Isotopes
8
Stable Isotopes
2
Stable Mass Numbers
3, 4
Natural isotopic abundance (IUPAC recommended values)
Isotopes of He8 known
| Nuclide | Z | N | Mass (AMU) | Half-life | Decay | Jπ | Abundance |
|---|---|---|---|---|---|---|---|
| 3Hestable | 2 | 1 | 3.016029 | Stable | stable (1%) | 1/2+ | 0.0200% |
| 4Hestable | 2 | 2 | 4.002603 | Stable | stable (1%) | 0+ | 9999.9800% |
| 5He | 2 | 3 | 5.012057 | Unknown | n (1%) | 3/2- | — |
| 8He | 2 | 6 | 8.033934 | 120 ms | β⁻ (1%), β⁻n (0.16%), B-t (0.009000000000000001%) | 0+ | — |
| 9He | 2 | 7 | 9.043946 | 2.50 zs | n (1%) | 1/2 | — |
| 10He | 2 | 8 | 10.052815 | Unknown | 2n (1%) | 0+ | — |