2HeHelium4.002602 u
Helium (He)
Helium is a noble gas with atomic number 2, in period 1 and group 18 of the periodic table. Its atomic mass is 4.002602 u and its electron configuration is 1s². Gas at 25 °C.
Key facts
| Atomic number | 2 |
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| Atomic mass | 4.002602 u |
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| Category | Noble Gas |
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| Period | 1 |
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| Group | 18 |
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| Block | s |
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| Electron configuration | 1s² |
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| Electrons per shell | 2 |
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| Electronegativity (Pauling) | --- |
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| Oxidation states | --- |
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| State at 25 °C | Gas |
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| Melting point | --- |
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| Boiling point | 4.22 K−268.93 °C |
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| Density | 0.1785 g/L |
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Atomic properties
| First ionization energy | 24.587 eV |
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| Electron affinity | 0 kJ/mol |
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| Atomic radius | 31 pm |
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| Covalent radius | 28 pm |
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| Van der Waals radius | 140 pm |
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| Common ions | --- |
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| Protons, neutrons, electrons | 2, 2, 2 |
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Physical and thermal properties
| Molar volume | 21 cm³/mol |
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| Heat of fusion | 0.02 kJ/mol |
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| Heat of vaporization | 0.083 kJ/mol |
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| Specific heat | 5193.1 J/(kg·K) |
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| Thermal conductivity | 0.1513 W/(m·K) |
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| Thermal expansion | --- |
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| Mohs hardness | --- |
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| Young’s modulus | --- |
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| Crystal structure | Hexagonal close-packed |
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| Magnetic type | Diamagnetic |
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Identification and history
| CAS number | 7440-59-7 |
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| Latin name | Helium |
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| Discovered by | P. Janssen, N. Lockyer · 1868 |
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| Named by | de Helios |
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Natural isotopes
| Isotope | Mass (u) | Abundance |
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| 3He | 3.016029 | 0.0002 % |
| 4He | 4.002603 | 99.9998 % |
Data checked against PubChem (NIH), IUPAC, the NIST Atomic Spectra Database and the IAEA (NUBASE and AME), the same data you see in the app.
A dash (---) means there is no reliable published measurement. The app never makes one up.
See Helium in the app
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Periodic Table