Helium dimer
| Names | |
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| Other names
dihelium
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| Identifiers | |
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3D model (Jmol)
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| ChEBI | |
| 48 | |
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PubChem CID
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| Properties | |
| He2 | |
| Molar mass | 8.0052 g/mol |
| Appearance | colorless gas |
| Thermochemistry | |
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Std enthalpy of
formation (ΔfH |
1.1×10−5 kcal/mol. |
| Related compounds | |
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Related van der Waals molecules
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LiHe NeHe2 He3 |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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 (what is   ?) |
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| Infobox references | |
The helium dimer is a van der Waals molecule with formula He2 consisting of two helium atoms. This molecule is the largest with two atoms, and is very weakly bound together. The bond is so weak that it will break if the molecule rotates, or vibrates too much. It can only exist at very low cryogenic temperatures.
Two excited helium atoms can also bond to each other in a form called an excimer. This was discovered from a spectrum of helium that contained bands first seen in 1912. Written as He2* with the * meaning an excited state, it is the first known Rydberg molecule.
Dihelium ions also exist with a negative, positive and double positive charge. Two helium atoms can be confined together without bonding in the cage of a fullerene.
Based on molecular orbital theory, He2 should not exist, and a chemical bond cannot form between the atoms. However, the van der Waals force exists between helium atoms as shown by the existence of liquid helium, and at a certain range of distances between atoms the attraction exceeds the repulsion. So a molecule composed of two helium atoms bound by the van der Waals force can exist. The existence of this molecule was proposed as early as 1930.
He2 is the largest known molecule of two atoms when in its ground state, due to its extremely long bond length. The He2 molecule has a large separation distance between the atoms of about 5200 pm. This is the largest for a diatomic molecule without ro-vibronic excitation. The binding energy is only about 1.3 mK, 10−7eV or 1.1×10−5 kcal/mol. The bond is 5000 times weaker than the covalent bond in the hydrogen molecule.
Both helium atoms in the dimer can be ionized by a single photon with energy 63.86 eV. The proposed mechanism for this double ionization is that the photon ejects an electron from one atom, and then that electron hits the other helium atom and ionizes that as well. The dimer then explodes as two helium cations ions repel each other with the same speed but opposite directions.
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Wikipedia