Element: Helium
General data
| Element name | Helium |
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| Chemical symbol | He |
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| Atomic number Z | 2 |
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| Year discovered | 1868 |
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| Standard state (25°C) | Gas |
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| Category | Noble gas |
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| Block | s |
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| Atomic mass | 4.0026 |
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| CPK color | #D9FFFF |
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Physical properties
| Melting point | 0.95 K (-272.20 °C) |
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| Boiling point | 4.22 K (-268.93 °C) |
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| Density | 0.0001785 g/cm³ |
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Energy properties
| First ionization energy | 24.587 eV |
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| Electron affinity | no data |
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Atomic structure
| Protons | 2 (Atomic number) |
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| Neutrons (He-4, most abundant isotope) | 2 |
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| Electrons | 2 |
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| Electron configuration | 1s² |
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| Oxidation states | no data |
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| Electronegativity (Pauling) | no data |
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| Atomic radius (van der Waals) | 140 pm |
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Ionization energies - NIST ASD (2 states)
Value type as given by NIST ASD: measured - from evaluated experimental data; calculated - from ab initio calculations (mostly highly ionised atoms that have not been measured; for the simplest ones, e.g. H I, the calculation is more accurate than measurement); semi-empirical - from interpolation, extrapolation or another semi-empirical method.
