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Pronunciation | (HEE-lee-əm) |
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Appearance | colorless gas, exhibiting a red-orange glow when placed in a high-voltage electric field |
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Group | group 18 (noble gases) |
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Period | period 1 |
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Block | s-block |
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Electron configuration | 1s2 |
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Electrons per shell | 2 |
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Phase at STP | gas |
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Melting point | 0.95 K (−272.20 °C, −457.96 °F) (at 2.5 MPa) |
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Boiling point | 4.222 K (−268.928 °C, −452.070 °F) |
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Density (at STP) | 0.1786 g/L |
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when liquid (at m.p.) | 0.145 g/cm3 |
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when liquid (at b.p.) | 0.125 g/cm3 |
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Triple point | 2.177 K, 5.043 kPa |
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Critical point | 5.1953 K, 0.22746 MPa |
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Heat of fusion | 0.0138 kJ/mol |
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Heat of vaporization | 0.0829 kJ/mol |
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Molar heat capacity | 5R/2 = 20.786 J/(mol·K) |
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Vapor pressure (defined by ITS-90)
P (Pa) |
1 |
10 |
100 |
1 k |
10 k |
100 k |
at T (K) |
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1.23 |
1.67 |
2.48 |
4.21 |
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Oxidation states | 0 |
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Electronegativity | Pauling scale: no data |
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Ionization energies | - 1st: 2372.3 kJ/mol
- 2nd: 5250.5 kJ/mol
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Covalent radius | 28 pm |
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Van der Waals radius | 140 pm |
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Spectral lines of helium |
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Natural occurrence | primordial |
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Crystal structure | hexagonal close-packed (hcp) |
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Speed of sound | 972 m/s |
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Thermal conductivity | 0.1513 W/(m⋅K) |
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Magnetic ordering | diamagnetic[3] |
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CAS Number | 7440-59-7 |
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Discovery | 1868 |
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First isolation | William Ramsay, Per Teodor Cleve, Abraham Langlet (1895) |
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Category: Helium | references |