|
|
| Appearance |
| colorless gas |
| General |
| Name, symbol, number |
helium, He, 2 |
| Element category |
noble gases |
| Group, period, block |
18, 1, s |
| Standard atomic weight |
4.002602(2) g·mol−1 |
| Electron configuration |
1s2 |
| Electrons per shell |
2 (Image) |
| Physical properties |
| Phase |
gas |
| Density |
(0 °C, 101.325 kPa)
0.1786 g/L |
| Melting point |
(at 2.5 MPa) 0.95 K, −272.20 °C, −457.96 °F |
| Boiling point |
4.22 K, −268.93 °C, −452.07 °F |
| Critical point |
5.19 K, 0.227 MPa |
| Heat of fusion |
0.0138 kJ·mol−1 |
| Heat of vaporization |
0.0829 kJ·mol−1 |
| Specific heat capacity |
(25 °C) 20.786 J·mol−1·K−1 |
| Vapor pressure (defined by ITS-90) |
| P/Pa |
1 |
10 |
100 |
1 k |
10 k |
100 k |
| at T/K |
|
|
1.23 |
1.67 |
2.48 |
4.21 |
|
| Atomic properties |
| Electronegativity |
no data (Pauling scale) |
| Ionization energies |
1st: 2372.3 kJ·mol−1 |
| 2nd: 5250.5 kJ·mol−1 |
| Covalent radius |
28 pm |
| Van der Waals radius |
140 pm |
| Miscellaneous |
| Crystal structure |
hexagonal close-packed |
| Magnetic ordering |
diamagnetic[1] |
| Thermal conductivity |
(300 K) 0.1513 W·m−1·K−1 |
| Thermal expansion |
(25 °C) { µm·m−1·K−1 |
| CAS registry number |
7440-59-7 |
| Most stable isotopes |
| Main article: Isotopes of helium |
|
|
|
|
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