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Helium (He)
Atomic number 2 · noble
| Atomic number | 2 |
|---|---|
| Atomic mass | 4.0026 u |
| Molar mass | 4.0026 g/mol |
| Category | noble |
| Group / period / block | 18 / 1 / s |
| Electron configuration | 1s2 |
| Electrons per shell | 2 |
| Atomic radius | 120.0 pm |
| Density | 0.00016 g/cm³ |
| First ionisation energy | 24.5874 eV |
| Boiling point | 4.22 K (-268.93 °C) |
| Abundance in Earth's crust | 0.008 ppm |
Uses
Balloons, cooling MRI magnets, deep-sea diving gas
Discovery
1895, Sir William Ramsey, Nils Langet, P.T.Cleve