He
Helium is the second lightest and second most abundant element in the observable universe.
Physical Properties
| Atomic Mass | 4.0026 u |
| Density | 0.0001785 g/cm³ |
| Melting Point | 0.95 K |
| Boiling Point | 4.22 K |
| Appearance | Colorless gas |
| State at Room Temp | Gas |
Chemical Properties
| 1st Ionization Energy | 2372.3 kJ/mol |
| Electron Affinity | -48.0 kJ/mol |
| Oxidation States | 0 |
Atomic Properties
| Electron Configuration | 1s2 |
| Atomic Radius | 31.0 pm |
| Covalent Radius | 28.0 pm |
| Van der Waals Radius | 140.0 pm |
Discovery
| Discovered By | Pierre Janssen, Joseph Norman Lockyer |
| Discovery Year | 1868 |
| Location | Guntur, India / London, England |
| Named After | Greek 'helios' meaning sun |
About Helium
Helium is a colorless, odorless, inert monatomic gas that heads the noble gas series. It has the lowest boiling point of all elements at 4.22 K and can only be solidified under pressure. Helium is formed on Earth through radioactive alpha decay of heavy elements. It was first detected spectroscopically in the Sun before being found on Earth.
Uses & Applications
Cryogenics, MRI machines, party balloons, deep-sea diving gas mixtures, leak detection, and semiconductor manufacturing.
Fun Fact
Helium is the only element that was discovered in space before it was found on Earth, first observed in the solar spectrum during a solar eclipse in 1868.
Isotopes
| Mass Number | Abundance | Half-Life | Stable |
| 3 | 0.00000134% | - | Yes |
| 4 | 0.99999866% | - | Yes |
Electron Configuration
1s2