Helium
Element 2 · Noble Gas
Helium carries atomic number 2 and the simplest full electron shell in nature: just two electrons filling the 1s orbital. That closed shell makes helium the least reactive element known — it forms no stable compounds under normal conditions. Helium is the second-lightest element and the second most abundant in the universe, born in the fusion furnaces of stars. Its claim to fame on Earth is cold: liquid helium boils at -268.93 °C, colder than any other substance, which makes it the coolant of choice for superconducting magnets in MRI scanners and particle accelerators. It also fills balloons, shields welds, and pressurizes rocket fuel tanks.
Key facts
| Atomic number | 2 |
|---|---|
| Atomic weight | 4.003 |
| Category | Noble Gas |
| Group / Period | 18 / 1 |
| Block | s |
| Electron configuration | 1s2 |
| Common oxidation states | 0 |
| State at STP | Gas |
| Atomic radius | 31 pm |
| Density | 0.00018 g/cm³ |
| Melting point | -272.20 °C |
| Boiling point | -268.93 °C |
Occurrence
Helium makes up roughly 24 percent of the universe's elemental mass, mostly in stars. On Earth it is rare in air but collects in some natural gas fields, where radioactive decay in rocks has produced it over millions of years; certain wells hold up to about 7 percent helium.
Applications
Cryogenic coolant for MRI scanners
Balloons and airships
Shielding gas for arc welding
Leak detection in vacuum systems
Breathing mixtures for deep-sea diving
Common compounds
- No stable neutral compounds (noble gas)
- Helium hydride ion (HeH+) detected in interstellar space
Helium was discovered in the Sun's spectrum in 1868 — 27 years before anyone found it on Earth. Its name comes from 'helios', the Greek word for the Sun.
Data source: IUPAC standard atomic weights 2021