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Helium (He), element 2
Helium (He) is the chemical element with atomic number 2: a noble gas in group 18, period 1. At 25 °C it is a gas, with an atomic mass of 4.003 u. It melts at -272.2 °C, one of the 3 lowest melting points known.
Physical and chemical data
| Atomic number | 2 |
|---|---|
| Symbol | He |
| Atomic mass | 4.003 u |
| Density | 0.0001785 g/cm³ |
| Melting point | -272.2 °C (0.95 K) |
| Boiling point | -268.93 °C (4.22 K) |
| Electronegativity (Pauling) | not defined |
| Electron configuration | 1s2 |
| Atomic radius (van der Waals) | 140 pm |
| Oxidation states | 0 |
| Physical state at 25 °C | gas |
| Family | noble gas |
| Group and period | 18 / 1 |
| Discoverer and year | Pierre Janssen and Norman Lockyer (1868) |
The discovery record belongs to Pierre Janssen and Norman Lockyer, in 1868.
Neighbours on the table
Horizontally, one proton less lands on Hydrogen (Z=1); one more lands on Lithium (Z=3). Vertically, group 18 gathers relatives with similar behaviour: Neon, Argon, Krypton, Xenon.
The story behind the name
It is the only element discovered off Earth before being found on it: in 1868, during an eclipse in India, an unknown yellow line showed up in the Sun's spectrum, and Lockyer named it after Helios, the Greek sun god. Only in 1895 did William Ramsay find the gas trapped in a uranium mineral down here. Helium is also the only element that never freezes at normal pressure: at 0.95 K it stays liquid, and it only solidifies squeezed under dozens of atmospheres.
Physical and chemical data: PubChem Periodic Table (NCBI, compiling NIST and IUPAC), public domain, snapshot of 2026-07-10. Temperatures converted from kelvin; historical notes cross checked against classic chemistry references.
Last updated: · Methodology and sources