Helium (He)
Helium (He) is a noble gas with atomic number 2 and a standard atomic weight of 4.0026. At room temperature it is a gas.
Helium key facts
Helium at a glance
Identity
| Symbol | He |
|---|---|
| Atomic number (Z) | 2 |
| Atomic weight | 4.0026 ± 0.0001 (CIAAW 2024) |
| Category | Noble gas |
| Group | 18 — noble gases |
| Period | 1 |
| Block | s-block |
| CAS number | 7440-59-7 |
Atomic properties
| Electron configuration | 1s2 |
|---|---|
| Electrons per shell | 2 |
| First ionisation energy | 24.587 eV (2,372.3 kJ/mol) 2nd: 54.418 eV |
| Atomic radius (empirical) | 120 pm |
| Covalent radius | 46 pm |
| Van der Waals radius | 140 pm |
Physical properties
| State at 25 °C | Gas |
|---|---|
| Boiling point | -268.9 °C · -452.1 °F · 4.22 K |
| Density | 0.1786 g/L (gas at 0 °C, 1 atm) |
| Specific heat capacity | 5.193 J/(g·K) |
| Thermal conductivity | 0.152 W/(m·K) |
| Crystal structure | Hexagonal |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 4He 99.9998%, 3He 0.0002% |
| Stable isotopes | 2 |
| Known isotopes | 8 |
| Abundance in Earth's crust | 0.008 mg/kg |
| Abundance in seawater | 0.000007 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Helium electron configuration
Helium has 2 electrons. Written in short form, its ground-state configuration is 1s2.
As a member of group 18 it has 2 valence electrons in its outer shell (shell 1), which largely decide its chemistry — a full outer shell, which is why it is so unreactive.
Discovery and name
1895 — William Ramsay; independently Per Teodor Cleve and Nils Abraham Langlet · Scotland / Sweden.
Greek hēlios, the Sun — first seen as a yellow line in the solar spectrum.
Known for
The second lightest and second most abundant element in the universe. Liquid helium (4.2 K) cools MRI magnets and research instruments; helium also fills balloons and protects welds.
Isotopes of helium
Naturally occurring helium is a mix of 2 isotopes. Each has 2 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 4He | 2 | 2 | 99.9998% | stable |
| 3He | 2 | 1 | 0.0002% | stable |
8 isotopes of helium are known in our data set (half-lives as published in nuclear data tables).
Helium compared with group 18
The other elements in group 18 (noble gases), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Helium | 2 | 4.0026 | — | -269 °C | 0.1786 g/L | — |
| Neon | 10 | 20.180 | -249 °C | -246 °C | 0.9002 g/L | — |
| Argon | 18 | 39.95 | -189 °C | -186 °C | 1.784 g/L | — |
| Krypton | 36 | 83.798 | -157 °C | -153 °C | 3.749 g/L | 3 |
| Xenon | 54 | 131.29 | -112 °C | -108 °C | 5.894 g/L | 2.6 |
| Radon | 86 | [222] | -71 °C | -62 °C | 9.73 g/L | 2.2 |
| Oganesson | 118 | [294] | — | — | 7 g/cm³ (pred.) | — |