Helium
Helium (He) is a noble gas with atomic number 2 and an atomic mass of 4.0026. Its electron configuration is 1s2, which gives it 2 valence electrons.
Helium key data
| Symbol | He |
|---|---|
| Atomic number | 2 |
| Atomic mass (standard atomic weight) | 4.0026 |
| Molar mass | 4.0026 g/mol |
| Protons | 2 |
| Neutrons (most abundant isotope, helium-4) | 2 |
| Electrons (neutral atom) | 2 |
| Valence electrons | 2 |
| Category | Noble gas |
| Period | 1 |
| Group | 18 |
| Block | s-block |
| Noble gas notation | 1s2 |
| Full electron configuration | 1s2 |
| Electrons per shell | 2 |
Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.
The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.
Measured properties of helium
From PubChem, the US National Institutes of Health. Public domain. PubChem’s periodic table gives no value for the rows not shown.
| Melting point (only under about 25 atm) | 0.95 K (−272.2 °C), 103rd of 103 by melting point |
|---|---|
| Boiling point | 4.22 K (−268.93 °C), 93rd of 93 by boiling point |
| Density | 0.0001785 g/cm³, 95th of 96 by density |
| State at room temperature | Gas |
| Van der Waals radius | 140 pm, joint 95th of 99 by atomic radius |
| First ionisation energy | 24.587 eV (2372.3 kJ/mol), 1st of 102 by first ionisation energy |
| Common oxidation states | 0 |
| Discovered | 1868 |
Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.
Helium electron configuration
Helium has no noble-gas core, so its electron configuration is simply 1s2.
Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.
Mass of one helium atom
A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.
Mass of one atom
- one atom = 4.0026 / (6.02214076 × 10²³)
- = 6.6465 × 10⁻²⁴ g
Atoms in one gram
- atoms per gram = 6.02214076 × 10²³ / 4.0026
- = 1.5046 × 10²³ atoms
Helium on the periodic table
Helium is in period 1, group 18, in the s-block.
The other elements of group 18 are neon, argon, krypton, xenon, radon and oganesson.