ElementsWhat is helium?
Helium is a very light gas that does not burn and does not react with other elements. It is the second most common element in the universe, but rare on Earth, where it is collected from natural gas wells.
What is helium used for?
Cooling the magnets of MRI scanners, filling balloons and airships, breathing mixes for deep-sea divers, and leak testing.
Where the name comes from
From the Greek word helios, the Sun. It was seen in sunlight during an eclipse in 1868, before it was found on Earth.
Key facts
- Symbol
- He
- Atomic number
- 2
- Atomic mass
- 4.00260 u
- Family
- Noble gas
- Period
- 1
- Group
- 18
- Block
- s
- State at room temperature
- Gas
- Melting point
- -272.2 °C (0.95 K, -458 °F)
- Boiling point
- -268.9 °C (4.22 K, -452 °F)
- Density
- 0.0001785 g/cm³ (as a gas at 0 °C)
- Electrons per shell
- 2
- Electron configuration
- 1s2
- Common oxidation states
- 0
- Electronegativity (Pauling)
- no value
- First ionisation energy
- 24.587 eV
- Electron affinity
- no value
- Atomic radius (van der Waals)
- 140 pm
- Discovered
- 1868
Place in the periodic table
Helium is element number 2. It sits in period 1 (row 1) of the periodic table, in group 18 (column 18), and belongs to the s-block. Just before it comes hydrogen (1), and just after it comes lithium (3). In the same column, neon is below it. Elements in one column tend to behave alike.
Family: Noble gases.
Physical properties
At room temperature helium is a gas. It boils at -268.9 °C, the lowest boiling point of any element, and it does not freeze at normal pressure however cold it gets. It turns solid only when squeezed. One litre of the gas weighs about 0.178 grams at 0 °C and normal pressure. That is lighter than air, which weighs about 1.29 grams a litre.
Inside the atom
An atom of helium has 2 protons in its nucleus and, when neutral, 2 electrons around it. The electrons are arranged in 1 shell: 2. The outermost shell holds 2 electrons. Written in full, the electron configuration is 1s2.
Removing one electron from an atom takes 24.587 electronvolts (about 2,372 kilojoules per mole). One mole of helium weighs 4.00260 grams, so a single gram contains about 1.50 × 1023 atoms.
The ring picture is a simple model that only counts the electrons in each shell. Real atoms do not look like this.
Next steps
- Work out the weight of a compound that contains helium with the molar mass calculator.
- Compare it with every other element in the sortable list.
- Test yourself with the symbol quiz.
Figures on this page are from PubChem (U.S. National Library of Medicine), as downloaded on 11 November 2026. See About and data sources.