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Helium nucleus: size, dimension and comparison

The adopted reference is about 3.4 fm.

approximate charge diameter

fm means femtometer: a unit used to measure the nuclei of atoms. Imagine 1 millimeter split into one trillion equal pieces: one piece measures 1 fm.

A particularly stable nucleus at the heart of helium-4.

Explore at this scale
Helium nucleus, interpreted illustration
Interpreted illustration · dimension stated in the fact sheet.

What does this number measure?

A particularly stable nucleus at the heart of helium-4. Helium-4 consists of two protons and two neutrons. Its nucleus is also called an alpha particle when emitted in certain radioactive decays.

Compare the stated dimensions

The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.

Compare with Classical electron radius in the explorer →

Source and limitations

Catalogue order of magnitude; a source is provided, but this measurement has not been individually audited.

Helium-4 nucleus: model of two protons and two neutrons. Spheres, colours and arrangement are schematic; approximate charge diameter.