A helium nucleus is the center of a helium atom, made of two protons and two neutrons. It is about 3.4 femtometers across, measured as twice its charge radius, the average radius of its electric charge. A femtometer is a millionth of a billionth of a meter (10⁻¹⁵ meters). A helium nucleus is also called an alpha particle.

Size

A nucleus has no hard edge, so its size is given as a charge radius. For the helium nucleus, the recommended value listed by the U.S. National Institute of Standards and Technology (NIST) is 1.6785 femtometers. That makes it 3.36 femtometers across.

The most precise measurement used helium nuclei orbited by a single muon instead of two electrons. A muon is a particle like an electron but about 200 times heavier. The result, reported in 2021 by a team working at the Paul Scherrer Institute in Switzerland, was 1.67824 femtometers. It agrees with earlier experiments that fired electrons at helium nuclei, and is 4.8 times as precise.

The helium nucleus is unusually compact. It is even smaller than the nucleus of deuterium, a heavy form of hydrogen, which holds only one proton and one neutron.

Size comparisons

A helium nucleus is about twice as wide as a single proton, which is about 1.7 femtometers across, and it has about four times the mass.
A uranium nucleus holds about 60 times as many protons and neutrons as a helium nucleus, but it is only about 3.5 times as wide.
If a helium nucleus were enlarged to the size of a basketball, a hydrogen atom on the same scale would be about 7.6 kilometers (4.7 miles) across.
About 30 billion helium nuclei lined up side by side would span the width of a human hair, about 0.1 millimeters.

Alpha particles

Many heavy radioactive atoms, including the three kinds of uranium found in nature, break down by throwing out a helium nucleus. This is called alpha decay. Helium is too light for Earth's gravity to hold in the atmosphere over billions of years. The helium in the air comes almost entirely from alpha decay inside the solid Earth.

Alpha particles were among the first tools of nuclear physics. In 1919, Ernest Rutherford fired them at nitrogen and knocked protons out of its nuclei. In 1930, alpha particles fired at the metal beryllium set off a penetrating radiation, which James Chadwick showed in 1932 to be made of neutrons.

Named after the Sun

Helium was found in the Sun before it was found on Earth. In 1868, the French astronomer Pierre Janssen detected it in light from the Sun during a solar eclipse. The English astronomer Joseph Norman Lockyer found it independently and named it after the Greek word for the Sun, "helios". In 1895, the Scottish chemist William Ramsay found helium on Earth, in a uranium mineral called cleveite.

Sources

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How big is the alpha particle? Laser spectroscopy provides new results with record precision. Max Planck Institute of Quantum Optics, 2021.
Nuclear Charge Radii. International Atomic Energy Agency, 2013.
Properties of Nuclei. In University Physics Volume 3. OpenStax, 2016.
Helium. Commission on Isotopic Abundances and Atomic Weights.
Uranium. Commission on Isotopic Abundances and Atomic Weights.
James Chadwick: Facts. Nobel Prize Outreach.
James Chadwick: Biographical. Nobel Prize Outreach.
Proton rms charge radius. NIST, 2022 CODATA recommended values, 2024.
Alpha particle-proton mass ratio. NIST, 2022 CODATA recommended values, 2024.
Bohr radius. NIST, 2022 CODATA recommended values, 2024.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.

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