A hydrogen atom is the simplest atom: a single electron held by a single proton. It is about 0.11 nanometers across, or 110 picometers, measured as twice the Bohr radius. A nanometer is a billionth of a meter, and a picometer is a thousandth of a nanometer.

Size

An atom has no hard edge. Its electron does not follow a fixed path like a planet. Quantum physics describes it instead as a cloud of probability: the electron is most likely to be found at one particular distance from the nucleus, and less likely farther out. Chemists therefore define the size of an atom in several ways, and each gives a different number.

The figure shown here is twice the Bohr radius. The Bohr radius, 52.9 picometers, is the distance from the proton at which the electron is most likely to be found. Twice that is 106 picometers, which rounds to 110. The name honors Niels Bohr, whose 1913 model of the atom placed the electron in a circular orbit of exactly that radius.

Other definitions give other sizes. Two hydrogen atoms joined into a molecule sit with their centers 74 picometers apart. Another measure, the van der Waals radius, is based on how close two separate atoms can come before they push each other away. By that measure, a hydrogen atom is about 220–240 picometers across.

Size comparisons

A hydrogen atom is about 60,000 times as wide as the proton at its center, which is about 1.7 femtometers across. A femtometer is a millionth of a nanometer.
If a hydrogen atom were enlarged to the length of an American football field, 110 meters (120 yards) including the end zones, its proton would be less than 2 millimeters (0.08 inches) across.
About 750,000–950,000 hydrogen atoms side by side would span the width of a human hair.
A hydrogen atom is 1.06 angstroms across, a little more than one. The angstrom, 100 picometers, is a unit used for the sizes of atoms and the lengths of chemical bonds.
No ordinary light microscope can show a hydrogen atom. The finest detail such a microscope can make out is about 0.2 micrometers, nearly 2,000 times the atom's width. A micrometer is a millionth of a meter.

The most common element

Hydrogen is the most abundant element in the universe. The Sun is about 75% hydrogen by mass. At the Sun's high temperatures, hydrogen nuclei fuse together to form helium, releasing vast amounts of energy. On Earth, most hydrogen is found in water.

Heavier forms of hydrogen

Atoms of the same element with different numbers of neutrons are called isotopes. In more than 99.97% of hydrogen atoms on Earth, the nucleus is a lone proton with no neutrons. In 1931, Harold Urey and his colleagues at Columbia University detected a rarer form, deuterium, with about twice the mass.

A third form, tritium, has two neutrons as well as its proton. It is radioactive, and half of any sample decays every 12.3 years. Tritium forms naturally high in the atmosphere, when cosmic rays strike nitrogen molecules in the air.

Sources

Bohr Radius. NIST, CODATA 2022.
Theoretical Calculation of Absolute Radii of Atoms and Ions. Part 1. The Atomic Radii. Ghosh and Biswas, International Journal of Molecular Sciences, 2002.
The Bohr Model. In Chemistry 2e. OpenStax, 2019.
The Hydrogen Atom. In University Physics Volume 3. OpenStax, 2016.
Hydrogen: Radii of Atoms and Ions. WebElements, University of Sheffield.
Hydrogen. Royal Society of Chemistry, Periodic Table.
Helium. Royal Society of Chemistry, Periodic Table.
Atomic Weight of Hydrogen. Commission on Isotopic Abundances and Atomic Weights.
Radionuclide Basics: Tritium. U.S. Environmental Protection Agency.
The International System of Units (SI). 9th edition, version 4.01. BIPM, 2026.
Proton RMS Charge Radius. NIST, CODATA 2022.
2026 NFL Rulebook. National Football League, 2026.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
The Nobel Prize in Chemistry 2014: Press Release. Royal Swedish Academy of Sciences, 2014.

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