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How big is a Hydrogen Atom?

110 picometers!

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Hydrogen Atom

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

Scale of the Universe > Hydrogen Atom
Angstrom
An angstrom, which is named after Swedish physicist Anders Jonas Ångström, is one tenth of a nanometer. The symbol for an angstrom is Å. The angstrom is used to express the size of atoms or transistors.
Helium Atom
Hydrogen atoms are larger than helium atoms because more protons in an atom's nucleus pull the electrons in closer. However, additional shells increase the size of the atoms the most. Cesium, the largest atom, has the most shells with the fewest electrons. (Francium has not been measured.)
Smallest Thing Visible to an Electron Microscope
Electron microscopes allow us to see very small things (like atoms) that would otherwise be invisible because they are smaller than the wavelength of visible light.
Carbon Atom
Carbon is the basis of all life on Earth. This is because carbon atoms can create four covalent bonds, which means it can bond in thousands of ways with oxygen, nitrogen, hydrogen, carbon, and other elements.
Water Molecule
I like to think that whenever I drink water, I drink Mickey Mouse heads. Do you, too? Oh, you don't? Well, you should.
Cesium Atom
Atom size is measured by the distance of the nuclei in covalent bonds. The shells don't actually look like this. Cesium is the largest atom, but its atomic number is not the highest. Why? See Helium Atom.

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