The angstrom is a unit of length equal to one ten-billionth of a meter: exactly 0.1 nanometers, or 100 picometers. It is not part of the modern metric system, the International System of Units (SI), but scientists still use it for the sizes of atoms, the lengths of the bonds between them and the wavelengths of light. Its symbol is Å, and it is named after the Swedish physicist Anders Jonas Ångström.

Definition

The International Bureau of Weights and Measures (BIPM), which maintains the SI, lists the angstrom among the units that are not part of the SI. It defines 1 angstrom as 0.1 nanometers, and groups it with other historical names for decimal multiples and fractions of SI units, such as the liter. A nanometer is a billionth of a meter, and a picometer is a trillionth.

In the United States, the National Institute of Standards and Technology (NIST) lists the angstrom among the non-SI units that may still be used alongside the SI. The international committee that oversees the SI allowed this in 1978, because the unit was well established in certain fields. NIST still discourages its own authors from using the angstrom, and says it should not be introduced in fields that do not already use it. NIST keeps the Swedish spelling, ångström.

Size comparisons

• A hydrogen atom is about 1 angstrom across: 1.06 angstroms, measured as twice the Bohr radius.
• Two carbon atoms joined by a single bond sit, on average, 1.54 angstroms apart, center to center. A double bond pulls them to 1.34 angstroms, and a triple bond to 1.20.
• Visible light has wavelengths of 4,000–7,000 angstroms, from violet to red.
• A human hair is 800,000–1,000,000 angstroms wide.
• If one angstrom were stretched to 1 millimeter (0.04 inches), a hair would be 0.8–1 kilometer (0.5–0.6 miles) wide. A person 1.7 meters (5 feet 7 inches) tall would stand 17,000 kilometers (10,600 miles) high, about one and a third times the width of Earth.

Who uses the angstrom

Bonds between atoms are generally 1–2 angstroms long, so distances at the scale of atoms come out as small, convenient numbers in this unit. Chemistry textbooks still list bond lengths in angstroms.

Electron microscopists often give the sharpness of their images in angstroms too. The TEAM 0.5 microscope, unveiled at Lawrence Berkeley National Laboratory in 2008, was built to resolve details half an angstrom apart. Astronomers who study visible light use both angstroms and nanometers for its wavelengths.

Where the name comes from

Anders Jonas Ångström (1814–1874) became a professor of physics at Uppsala University in Sweden in 1858. He was one of the founders of spectroscopy, the study of the colors of light that substances give off or absorb. He was also the first to study the spectrum of the northern lights.

In 1868, Ångström published a detailed study of the Sun's spectrum, Recherches sur le spectre solaire ("Researches on the Solar Spectrum"), which long served as a standard work. He introduced a unit for the wavelengths of light, later adopted internationally and named the angstrom in his honor.

Sources

• The International System of Units (SI). 9th edition, version 4.01. BIPM, 2026.
• NIST Guide to the SI, Chapter 5: Units Outside the SI. NIST.
• Anders and Knut Ångström. Uppsala University.
• Strengths of Ionic and Covalent Bonds. In Chemistry 2e. OpenStax, 2019.
• Electron Microscope Detector Achieves Record Resolution. Cornell University College of Arts and Sciences, 2018.
• Debut of TEAM 0.5, the World's Best Microscope. Lawrence Berkeley National Laboratory, 2008.
• Electromagnetic Spectrum: Introduction. NASA Imagine the Universe, 2013.
• Bohr Radius. NIST, CODATA 2022.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.
• Earth Fact Sheet. NASA, 2024.