Definition of Angstrom in Physics and Chemistry

An angstrom or ångström is a unit of length used to measure very small distances. One angstrom is equal to 10⁻¹⁰ m (one ten-billionth of a meter or 0.1 nanometers). Although the unit is recognized world-wide, it is not an International System (SI) or metric unit.

The symbol for angstrom is Å, which is a letter in the Swedish alphabet.

• 1 Å = 10^-10 meters

Uses of the Angstrom

The diameter of an atom is on the order of 1 angstrom, so the unit is particularly handy when referring to the atomic and ionic radius or size of molecules and spacing between planes of atoms in crystals. The covalent radius of atoms of chlorine, sulfur, and phosphorus are about one angstrom, while the size of a hydrogen atom is about half of an angstrom. The angstrom is used in solid-state physics, chemistry, and crystallography. The unit is used to cite wavelengths of light, chemical bond length, and the size of microscopic structures using the electron microscope. X-ray wavelengths may be given in angstroms, as these values typically range from 1 to 10 Å.

Angstrom History

The unit is named for Swedish physicist Anders Jonas Ångström, who used it to produce a chart of the wavelengths of electromagnetic radiation in sunlight in 1868. His use of units made it possible to report the wavelengths of visible light (4000 to 7000 Å) without having to use decimals or fractions. The chart and unit became widely used in solar physics, atomic spectroscopy, and other sciences that deal with extremely small structures.

Although the angstrom is 10⁻¹⁰ meters, it was precisely defined by its own standard because it is so small. The error in the meter standard was larger than the angstrom unit! The 1907 definition of the angstrom was the wavelength of the red line of cadmium set to be 6438.46963 international ångströms. In 1960, the standard for the meter was redefined in terms of spectroscopy, finally basing the two units on the same definition.

Multiples of the Angstrom

Other units based on the angstrom are the micron (10⁴ Å) and the millimicron (10 Å). These units are used to measure thin film thicknesses and molecular diameters.

Writing the Angstrom Symbol

Although the symbol for the angstrom is easy to write on paper, some code is needed to produce it using digital media. In older papers, the abbreviation "A.U." was sometimes used. Methods of writing the symbol include:

• Typing the symbol U+212B or U+00C5 in Unicode
• Using the symbol &#8491 or &#197 in HTML
• Using the code Å in HTML

Sources

• International Bureau of Weights and Measures. The International System of Units (SI) (8th ed.). 2006, p. 127. ISBN 92-822-2213-6.
• Wells, John C. Longman Pronunciation Dictionary (3rd ed.). Longman, 2008. ISBN 9781405881180.