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| Unit of... | Length |
| Symbol: | Å |
| Named after: | Anders Jonas Ångström |
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| 1 Å in... | is equal to... |
| meters | 10-10 m |
| centimeters | 10-8 cm |
| nanometers | 0.1 nm |
| picometers | 100 pm |
The angstrom or ångström /ˈæŋstrəm/ is a unit of length equal to 1/10,000,000,000 (one ten billionth) of a meter (1×10−10 m or 100 pm). Its symbol is the Swedish letter Å.
Although internationally recognized, the angstrom is not formally a part of the International System of Units (SI); the closest SI unit is the nanometre (1×10−9 m). Nevertheless, the angstrom is often used in the natural sciences and technology to express the sizes of atoms, molecules, and microscopic biological structures, the lengths of chemical bonds, the arrangement of atoms in crystals, the wavelengths of electromagnetic radiation, and the dimensions of integrated circuit parts.
The unit was named after the Swedish physicist Anders Jonas Ångström ([ˈɔŋstrøm]; 1814–1874). The symbol is always written with a ring diacritic, as in the Swedish letter. Although the unit's name is often written in English without the diacritics,[1] the official definitions contain diacritics.[2][3]
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Anders Jonas Ångström was one of the pioneers in the field of spectroscopy, and is known also for studies of astrophysics, heat transfer, terrestrial magnetism, and the aurora borealis.
In 1868, Ångström created a chart of the spectrum of solar radiation that expressed the wavelengths of electromagnetic radiation in the electromagnetic spectrum in multiples of one ten-millionth of a millimetre (or 1×10−10 m). Since the human eye is sensitive to wavelengths from about 4,000 to 7,000 angstroms, that choice of unit allowed sufficiently accurate measurements of visible wavelengths without resort to fractional numbers. The unit then spread to other sciences that deal with atomic-scale structures.
Although intended to correspond to 1×10−10 metres, for precise spectral analysis the angstrom needed to be defined more accurately than the metre which until 1960 was still defined based on the length of a bar of metal held in Paris. In 1907 the International Astronomical Union defined the international angstrom by declaring the wavelength of the red line of cadmium in air equal to 6438.46963 international angstroms, and this definition was endorsed by the International Bureau of Weights and Measures in 1927. From 1927 to 1960, the angstrom remained a secondary unit of length for use in spectroscopy, defined separately from the metre. In 1960 the metre itself was redefined in spectroscopic terms, and then the angstrom was redefined as being exactly 0.1 nanometres.
Today the use of the angstrom is declining in favour of the nanometre. Its use is officially discouraged by the International Committee for Weights and Measures and is not included in the European Union's catalogue of units of measure that may be used within its Internal Market.[4]
Unicode includes the "angstrom sign" at U+212B ANGSTROM SIGN (Å). However, the "angstrom sign" is normalized into U+00C5 LATIN CAPITAL LETTER A WITH RING ABOVE (Å), which is part of the ISO-Latin-1 segment (octal 305, hexadecimal C5).[5]