|Named after||Anders Jonas Ångström|
|1 Å in ...||... is equal to ...|
The ångström 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. Atoms of phosphorus, sulfur, and chlorine are 1 Å in covalent radius, while a hydrogen atom is 0.25 Å; see atomic radius.
The unit was named after the Swedish physicist Anders Jonas Ångström (1814–1874). The symbol is always written with a ring diacritic, as the letter in the Swedish alphabet. Although the unit's name is often written in English without the diacritics, the official definitions contain diacritics.
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 10−7 mm.) Since the human eye is sensitive to wavelengths from about 4000 to 7000 Å, what we commonly call visible light, that choice of unit allowed sufficiently accurate measurements of visible wavelengths without resorting to fractional numbers. The unit then spread to other sciences that deal with atomic-scale structures.
Although intended to correspond to 10−10 metres, for precise spectral analysis the ångström 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. The use of metal bars had been involved in an early error in the value of the ångström of about one part in 6000. Ångström took the precaution of having the standard bar he used checked against a standard in Paris, but the metrologist Henri Tresca reported it to be so much shorter than it really was that Ångström's corrected results were more in error than the uncorrected ones.
In 1907, the International Astronomical Union defined the international ångström by declaring the wavelength of the red line of cadmium in air equal to 6438.46963 international ångströms, and this definition was endorsed by the International Bureau of Weights and Measures in 1927. From 1927 to 1960, the ångström 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 ångström was redefined as being exactly 0.1 nanometres.
Although internationally recognized, the ångström is not formally a part of the International System of Units (SI); the closest SI unit is the nanometre (10−9 m). 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.
Unicode includes the formal symbol at U+212B Å angstrom sign (HTML:
Å). However, the ångström sign is also normalized into U+00C5 Å latin capital letter a with ring above (HTML:
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- Webster′s Encyclopedic Unabridged Dictionary of the English Language. Portland House, 1989.
- International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 127, ISBN 92-822-2213-6
- Thompson, A.; Taylor, B. N (5 October 2010). "B.8 Factors for Units Listed Alphabetically". NIST Guide to the SI. NIST. Retrieved 21 September 2011.
- "Kungliga Vetenskapsakademiens handlingar", roughly translated as "Transactions of the Royal Academy of Sciences", published between 1739 and 1974, see Vetenskapsakademiens Handlingar (in Swedish)
- "A Brief (Incomplete) History of Light and Spectra". ChemTeam.
- Brand, J.C.D. (1995). Lines of Light: Sources of Dispersive Spectroscopy, 1800-1930. CRC Press. p. 47. ISBN 9782884491631.
- The Council of the European Communities (27 May 2009). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". Retrieved 23 September 2011.
- The Unicode Consortium (2007). "Symbols". The Unicode Standard, Version 5.0. Addison-Wesley. p. 493. ISBN 0-321-48091-0. OCLC 145867322.