Angstrom

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Ångström
Unit of Length
Symbol Å
Named after Anders Jonas Ångström
Unit conversions
1 Å in... is equal to...
   metres    10−10 m
   centimetres    10−8 cm
   nanometres    0.1 nm
   picometres    100 pm

The angstrom or ångström ([ˈɔŋstrøm]) is a unit of length equal to 10−10 m (one ten-billionth of a metre) or 0.1 nm. Its symbol is the Swedish letter Å.

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 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]

History[edit]

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 10−7 mm.)[4] 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.[citation needed] 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. 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.[5]

Symbol[edit]

Example of the Unicode codification

Unicode includes the formal symbol at U+212B angstrom sign (HTML: Å). However, the ångström sign is also normalized into U+00C5 Å latin capital letter a with ring above (HTML: Å Å)[6]

See also[edit]

References[edit]

  1. ^ Webster′s Encyclopedic Unabridged Dictionary of the English Language. Portland House, 1989.
  2. ^ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 127, ISBN 92-822-2213-6 
  3. ^ 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. 
  4. ^ "A Brief (Incomplete) History of Light and Spectra". ChemTeam. 
  5. ^ 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. 
  6. ^ The Unicode Consortium (2007). "Symbols". The Unicode Standard, Version 5.0. Addison-Wesley. p. 493. ISBN 0-321-48091-0. OCLC 145867322.