The ångström or angstrom (symbol Å) (pronounced /ˈɔːŋstrəm/; Swedish: IPA: [ˈɔŋstrˈøm]) is an internationally recognized unit of length equal to 0.1 nanometre or 1 × 10⁻¹⁰ metres. It is named after Anders Jonas Ångström. Although accepted for use, it is not formally defined within the International System of Units (SI).

The ångström is often used in the natural sciences for expressing the sizes of atoms, lengths of chemical bonds and the wavelengths of electromagnetic radiation, and in technology for the dimensions of parts of integrated circuits. It is also commonly applied in structural biology.

Contents 1 History 2 Unicode symbol 3 See also 4 References

[edit] History

The ångström is named after the Swedish physicist Anders Jonas Ångström (1814–1874), one of the pioneers in the field of spectroscopy, who 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 expresses the wavelength of electromagnetic radiation in the electromagnetic spectrum in multiples of one ten-millionth of a millimetre, or 1 × 10⁻¹⁰ metres. This unit of length became known as the Ångström unit, and later simply as the ångström.

The visual sensitivity of the human eye is from about 4,000 ångströms (violet) to 7,000 ångströms (dark red) so the use of the ångström as a unit provided a fair amount of discrimination without resort to fractional numbers. Because of its closeness to the scale of atomic and molecular structures it also became popular in chemistry and crystallography.

Although intended to correspond to 1 × 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, but in 1960, the length unit metre itself was redefined in spectroscopic terms, thus aligning the ångström as a submultiple of the metre. In short, one nanometre is equal to ten ångströms.

Today the use of the ångström as a unit is less popular than it used to be, and the nanometre is often used instead. The ångström is officially discouraged by the International Committee for Weights and Measures.

[edit] Unicode symbol

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), and is thereby seen as a (pre-existing) encoding mistake. It is understood that the use of U+00C5 (Å) directly is more appropriate for this reason.^[1]

[edit] See also

• Conversion of units
• 1 E-10 m

[edit] References