The angstrom or ångström is a unit of length equal to 10−10 m or 0.1 nm. Its symbol
is Å, a letter in the Scandinavian alphabets. 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. 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.
History 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 1852 Ångström formulated in Optiska undersökningar,
in English translation Optical researches, a law of absorption, later to be modified
somewhat and to become known as Kirchhoff's law of thermal radiation.
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 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; the closest SI unit is 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.
Symbol
Unicode includes the formal symbol at U+212B Å angstrom sign. However, the ångström
sign is also normalized into U+00C5 Å latin capital letter a with ring above
See also 100 picometres
Conversion of units References