The
angstrom or
ångström, is a unit of length equal to 1/10,000,000,000 (one ten billionth) of a meter ( or 100
pmA picometre is a unit of length in the metric system, equal to one trillionth, i.e. of a metre, which is the current SI base unit of length...
). Its symbol is the
Swedish letterModern Swedish is written with a 29-letter Latin alphabet:Prior to the 13th edition of Svenska Akademiens ordlista in 2006, the letters and were collated together....
ÅÅ represents various sounds in several languages. Å is part of the alphabets used for the Alemannic and the Bavarian-Austrian dialects of German...
.
Although internationally recognized, the angstrom is not formally a part of the
International System of UnitsThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
(SI); the closest SI unit is the
nanometreA nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
. Nevertheless, the angstrom is often used in the natural sciences and technology to express the sizes of
atomThe atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s, molecules, and
microscopicA microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
biological structuresStructural biology is a branch of molecular biology, biochemistry, and biophysics concerned with the molecular structure of biological macromolecules, especially proteins and nucleic acids, how they acquire the structures they have, and how alterations in their structures affect their function...
, the lengths of
chemical bondA chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
s, the
arrangement of atoms in crystalsCrystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
, the wavelengths of
electromagnetic radiationElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
, and the dimensions of
integrated circuitAn integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
parts.
The unit was named after the Swedish
physicistA physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Anders Jonas ÅngströmAnders Jonas Ångström was a Swedish physicist and one of the founders of the science of spectroscopy.-Biography:...
(ˈɔŋstrøm; 1814–1874). The symbol is always written with a
ring diacriticA ring diacritic may appear above or below letters. It may be combined with some letters of the extended Latin alphabets in various contexts.-Ring above:...
, as in the Swedish letter. Although the unit's name is often written in English without the
diacriticsdiacritics is a quarterly academic journal established in 1971 at Cornell University and published by the Johns Hopkins University Press. Articles serve to review recent literature in the field of literary criticism, and have covered topics in gender studies, political theory, psychoanalysis, queer...
, the official definitions contain diacritics.
History
Anders Jonas Ångström was one of the pioneers in the field of
spectroscopySpectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, and is known also for studies of
astrophysicsAstrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
,
heatIn physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
transfer,
terrestrial magnetismEarth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
, and the
aurora borealisAn aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...
.
In 1868, Ångström created a chart of the spectrum of
solar radiationSunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
that expressed the
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s of
electromagnetic radiationElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
in the
electromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
in multiples of one ten-millionth of a
millimetreThe millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....
(or ). 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 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 UnionThe International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
defined the international angstrom by declaring the wavelength of the red line of
cadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
in
airThe atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
equal to 6438.46963 international angstroms, and this definition was endorsed by the
International Bureau of Weights and MeasuresThe International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...
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 MarketThe European Union's Internal Market seeks to guarantee the free movement of goods, capital, services, and people – the EU's four freedoms – within the EU's 27 member states.The Internal Market is intended to be conducive to increased competition, increased specialisation, larger...
.
Symbol
UnicodeUnicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
includes the "angstrom sign" at U+212B ANGSTROM SIGN (
Å). However, the "angstrom sign" is
normalizedUnicode equivalence is the specification by the Unicode character encoding standard that some sequences of code points represent essentially the same character...
into U+00C5 LATIN CAPITAL LETTER A WITH RING ABOVE (
Å), which is part of the
ISO-Latin-1ISO/IEC 8859-1:1998, Information technology — 8-bit single-byte coded graphic character sets — Part 1: Latin alphabet No. 1, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1987. It is informally referred to as Latin-1. It is generally...
segment (octal 305, hexadecimal C5).