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The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.merriam-webster.com/dictionary/angstrom. (, ; , ) or ångström is a metric
unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary uni ...
equal to m; that is, one ten-billionth ( US) of a
metre The metre (British spelling) or meter (American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its prefi ...
, a hundred-millionth of a
centimetre 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its deriveds scales. The Microwave are in-between 1 meter to 1 millimeter. A centimetre (international spelling) or centimeter (American spellin ...
,Entry "angstrom" in the Oxford English Dictionary, 2nd edition (1986). Retrieved on 2021-11-22 from https://www.oed.com/oed2/00008552. 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet. The unit is named after the
Swedish Swedish or ' may refer to: Anything from or related to Sweden, a country in Northern Europe. Or, specifically: * Swedish language, a North Germanic language spoken primarily in Sweden and Finland ** Swedish alphabet, the official alphabet used by ...
physicist
Anders Jonas Ångström Anders Jonas Ångström (; 13 August 181421 June 1874) was a Swedish physicist and one of the founders of the science of spectroscopy.P.Murdin (2000): "Angstrom" chapter in ''Encyclopedia of Astronomy and Astrophysics''. Ångström is also wel ...
(1814–1874). The angstrom is often used in the natural sciences and
technology Technology is the application of knowledge to reach practical goals in a specifiable and Reproducibility, reproducible way. The word ''technology'' may also mean the product of such an endeavor. The use of technology is widely prevalent in me ...
to express sizes of
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...
s,
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s, microscopic biological structures, and lengths of
chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
s, arrangement of atoms in crystals,Arturas Vailionis (2015):
Geometry of Crystals
Lecture slides for MatSci162_172, Geometry; Stanford University
archived on 2015-03-19
/ref>
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
s of electromagnetic radiation, and dimensions of integrated circuit parts. The atomic (covalent) radii of
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, sulfur, and
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
are about 1 angstrom, while that of
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
is about 0.5 angstroms.
Visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
has wavelengths in the range of 4000–7000 Å. In the late 19th century, spectroscopists adopted of a metre as a convenient unit to express the wavelengths of characteristic
spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s ( monochromatic components of the emission spectrum) of
chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...
s. However, they soon realized that the definition of the metre at the time, based on a material artifact, was not accurate enough for their work. So, around 1907 they defined their own unit of length, which they called "Ångström", based on the wavelength of a specific spectral line. It was only in 1960, when the metre was redefined in the same way, that the angstrom became again equal to meter. Even though it is a decimal power fraction of the metre, the angstrom was never part of the SI system of units, and it has been increasingly replaced by the nanometre or picometre. Up to 2019 it was listed as a compatible unit by both the International Bureau of Weights and Measures (BIPM) and the US
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...
(NIST), but it is not mentioned in the 9th edition of the official SI document, the "BIPM Brochure" (2019)Bureau international des poids et measures (2019)
Le système international d'unités
complete brochure, 9th edition.
or in the NIST version of the same.NIST (2019):
Special Publication 330: The International System of Units (SI) 2019 Edition
'.
The 8th edition of the BIPM brochure (2006) and the NIST guide 811 (2008)Ambler Thompson and Barry N. Taylor (2009):
B.8 Factors for Units Listed Alphabetically
. ''NIST Guide to the SI'', National Institutes of Standards and Technology. Accessed on 2019-03-02
used the spelling ''ångström'', with Swedish letters; however, this form is rare in English texts. Some popular US dictionaries list only the spelling ''angstrom''. The accepted symbol is "Å", no matter how the unit is spelled.''Webster's Encyclopedic Unabridged Dictionary of the English Language''. Portland House, 1989 However, "A" is often used in less formal contexts or typographically limited media.


History

In 1868,
Swedish Swedish or ' may refer to: Anything from or related to Sweden, a country in Northern Europe. Or, specifically: * Swedish language, a North Germanic language spoken primarily in Sweden and Finland ** Swedish alphabet, the official alphabet used by ...
physicist
Anders Jonas Ångström Anders Jonas Ångström (; 13 August 181421 June 1874) was a Swedish physicist and one of the founders of the science of spectroscopy.P.Murdin (2000): "Angstrom" chapter in ''Encyclopedia of Astronomy and Astrophysics''. Ångström is also wel ...
created a chart of the spectrum of sunlight, in which he expressed the wavelengths of electromagnetic radiation in the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
in multiples of one ten-millionth of a millimetre (or .) Ångström's chart and table of wavelengths in the solar spectrum became widely used in solar physics community, which adopted the unit and named it after him. It subsequently spread to the fields of
astronomical spectroscopy Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light, ultraviolet, X-ray, infrared and radio waves that radiate from stars a ...
, atomic spectroscopy, and then to other sciences that deal with atomic-scale structures. Although intended to correspond to  metres, that definition was not accurate enough for spectroscopy work. Until 1960 the metre was defined as the distance between two scratches on a bar of platinum-
iridium Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of ...
alloy, kept at the BIPM in Paris in a carefully controlled environment. Reliance on that material standard had led to an early error of about one part in 6000 in the tabulated wavelengths. Ångström took the precaution of having the standard bar he used checked against a standard in Paris, but the
metrologist Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in Fran ...
Henri Tresca Henri Édouard Tresca (12 October 1814 – 21 June 1885) was a French mechanical engineer, and a professor at the Conservatoire National des Arts et Métiers in Paris. Work on plasticity He is the father of the field of plasticity, or non-recov ...
reported it to be so incorrect that Ångström's corrected results were more in error than the uncorrected ones. In 1892–1895,
Albert A. Michelson Albert Abraham Michelson FFRS HFRSE (surname pronunciation anglicized as "Michael-son", December 19, 1852 – May 9, 1931) was a German-born American physicist of Polish/Jewish origin, known for his work on measuring the speed of light and esp ...
and Jean-René Benoît, working at the BIPM with specially developed equipment, determined that the length of the international metre standard was equal to times the wavelength of the red line of the emission spectrum of electrically excited
cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
vapor. In 1907, the International Union for Cooperation in Solar Research (which later became the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
) defined the international angstrom as precisely 1/6438.4696 of the wavelength of that line (in dry air at 15 °C (hydrogen scale) and 760  mmHg under a gravity of 9.8067 m/s2). This definition was endorsed at the 7th General Conference on Weights and Measures (CGPM) in 1927, but the material definition of the metre was retained until 1960. 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, which allowed the angstrom to be redefined as being exactly 0.1 nanometres. Although still widely used in physics and chemistry, the angstrom is not a formal part of the International System of Units (SI). The closest SI unit is the nanometre (). The International Committee for Weights and Measures officially discouraged its use, and does not even mention it in the 9th edition of the official standard (2019). The angstrom is also not included in the European Union's catalogue of units of measure that may be used within its internal market.


Angstrom star

After the redefinition of the meter in spectroscopic terms, the Angstrom was formally redefined to be 0.1 nanometers. However, there was briefly thought to be a need for a separate unit of comparable size defined directly in terms of spectroscopy. In 1965, J.A. Bearden defined the ''Angstrom Star'' (symbol: Å*) as 0.202901 times the wavelength of the tungsten parts per million In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, th ...
of the version derived from the new meter. Within ten years, the unit had been deemed both insufficiently accurate (with accuracies closer to 15 parts per million) and obsolete due to higher precision measuring equipment.


Symbol

For compatibility reasons,
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, wh ...
includes the formal symbol at U+212B (ANGSTROM SIGN;
HTML entity In SGML, HTML and XML documents, the logical constructs known as ''character data'' and ''attribute values'' consist of sequences of characters, in which each character can manifest directly (representing itself), or can be represented by a series ...
"Å", "Å", or "Å"), which is deprecated.The Unicode Standard 14
Chapter 22.2 ''Letterlike Symbols'', p. 839
The angstrom sign is normalized into U+00C5 (LATIN CAPITAL LETTER A WITH RING ABOVE; HTML entity "Å", "Å", or "Å").The Unicode Consortium (2008):
The Unicode Standard, Version 5.0
' Chapter
Symbols
.
The Unicode consortium recommends to use the latter. Before digital typesetting, the angstrom (or angstrom unit) was sometimes written as "A.U.". This use is evident in Bragg's paper on the structure of ice, which gives the c- and a-axis lattice constants as 4.52 A.U. and 7.34 A.U., respectively. Ambiguously, the abbreviation "a.u." may also refer to the atomic unit of length, the bohr—about 0.53 Å—or the much larger
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
(about ).


See also

* (for objects on this scale) *
Conversion of units Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors which change the measured quantity value without changing its effects. Overview The process ...
* X unit


References


External links

{{DEFAULTSORT:Angstrom Non-SI metric units Units of length