{{Other uses}}
{{Short description|Unit of length}}
{{infobox unit
| name = angstrom
| image = Hydrogen atom.svg
| caption = The width of a hydrogen atom is about 1.1 angstroms; the width of a proton (in black) is {{val|1.7|e=-5|}} angstroms.
| standard = Non-SI metric unit
| quantity = [[Length]]
| symbol = Å
| namedafter = [[Anders Jonas Ångström]]
| units1 = [[SI Units]]
| inunits1 = {{val|e=-10|u=m}} = {{val|0.1|u=nm}}
| units2 = [[CGS Units]]
| inunits2 = {{val|e=-8|u=cm}}
| units3 = [[Imperial units]]
| inunits3= {{cvt|1|Å|in|disp=out|sigfig=7}}
| units4 = [[Planck units]]
| inunits4 = {{val|6.187|e=24|u=''l''P}}
}}
The '''angstrom'''[ ({{IPAc-en|ˈ|æ|ŋ|s|t|r|əm}};][ {{respell|ANG|strəm}}][) or '''ångström''' ({{IPAc-en|ˈ|ɒ|ŋ|s|t|r|əm}})][ is a [[metric units|metric]] [[unit of length]] equal to {{val|e=-10}} [[metre|m]]; that is, one ten-billionth ([[short and long scale|US]]) of a [[metre]], a hundred-millionth of a [[centimetre]],][ 0.1 [[nanometre]], or 100 [[picometre]]s. Its symbol is [[Å]], a letter of the [[Swedish alphabet]]. The unit is named after the [[Swedish people|Swedish]] physicist [[Anders Jonas Ångström]] (1814–1874).][
The angstrom is often used in the [[natural science]]s and [[technology]] to express sizes of [[atom]]s, [[molecule]]s, microscopic biological structures, and lengths of [[chemical bond]]s, [[crystallography|arrangement of atoms in crystals]],][ [[wavelength]]s of [[electromagnetic radiation]], and dimensions of [[integrated circuit]] parts. The [[covalent radius|atomic (covalent) radii]] of [[phosphorus]], [[sulfur]], and [[chlorine]] are about 1 angstrom, while that of [[hydrogen]] is about 0.5 angstroms. [[Visible light]] has wavelengths in the range of 4000–7000 Å.
In the late 19th century, spectroscopists adopted {{val|e=-10}} of a metre as a convenient unit to express the wavelengths of characteristic [[spectral line]]s ([[monochromatic radiation|monochromatic]] components of the [[emission spectrum]]) of [[chemical element]]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 {{val|e=-10}} metre.
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]] (NIST),][ but it is not mentioned in the 9th edition of the official SI document, the "BIPM Brochure" (2019)][ or in the NIST version of the same.][
The 8th edition of the BIPM brochure (2006)][ and the NIST guide 811 (2008)][ used the spelling ''ångström'', with Swedish letters; however, this form is rare in [[English language|English]] texts. Some popular US dictionaries list only the spelling ''angstrom''.][
The accepted symbol is "Å", no matter how the unit is spelled.][ However, "A" is often used in less formal contexts or [[typography|typographically]] limited media.{{citation needed|date=April 2020}}
==History==
[[File:Anders Ångström painting.jpg|thumb|Portrait of Anders Ångström][]]
In 1868, [[Swedish people|Swedish]] physicist [[Anders Jonas Ångström]] created a chart of the spectrum of [[sunlight]], in which he expressed the wavelengths of electromagnetic radiation in the [[electromagnetic spectrum]] in multiples of one ten-millionth of a millimetre (or {{val|e=-7|u=mm}}.)][ 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.{{citation needed|date=April 2020}} It subsequently spread to the fields of [[astronomical spectroscopy]], [[atomic spectroscopy]], and then to other sciences that deal with atomic-scale structures.
Although intended to correspond to {{val|e=-10}} 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]] alloy, kept at the [[International Bureau of Weights and Measures|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]] [[Henri Tresca]] 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]] and [[Jean-René Benoît]], working at the BIPM with specially developed equipment, determined that the length of the international metre standard was equal to {{gaps|1|553|163.5}} times the wavelength of the red line of the [[emission spectrum]] of electrically excited [[cadmium]] vapor.][ In 1907, the International Union for Cooperation in Solar Research (which later became the [[International Astronomical Union]]) 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 [[millimetres of mercury|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,{{citation needed|date=April 2020}} 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.{{citation needed|date=November 2021}} In 1960, the metre itself was redefined in spectroscopic terms, which allowed the angstrom to be redefined as being exactly 0.1 nanometres.{{citation needed|date=April 2020}}
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]] ({{val|e=-9|u=m}}). 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 units of measurement directives|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 ]