The angstrom[1][2][3][4] (/ˈæŋstrəm/;[3][5][6] ANG-strəm[5]) is a unit of length equal to 10−10 m; that is, one ten-billionth of a metre, a hundred-millionth of a centimetre,[7] 0.1 nanometre, or 100 picometres.

In the late 19th century, spectroscopists adopted 10−10 of a metre as a convenient unit to express the wavelengths of characteristic spectral lines (monochromatic components of the emission spectrum) of chemical elements.

So, around 1907 they defined their own unit of length, which they called "Ångström", based on the wavelength of a specific spectral line.

Until 1960 the metre was defined as the distance between two scratches on a bar of platinum-iridium alloy, kept at the BIPM in Paris in a carefully controlled environment.

[18] 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 1553163.5 times the wavelength of the red line of the emission spectrum of electrically excited cadmium vapor.

[citation needed] After the redefinition of the metre in spectroscopic terms, the Angstrom was formally redefined to be 0.1 nanometres.

However, there was briefly thought to be a need for a separate unit of comparable size defined directly in terms of spectroscopy.

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.

[23] Although still widely used in physics and chemistry, the angstrom is not officially a part of the International System of Units (SI).

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).