Angstrom
The angstrom (/ˈæŋstrəm/, /ˈæŋstrʌm/; ANG-strəm, ANG-strum) or ångström is a metric unit of length equal to 10−10 m; that is, one ten-billionth (US) of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874). The angstrom is often used in the natural sciences and technology to express sizes of atoms, molecules, microscopic biological structures, and lengths of chemical bonds, arrangement of atoms in crystals, wavelengths of electromagnetic radiation, and dimensions of integrated circuit parts. The 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 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. 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 10−10 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 (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 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 typographically limited media. (Wikipedia).
