# Ångström (unit)

Physical unit
Unit name Ångström
Unit symbol ${\ displaystyle \ mathrm {\ AA}}$
Physical quantity (s) length
Formula symbol ${\ displaystyle \ lambda}$, , ,${\ displaystyle a}$${\ displaystyle b}$${\ displaystyle c}$
dimension ${\ displaystyle {\ mathsf {L}}}$
In SI units ${\ displaystyle \ mathrm {1 \; \ mathrm {\ AA} = 10 ^ {- 10} \; m = 100 \; pm}}$
Named after Jonas Ångström is different

The Ångström [ ˈɔŋstrø: m ] (after the Swedish physicist Anders Jonas Ångström ) is a unit of length . The unit symbol is  Å ( A with a ring ). One Ångström is equal to ten millionth of a millimeter . The Ångström is not an SI unit.

1 Å = 100  pm  = 0.1 nm = 10 −10  m

The Ångström is used in particular in crystallography and chemistry to be able to work with "simple" numerical values. 1 Å is the typical order of magnitude for atomic radii and distances between atoms in crystal structures and bond lengths in molecules . The radius of isolated neutral atoms is between 0.3 and 3 Å. Therefore, the angstrom is often used as a unit for distances on an atomic scale, e.g. B. for the thickness of very thin layers , for specifying the wavelength of the X-ray radiation used in their determination in X-ray diffraction experiments such as crystal structure analysis, and for the pore size of stationary phases in liquid chromatographic columns for high-performance liquid chromatography (HPLC).

In thermodynamics , the mean free path of moving molecules is often given in angstroms. It is also used in optics and astronomy to specify a wavelength (although less in German-language, but more in English-language specialist publications).

In 1925 Manne Siegbahn made a similar attempt to come up with easily manageable numerical values with the definition of the X-unit , which  corresponded to about 10 −13 meters. But the Ångström prevailed.

Since the Ångström is not listed in the Units Directive, it is not a legal unit in the EU , nor in Switzerland according to the Swiss Units Ordinance. In DIN  1301-3 it is explicitly listed as a no longer permitted unit.

## Representation in computer systems

Representations of Å in Unicode. The latter option should not be used.

According to the Unicode standard, the unit of length Ångström should be represented by the capital letter Å ( U+00C5). Unicode also contains a character called ANGSTROM SIGN (Ångström character ,:U+212B Å), but this was only included for compatibility with older character coding standards and should not be used in newly created texts .