angstrom
Lengthexact by definition
The angstrom is exactly 10⁻¹⁰ metres, or 100 picometres. It is the natural ruler of atomic structure: a carbon-carbon bond is 1.54 Å, a water molecule is about 2.8 Å across, and the spacing of planes in a crystal lattice is a few Å, which is why X-rays of comparable wavelength diffract from them.
Watch out: It is not an SI unit and the CGPM has never accepted it. Journals increasingly ask for picometres or nanometres, but no crystallographer speaks that way, and the angstrom shows no sign of leaving. The conversion is exact by definition.
| 1 Å | 1.0000000e-10 m |
Named for Anders Jonas Ångström, who in 1868 published a map of the solar spectrum with wavelengths given in units of 10⁻¹⁰ m. Spectroscopists adopted the unit wholesale and structural biology inherited it, which is why protein structures are still quoted at "1.8 Å resolution".