The Bohr radius, symbolized a , is the mean radius of the orbit of an electron around the nucleus of a hydrogen atom at its ground state (lowest-energy level). The value of this radius is a physical constant; a is approximately equal to 5.29177 x 10 -11 meter (m). This is 5.29177 x 10 -9 centimeter (cm) or 0.0529177 nanometer (nm). This is a span equivalent to about 1/10,000 of the wavelength of a ray of blue visible light.
The Bohr radius is based on the so-called Bohr model of the atom, named after the Danish physicist and philosopher Niels Bohr (1884-1962). Bohr envisioned atoms as consisting of small, dense nuclei with positive electric charge, around which negatively charged electrons orbit in circular paths. Nowadays, physicists consider this an oversimplification; the electrons are thought to surround the nucleus in spherical probability zones called shells. However, the Bohr radius is still a useful constant because, in a sense, it represents the smallest mean radius normally attainable by a neutral atom.
Also see Table of Physical Units and Constants.