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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.