The Bohr radius is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an atom. The precise definition of the Bohr radius is:
a0 = 4пє0h2 / mee2 = h/mec K
where: є0 is the permittivity of free space h is the reduced Planck's constant me is the electron rest mass e is the elementary charge c is the speed of light in vacuum K is the fine structure constant.