Bohr radius, in N. Bohr's theory of the hydrogen atom — the radius of the electron orbit nearest to the nucleus (the proton). The Bohr radius a0=h2/me2=5.2917706 (44)·10-11m (as of 1980), where m and e are the mass and the charge of the electron. In quantum mechanics the Bohr radius is defined as the distance from the nucleus at which an electron in an unexcited hydrogen atom can be found with the greatest probability (see Atom).