5.3 a₀ (Bohr radius) is equal to 0.00000028046394196687187 mm (millimeter). To convert from Bohr radius to millimeter, multiply by 0.00000005291772489940979 or divide by 18,897,259.886.
physical constant; the most probable distance between an electron and the nucleus in a nonrelativistic model of the hydrogen atom with infinitely heavy nucleus