  We propose that any black hole has atomic structure in its inside and has no
horizon as a model of black holes. Our proposal is founded on a mean field
approximation of gravity. The structure of our model consists of a (charged)
singularity at the center and quantum fluctuations of fields around the
singularity, namely, it is quite similar to that of atoms. Any properties of
black holes, e.g. entropy, can be explained by the model. The model naturally
quantizes black holes. In particular, we find the minimum black hole, whose
structure is similar to that of the hydrogen atom and whose Schwarzschild
radius is approximately 1.1287 times the Planck length. Our approach is
conceptually similar to the Bohr's model of the atomic structure, and the
concept of the minimum Schwarzschild radius is similar to that of the Bohr
radius. The model predicts that black holes carry baryon number, and the baryon
number is rapidly violated. This baryon number violation can be used as
verification of the model.
