  We solve the condundrum on whether the molecules of the Reissner-Nordstr\"om
black hole interact through the Ruppeiner thermodynamic geometry, basing our
study on the concept of the black hole molecule proposed in [Phys. Rev. Lett.
115 (2015) 111302] and choosing the appropriate extensive variables. Our
results show that the Reissner-Nordstr\"om black hole is indeed an interaction
system that may be dominated by repulsive interaction. More importantly, with
the help of a novel quantity, namely the thermal-charge density, we describe
the fine micro-thermal structures of the Reissner-Nordstr\"om black hole in
detail. Three different phases are presented, namely the free, interactive, and
balanced phases. The thermal-charge density plays a role similar to the order
parameter, and the back hole undergoes a new phase transition between the free
phase and interactive phase. The competition between the free phase and
interactive phase exists, which leads to extreme behavior of the temperature of
the Reissner-Nordstr\"om black hole. For the extreme Reissner-Nordstr\"om black
hole, the entire system is completely in the interactive phase. More
importantly, we provide the thermodynamic micro-mechanism for the formation of
the naked singularity of the Reissner-Nordstr\"om black hole.
