  We discuss, from a historiographical point of view, which was the degree of
certainty that the physicists directly involved in the birth of Quantum
Mechanics (Heisenberg, Born, Jordan, Dirac and Schr\"odinger) gave to the
atomistic hypothesis, starting from 1925, and until the celebration of the 5th
Solvay Conference, in October 1927. In particular, we analyze how was tackled
the problem of the free particle, and how that affected decisively the
interpretations proposed for the new mechanics. We show that, despite focusing
on such a narrow period of time, so close to those seminal years, practically
all the questions which still prevail nowadays, were then already posed. Except
in the case of Schr\"odinger, there hardly were changes of appreciation of the
atomistic hypothesis.
