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.

Comment: To be published in the Proceedings of the X International Ontology Congress