jan 1, 1803 - John Dalton came up with his atomic theory.
This theory was the real start of the discovery of the atom and gave a guideline for what atoms were after he completed multiple experiments. In previous years, Democritus was the main scientist studying the atom and his ideas mostly agreed with later scientific theories. However, they were not based on the scientific method so they lacked experimental support. When John Dalton took his turn in studying the atom he used experimental methods and changed his ideas on atoms into a scientific theory which is how his atomic theory became a very important part for further studies of the atom.
Added to timeline:
The Atom vs the Telephone