Who were the important figures in the development of the atomic theory?
Dalton, Thomson, Rutherford, Bohr, Schrodinger.
That's the rough order of historical development. You will have to look up the details.
The list quite rightly omits the Greek philosopher, Democritus, who coined the idea of the indivisible atom in the the 5th century BC. The individuals on the list were scientists; they tried to interpret natural phenomena by experiment.