This paper examines the educational and formative value of nineteenth-century atomic theory, with particular reference to the thought of Stanislao Cannizzaro. Through the historical-conceptual journey from Dalton to Avogadro and Cannizzaro’s subsequent clarification, it is argued that a historical-critical approach represents a privileged tool for developing scientific thinking in students. Atomic theory emerges not as an absolute truth, but as an interpretive framework capable of giving meaning to experimental data.