| Resumo: | Understanding Chemistry requires not only learning scientific concepts but also developing epistemic knowledge about how chemical knowledge is constructed, evaluated, and validated. Explicitly addressing epistemic aspects in chemistry lessons is essential for enhancing students' understanding and their performances. In this sense, this study examined how secondary school students employed epistemic knowledge involved in the scientific practice of modelling when analysing different atomic models. A qualitative design was adopted to examine in detail how participants engaged with some epistemic aspects of modelling. The participants were seventeen 15–16-year-old students enrolled in a Physics and Chemistry course at a rural public school in Spain. The intervention was conducted in the middle of semester over four 50 minute-sessions and consisted of a teaching unit on atomic models. It began with a questionnaire aimed at identifying students’ initial ideas about the atomic structure of matter with an epistemic focus, followed by several teaching activities addressing the different atomic models, with emphasis on their scientific development and historical context. At the end of the unit, an application activity was conducted to assess students’ ability to apply their epistemic and scientific knowledge of the different atomic models. The data, consisting of individual written responses, were analysed through qualitative content analysis focusing on disciplinary epistemic knowledge (recognising representational plurality, model tentativeness, and explanatory scope) and social epistemic practices (model comparison). The main findings reveal persistent challenges in students’ performances, especially in identifying the cumulative explanatory power of atomic models and in connecting phenomena with supporting evidence to each atomic model. The results suggest the need of new instructional experiences as well as strategies to explicitly incorporate epistemic aspects in Chemistry lessons together with content knowledge.
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